[RFC PATCH v4 0/3] trace: stack trace deduplication for ftrace ring buffer

[RFC PATCH v4 0/3] trace: stack trace deduplication for ftrace ring buffer

[Li Pengfei]

From: Pengfei Li <lipengfei28@xiaomi.com>

Hi Masami, Steven, all,

This is v4 of the ftrace stackmap series. It is sent as a new thread.

v3: https://lore.kernel.org/all/cover.1779769138.git.lipengfei28@xiaomi.com/

The series adds stack trace deduplication to ftrace. When the
'stackmap' option is enabled alongside 'stacktrace', the ring buffer
stores a 4-byte stack_id instead of a full kernel stack trace, and the
full stacks are exported once via tracefs (stack_map / stack_map_bin).

Rebased onto v7.1-rc5 (e8c2f9fdadee).

Motivation
==========

The target use case is long-duration, from-boot kernel tracing where
the same stacks recur enormously often and the bottleneck is ring
buffer space, not CPU.

Concretely: tracing the slab allocator from boot for hours to study
memory aging and to catch the allocation backtraces behind a usage
peak. With a stacktrace trigger on the slab tracepoints, every event
today carries a full kernel stack (~80-160 bytes). On a fixed-size
ring buffer that bounds how far back in time the trace reaches: the
buffer wraps in seconds to minutes and the early-boot history -- the
part we care about -- is overwritten before it can be consumed.

In this workload the set of distinct stacks is small and highly
repetitive, so storing a 4-byte stack_id per event and the full stack
only once dramatically increases the time span a given buffer covers.
The intended operating model is exactly the low-overhead one ftrace is
good at: let the trace run for a long time producing a comparatively
small, dense log, then resolve stack_ids offline (cat stack_map, or
parse stack_map_bin with the included tool) during analysis.

This is complementary to, not a replacement for, the existing full
stack recording: deep stacks and the early pre-init window still fall
back to full stacks (see below).

Effect on retention
====================

Same fixed per-CPU buffer, slab allocation workload with a shallow
kernel stack (kmem_cache_alloc), stackmap OFF vs ON:

                  retained events   bytes/event   time span
  stackmap OFF        645,068          ~104 B        15.0 s
  stackmap ON       1,397,741           ~48 B        27.7 s
                     2.17x             2.17x          1.85x

The buffer holds ~2.17x more events and reaches ~1.85x further back in
time for the same memory. The win grows with stack depth and with how
repetitive the stacks are; for deep, highly-repeated stacks the
per-event size approaches the 4-byte stack_id plus event header.

Changes since v3
================

Correctness:
  - Deep stacks are never silently truncated or merged. A stack deeper
    than FTRACE_STACKMAP_MAX_DEPTH (64) now falls back to a full stack
    trace; ftrace_stackmap_get_id() returns -E2BIG rather than
    truncating, so two distinct stacks sharing their first 64 frames
    can no longer collapse to one stack_id.
  - reset is now genuinely destructive and coherent: under the
    reader_sem write lock it clears the owning trace_array's ring
    buffer (and snapshot) BEFORE clearing the map, and uses
    tracing_reset_all_cpus() rather than _online_cpus() so a
    TRACE_STACK_ID written by a now-offline CPU cannot survive and
    dangle against a cleared map.
  - __ftrace_trace_stack() reserves the TRACE_STACK_ID ring-buffer
    slot before inserting into the map, so stack_map_stat counters and
    ref_count stay consistent with what the ring buffer actually
    references (failed reservation -> full stack, map untouched).
  - ref_count / successes / drops now saturate (INT_MAX / LONG_MAX)
    instead of wrapping on multi-hour, billions-of-hits traces.

Global-instance gating:
  - Enabling 'stackmap' on a secondary instance via the aggregate
    trace_options file is now rejected, not just hidden in the
    per-instance options/ directory.
  - tracefs init is failure-atomic: the required stack_map file is
    created before the map pointer is published; if it cannot be
    created the map is destroyed and never published. An init-state
    (PENDING/DONE/FAILED) lets boot-time trace_options=stackmap set
    the flag before the map exists (hot path falls back until it is
    published) while still rejecting enables after a permanent init
    failure, so options/stackmap never reports an enabled no-op.

ABI / tooling:
  - Binary magic corrected to 0x46534D42 ('FSMB'); version is 1 (first
    upstream ABI). Documentation, tool and selftest updated to match.
  - Text and binary exports now follow the same trampoline-marker and
    trace_adjust_address() handling as the normal stack print path.
  - stackmap_dump.py emits hex addresses in 'ips' and shows the ftrace
    trampoline marker only in the resolved 'symbols'.

Selftests:
  - New stackmap-reset.tc: verifies reset clears stale <stack_id N>
    from the trace buffer and checks the stack_map_bin magic/version.
  - stackmap-instance-gate.tc extended to verify the trace_options
    write path is rejected on a secondary instance.
  - stackmap-basic.tc no longer treats a nonzero drops count as a
    failure (drops is a by-design fallback); only zero successes with
    nonzero drops is fatal.

Open questions for maintainers
==============================

Two design points where I would value direction before polishing
further:

1. Eager vs lazy allocation. The element pool is allocated at
   fs_initcall when CONFIG_FTRACE_STACKMAP=y, regardless of whether
   userspace ever enables the option (~8 MB at the default bits=14,
   up to ~135 MB at bits=18). This keeps the hot path allocation-free
   with no allocation-failure path under tracing pressure. Is eager
   allocation acceptable, or would you prefer lazy allocation on the
   first 'echo 1 > options/stackmap'?

2. Binary ABI now or later. stack_map_bin is a new tracefs binary
   interface (magic 0x46534D42, version 1). Is it acceptable to
   introduce it now, or would you prefer the first version ship with
   the text stack_map interface only and add the binary export once
   trace-cmd / libtraceevent integration is designed?

Test results
============

QEMU (aarch64 virt, v7.1-rc5 + this series), boot to init smoke test:
  - stackmap functional suite: 16/16 PASS, including reset clearing the
    trace buffer (stale <stack_id> count 48 -> 0), stack_map_bin
    magic/version, global-vs-secondary instance gating, and the
    trace_options rejection on a secondary instance.
  - boot-time activation (trace_options=stackmap,stacktrace on the
    kernel cmdline): 3/3 PASS -- the option survives the
    pre-initialization window and the map is live once published.
  - ftrace startup self-tests pass with the new TRACE_STACK_ID entry.

Device retention numbers above were collected on a Xiaomi SM8850
(ARM64) running an Android workload, comparing the same buffer with
the option off and on.

Known limitations
=================

- Per-instance stackmap support is not included; the option is gated
  to the global instance (in the tracefs layout and at the
  set_tracer_flag() write path). Per-instance maps are a follow-up.
- Deduplication is best-effort, not strict: under heavy concurrent
  contention two CPUs racing with the same stack hash may each claim a
  different slot, producing a few duplicate entries; ref_count is then
  split across them. This keeps the hot path lock-free.
- The stackmap covers kernel stacks only.
- stack_map_bin is a best-effort snapshot, serialized against reset
  but not a fully atomic export.
- trace-cmd / libtraceevent integration is left for follow-up once the
  binary format settles (see open question 2).

Usage
=====

  echo 1 > /sys/kernel/debug/tracing/options/stackmap
  echo 1 > /sys/kernel/debug/tracing/options/stacktrace

Pengfei Li (3):
  trace: add lock-free stackmap for stack trace deduplication
  trace: integrate stackmap into ftrace stack recording path
  trace: add documentation, selftest and tooling for stackmap

 Documentation/trace/ftrace-stackmap.rst       | 177 ++++
 Documentation/trace/index.rst                 |   1 +
 kernel/trace/Kconfig                          |  22 +
 kernel/trace/Makefile                         |   1 +
 kernel/trace/trace.c                          | 216 ++++-
 kernel/trace/trace.h                          |  17 +
 kernel/trace/trace_entries.h                  |  15 +
 kernel/trace/trace_output.c                   |  23 +
 kernel/trace/trace_selftest.c                 |   1 +
 kernel/trace/trace_stackmap.c                 | 889 ++++++++++++++++++
 kernel/trace/trace_stackmap.h                 |  57 ++
 .../ftrace/test.d/ftrace/stackmap-basic.tc    | 111 +++
 .../test.d/ftrace/stackmap-instance-gate.tc   |  54 ++
 .../ftrace/test.d/ftrace/stackmap-reset.tc    |  76 ++
 tools/tracing/stackmap_dump.py                | 164 ++++
 15 files changed, 1821 insertions(+), 3 deletions(-)
 create mode 100644 Documentation/trace/ftrace-stackmap.rst
 create mode 100644 kernel/trace/trace_stackmap.c
 create mode 100644 kernel/trace/trace_stackmap.h
 create mode 100644 tools/testing/selftests/ftrace/test.d/ftrace/stackmap-basic.tc
 create mode 100644 tools/testing/selftests/ftrace/test.d/ftrace/stackmap-instance-gate.tc
 create mode 100644 tools/testing/selftests/ftrace/test.d/ftrace/stackmap-reset.tc
 create mode 100755 tools/tracing/stackmap_dump.py

-- 
2.34.1

[RFC PATCH v4 1/3] trace: add lock-free stackmap for stack trace deduplication

[Li Pengfei]

From: Pengfei Li <lipengfei28@xiaomi.com>

Add a lock-free hash map (ftrace_stackmap) that deduplicates kernel
stack traces for the ftrace ring buffer. Instead of storing full
stack traces (80-160 bytes each) in the ring buffer for every event,
ftrace can store a 4-byte stack_id when the stackmap option is enabled.

The implementation is modeled after tracing_map.c (used by hist
triggers), using the same lock-free design based on Dr. Cliff Click's
non-blocking hash table algorithm:

- Lock-free insert via cmpxchg, safe in NMI/IRQ/any context
- Pre-allocated element pool (zero allocation on hot path)
- Linear probing with 2x over-provisioned table; probe length is
  bounded by FTRACE_STACKMAP_MAX_PROBE so worst-case insert/lookup
  is O(1) even when the table is heavily loaded with claimed-but-
  empty slots from pool exhaustion
- Single global instance (initialized for the global trace array)

The Kconfig depends on ARCH_HAVE_NMI_SAFE_CMPXCHG, matching the
existing tracing_map / hist_triggers requirement: the lock-free
hot path uses cmpxchg in a context that may be reached from NMI.

The stackmap is exported via three tracefs nodes:
- stack_map: text export with symbol resolution (mode 0640)
- stack_map_stat: counters (entries, successes, drops, success_rate)
- stack_map_bin: binary export (magic 0x46534D42 'FSMB', version 1,
  all fields native-endian)

ftrace_stackmap_get_id() never truncates: a stack deeper than
FTRACE_STACKMAP_MAX_DEPTH (64) returns -E2BIG so the caller records a
full stack instead. This prevents two distinct traces that share their
first 64 frames from being merged into one stack_id.

Hot-path counters use per-CPU local_t (NMI-safe single-instruction
increments) instead of atomic64_t. atomic64_t falls back to
raw_spinlock_t-based emulation on 32-bit GENERIC_ATOMIC64 systems,
which would deadlock if an NMI hit while the spinlock was held.
local_t avoids this hazard. All counters saturate rather than wrap on
long (from-boot, multi-hour) traces: ref_count via
atomic_add_unless(.., INT_MAX) and successes/drops via
local_add_unless(.., LONG_MAX).

Reset semantics:
- Reset is a control-path operation only allowed when tracing is
  stopped on the owning trace_array. Online reset (with tracing
  active) is intentionally not supported.
- Reset is destructive: under the reader_sem write lock it clears the
  owning trace_array's ring buffer (and snapshot buffer) BEFORE the
  map, so an external observer never sees "trace still has
  <stack_id N> but the map is already empty". The buffers are cleared
  with tracing_reset_all_cpus() rather than _online_cpus() so a
  TRACE_STACK_ID written by a now-offline CPU cannot survive a reset.
- Reset uses atomic_cmpxchg() to claim the resetting flag, then
  verifies tracer_tracing_is_on() returns false.
- synchronize_rcu() drains in-flight get_id() callers from the
  ftrace callback path (which runs preempt-disabled).
- The reader_sem (rw_semaphore) serializes the clearing against
  tracefs readers (seq_file iteration and stack_map_bin snapshot),
  which run in process context and aren't covered by
  synchronize_rcu(). Readers take it shared, reset takes it
  exclusive, so a reset cannot tear an iteration in progress. The
  hot path doesn't take this lock.
- Reset clears the resetting flag with atomic_set_release() so a
  subsequent get_id() observes a fully cleared map.
- get_id() uses atomic_read_acquire() on resetting so subsequent
  loads of entry->key/val are properly ordered after the check
  (control dependencies only order stores per LKMM).
- Concurrent reset, or reset while tracing is active, returns -EBUSY.

Concurrency notes:
- entry->val publication uses smp_store_release() paired with
  smp_load_acquire() in all dereferencing readers.
- entry->key reads (in get_id, seq_start/next, bin_open) use
  READ_ONCE() to avoid LKMM data races with the cmpxchg writer.
- elt->nr is read with READ_ONCE() and clamped to MAX_DEPTH before
  use in seq_show and bin_open.
- Pool exhaustion: stackmap_get_elt() short-circuits via
  atomic_read() before the contended atomic RMW, avoiding cacheline
  contention once the pool is full. Slots that win cmpxchg but
  cannot get an elt are left 'claimed but empty'; subsequent
  lookups treat val==NULL as a miss and probe past them.

Hash key:
- Per-instance random seed stored in the stackmap struct (no
  global state), seeded at create time.
- 32-bit jhash is forced to 1 if it lands on 0 (which is the
  free-slot sentinel). Full memcmp confirms matches.

Memory:
- Single flat vmalloc for the element pool (no per-elt kzalloc).
- bits parameter clamped to [10, 18]: at the maximum bits=18, the
  element pool is ~135 MB and a stack_map_bin snapshot may briefly
  allocate another ~135 MB.
- struct stackmap_bin_snapshot uses u64 (not size_t) for its size
  field so data[] is 8-byte aligned on both 32-bit and 64-bit
  architectures, avoiding alignment faults when writing u64 IPs
  on strict-alignment architectures.

Kernel command line parameter:
- ftrace_stackmap.bits=N: set map capacity (2^N unique stacks,
  range 10-18, default 14)

Signed-off-by: Pengfei Li <lipengfei28@xiaomi.com>
---
 kernel/trace/Kconfig          |  22 +
 kernel/trace/Makefile         |   1 +
 kernel/trace/trace_stackmap.c | 889 ++++++++++++++++++++++++++++++++++
 kernel/trace/trace_stackmap.h |  57 +++
 4 files changed, 969 insertions(+)
 create mode 100644 kernel/trace/trace_stackmap.c
 create mode 100644 kernel/trace/trace_stackmap.h

diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
index e130da35808f..e49cae886ff0 100644
--- a/kernel/trace/Kconfig
+++ b/kernel/trace/Kconfig
@@ -412,6 +412,28 @@ config STACK_TRACER
 
 	  Say N if unsure.
 
+config FTRACE_STACKMAP
+	bool "Ftrace stack map deduplication"
+	depends on TRACING
+	depends on STACKTRACE
+	depends on ARCH_HAVE_NMI_SAFE_CMPXCHG
+	select KALLSYMS
+	help
+	  This enables a global stack trace hash table for ftrace, inspired
+	  by eBPF's BPF_MAP_TYPE_STACK_TRACE. When enabled, ftrace can store
+	  only a stack_id in the ring buffer instead of the full stack trace,
+	  significantly reducing trace buffer usage when the same call stacks
+	  appear repeatedly.
+
+	  The deduplicated stacks are exported via:
+	    /sys/kernel/debug/tracing/stack_map
+
+	  Writing to this file resets the stack map. Reading shows all unique
+	  stacks with their stack_id and reference count.
+
+	  Say Y if you want to reduce ftrace buffer usage for stack traces.
+	  Say N if unsure.
+
 config TRACE_PREEMPT_TOGGLE
 	bool
 	help
diff --git a/kernel/trace/Makefile b/kernel/trace/Makefile
index 8d3d96e847d8..c2d9b2bf895a 100644
--- a/kernel/trace/Makefile
+++ b/kernel/trace/Makefile
@@ -85,6 +85,7 @@ obj-$(CONFIG_HWLAT_TRACER) += trace_hwlat.o
 obj-$(CONFIG_OSNOISE_TRACER) += trace_osnoise.o
 obj-$(CONFIG_NOP_TRACER) += trace_nop.o
 obj-$(CONFIG_STACK_TRACER) += trace_stack.o
+obj-$(CONFIG_FTRACE_STACKMAP) += trace_stackmap.o
 obj-$(CONFIG_MMIOTRACE) += trace_mmiotrace.o
 obj-$(CONFIG_FUNCTION_GRAPH_TRACER) += trace_functions_graph.o
 obj-$(CONFIG_TRACE_BRANCH_PROFILING) += trace_branch.o
diff --git a/kernel/trace/trace_stackmap.c b/kernel/trace/trace_stackmap.c
new file mode 100644
index 000000000000..9e9fdf85071d
--- /dev/null
+++ b/kernel/trace/trace_stackmap.c
@@ -0,0 +1,889 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Ftrace Stack Map - Lock-free stack trace deduplication for ftrace
+ *
+ * Modeled after tracing_map.c (used by hist triggers), this provides
+ * a lock-free hash map optimized for the ftrace hot path. The design
+ * is based on Dr. Cliff Click's non-blocking hash table algorithm.
+ *
+ * Key properties:
+ * - Lock-free insert via cmpxchg, safe in NMI/IRQ/any context
+ * - Pre-allocated element pool (zero allocation on hot path)
+ * - Linear probing with 2x over-provisioned table; probe length
+ *   bounded by FTRACE_STACKMAP_MAX_PROBE to keep worst-case lookup
+ *   cost constant even when the table is heavily loaded
+ * - Single global instance (initialized for the global trace array)
+ *
+ * Reset is a control-path operation, only allowed when tracing is
+ * stopped on the owning trace_array. The protocol is:
+ *
+ *   - atomic_cmpxchg(&resetting, 0, 1) atomically claims reset rights
+ *     and blocks new get_id() callers (they observe resetting=1 and
+ *     return -EINVAL).
+ *   - trace_types_lock serializes the tracer_tracing_is_on() check and
+ *     the destructive ring-buffer reset against tracefs writes to
+ *     tracing_on.
+ *   - synchronize_rcu() drains in-flight get_id() callers from the
+ *     ftrace callback path, which runs with preemption disabled.
+ *
+ * Online reset (with tracing active) is intentionally not supported
+ * to keep the design simple and the proof obligations small.
+ *
+ * The 32-bit jhash of the stack IPs is the hash table key. On hash
+ * collision, linear probing finds the next slot and full memcmp
+ * confirms the match.
+ *
+ * Concurrent userspace readers (cat stack_map / stack_map_bin) get
+ * a best-effort snapshot. They are coherent with the hot path
+ * (smp_load_acquire on entry->val); they are also serialized
+ * against reset via smap->reader_sem (readers take it in shared
+ * mode, reset in exclusive mode), so a reset cannot tear an
+ * iteration in progress -- it waits for active readers to drop
+ * the rwsem before clearing the map. The hot path is coordinated
+ * with reset separately, via acquire/release on smap->resetting.
+ */
+
+#include <linux/kernel.h>
+#include <linux/slab.h>
+#include <linux/jhash.h>
+#include <linux/seq_file.h>
+#include <linux/kallsyms.h>
+#include <linux/vmalloc.h>
+#include <linux/atomic.h>
+#include <linux/local_lock.h>
+#include <linux/percpu.h>
+#include <linux/random.h>
+#include <linux/rcupdate.h>
+#include <linux/log2.h>
+#include <asm/local.h>
+
+#include "trace.h"
+#include "trace_stackmap.h"
+
+/*
+ * Bound the linear-probe scan length. With a 2x over-provisioned table,
+ * a well-distributed hash gives very short probe chains. Capping at 64
+ * keeps worst-case lookup O(1) even when the table is heavily loaded
+ * with claimed-but-empty slots from pool exhaustion.
+ */
+#define FTRACE_STACKMAP_MAX_PROBE	64
+
+/*
+ * Memory ordering of entry->val: published with smp_store_release()
+ * by the inserter; consumed with smp_load_acquire() by every reader
+ * that dereferences the elt (get_id, seq_show, bin_open). This pairs
+ * the writes to elt->{nr,ips,ref_count} (initialized BEFORE the
+ * publish) with the reads of those fields (which happen AFTER the
+ * load). seq_start / seq_next only test val for NULL and use the
+ * acquire load purely to keep memory ordering symmetric.
+ */
+
+/*
+ * Each pre-allocated element holds one unique stack trace.
+ * Fixed size: MAX_DEPTH entries regardless of actual depth.
+ */
+struct stackmap_elt {
+	u32		nr;		/* actual number of IPs */
+	atomic_t	ref_count;
+	unsigned long	ips[FTRACE_STACKMAP_MAX_DEPTH];
+};
+
+/*
+ * Hash table entry: a 32-bit key (jhash of stack) + pointer to elt.
+ * key == 0 means the slot is free.
+ */
+struct stackmap_entry {
+	u32			key;	/* 0 = free, non-zero = jhash */
+	struct stackmap_elt	*val;	/* NULL until fully published */
+};
+
+static struct stackmap_elt *stackmap_load_elt(struct stackmap_entry *entry)
+{
+	/*
+	 * Pairs with the smp_store_release() that publishes entry->val
+	 * after fully initializing the element payload.
+	 */
+	return smp_load_acquire(&entry->val);
+}
+
+struct ftrace_stackmap {
+	struct trace_array	*tr;		/* owning trace_array */
+	unsigned int		map_bits;
+	unsigned int		map_size;	/* 1 << (map_bits + 1) */
+	unsigned int		max_elts;	/* 1 << map_bits */
+	u32			hash_seed;	/* per-instance jhash seed */
+	atomic_t		next_elt;	/* index into elts pool */
+	struct stackmap_entry	*entries;	/* hash table */
+	struct stackmap_elt	*elts;		/* flat element pool */
+	atomic_t		resetting;
+	/*
+	 * Reader/reset serialization. Held in shared mode (read lock)
+	 * across seq_file iteration and binary snapshot construction;
+	 * held in exclusive mode (write lock) by reset's clearing
+	 * phase. The hot path (get_id) does not take this lock — it
+	 * uses smp_load_acquire/smp_store_release on entry->val and
+	 * the resetting flag for the lock-free protocol.
+	 */
+	struct rw_semaphore	reader_sem;
+	/*
+	 * Per-CPU counters using local_t. local_t increments are NMI-
+	 * safe on all architectures (single-instruction or interrupt-
+	 * masked) and avoid the raw_spinlock_t fallback that
+	 * atomic64_t uses on 32-bit GENERIC_ATOMIC64 — which would
+	 * deadlock if an NMI hit while the spinlock was held.
+	 */
+	local_t __percpu	*successes;	/* events served (hits + new inserts) */
+	local_t __percpu	*drops;
+};
+
+/*
+ * Cap the bits parameter to keep worst-case allocations bounded:
+ *   bits=18 → 256K elts, 512K slots, ~130 MB elt pool, ~130 MB bin
+ *             export.
+ * Smaller workloads should use the default (14) which gives 16K elts
+ * (~8 MB pool); bump bits via the ftrace_stackmap.bits= kernel
+ * parameter for higher unique-stack capacity.
+ */
+#define FTRACE_STACKMAP_BITS_MIN	10
+#define FTRACE_STACKMAP_BITS_MAX	18
+#define FTRACE_STACKMAP_BITS_DEFAULT	14
+
+static unsigned int stackmap_map_bits = FTRACE_STACKMAP_BITS_DEFAULT;
+static int __init stackmap_bits_setup(char *str)
+{
+	unsigned long val;
+
+	if (kstrtoul(str, 0, &val))
+		return -EINVAL;
+	val = clamp_val(val, FTRACE_STACKMAP_BITS_MIN, FTRACE_STACKMAP_BITS_MAX);
+	stackmap_map_bits = val;
+	return 0;
+}
+early_param("ftrace_stackmap.bits", stackmap_bits_setup);
+
+/* --- Element pool --- */
+
+static struct stackmap_elt *stackmap_get_elt(struct ftrace_stackmap *smap)
+{
+	int idx;
+
+	/*
+	 * Fast-path early-out once the pool is fully consumed. Avoids
+	 * the contended atomic RMW on next_elt for every traced event
+	 * after the pool is exhausted.
+	 */
+	if (atomic_read(&smap->next_elt) >= smap->max_elts)
+		return NULL;
+
+	idx = atomic_fetch_add_unless(&smap->next_elt, 1, smap->max_elts);
+	if (idx < smap->max_elts)
+		return &smap->elts[idx];
+	return NULL;
+}
+
+/* --- Create / Destroy / Reset --- */
+
+struct ftrace_stackmap *ftrace_stackmap_create(struct trace_array *tr)
+{
+	struct ftrace_stackmap *smap;
+	unsigned int bits;
+
+	smap = kzalloc_obj(*smap, GFP_KERNEL);
+	if (!smap)
+		return ERR_PTR(-ENOMEM);
+
+	/* Defensive clamp: reject bogus bits even if early_param is bypassed. */
+	bits = clamp_val(stackmap_map_bits,
+			 FTRACE_STACKMAP_BITS_MIN,
+			 FTRACE_STACKMAP_BITS_MAX);
+
+	smap->tr = tr;
+	smap->map_bits = bits;
+	smap->max_elts = 1U << bits;
+	smap->map_size = 1U << (bits + 1);	/* 2x over-provision */
+
+	smap->entries = vzalloc(sizeof(*smap->entries) * smap->map_size);
+	if (!smap->entries) {
+		kfree(smap);
+		return ERR_PTR(-ENOMEM);
+	}
+
+	/*
+	 * Single large vmalloc of the element pool, indexed flat.
+	 * At bits=18 this is 256K * sizeof(struct stackmap_elt). The
+	 * struct is ~520 B (8 + 4 + 4 + 64*8), so total ~135 MB.
+	 */
+	smap->elts = vzalloc(sizeof(*smap->elts) * (size_t)smap->max_elts);
+	if (!smap->elts) {
+		vfree(smap->entries);
+		kfree(smap);
+		return ERR_PTR(-ENOMEM);
+	}
+
+	smap->successes = alloc_percpu(local_t);
+	if (!smap->successes) {
+		vfree(smap->elts);
+		vfree(smap->entries);
+		kfree(smap);
+		return ERR_PTR(-ENOMEM);
+	}
+	smap->drops = alloc_percpu(local_t);
+	if (!smap->drops) {
+		free_percpu(smap->successes);
+		vfree(smap->elts);
+		vfree(smap->entries);
+		kfree(smap);
+		return ERR_PTR(-ENOMEM);
+	}
+
+	smap->hash_seed = get_random_u32();
+	atomic_set(&smap->next_elt, 0);
+	atomic_set(&smap->resetting, 0);
+	init_rwsem(&smap->reader_sem);
+
+	return smap;
+}
+
+void ftrace_stackmap_destroy(struct ftrace_stackmap *smap)
+{
+	if (!smap || IS_ERR(smap))
+		return;
+	free_percpu(smap->drops);
+	free_percpu(smap->successes);
+	vfree(smap->elts);
+	vfree(smap->entries);
+	kfree(smap);
+}
+
+/**
+ * ftrace_stackmap_reset - clear all entries in the stackmap
+ * @smap: the stackmap to reset
+ *
+ * Returns 0 on success, -EBUSY if another reset is already in
+ * progress, or if tracing is currently active on the owning
+ * trace_array.
+ *
+ * Online reset (with tracing active) is not supported. Caller must
+ * stop tracing first (echo 0 > tracing_on).
+ *
+ * Caller is process context (typically sysfs write handler).
+ *
+ * Protocol:
+ *   1. Atomically claim reset rights via cmpxchg on @resetting.
+ *   2. Take trace_types_lock to serialize against tracefs writes to
+ *      tracing_on.
+ *   3. Verify tracing is stopped on @smap->tr; if not, release the
+ *      claim and return -EBUSY. The resetting flag itself blocks
+ *      any subsequent get_id() callers.
+ *   4. synchronize_rcu() drains in-flight get_id() callers from the
+ *      ftrace callback path (which runs preempt-disabled).
+ *   5. Reset the ring buffer(s), then memset entries, elts, and
+ *      counters.
+ *   6. Release the resetting flag with release semantics so any new
+ *      get_id() observes a fully cleared map.
+ */
+int ftrace_stackmap_reset(struct ftrace_stackmap *smap)
+{
+	struct trace_array *tr;
+	int ret = 0;
+
+	if (!smap)
+		return 0;
+
+	if (atomic_cmpxchg(&smap->resetting, 0, 1) != 0)
+		return -EBUSY;
+
+	mutex_lock(&trace_types_lock);
+
+	tr = smap->tr;
+	if (tr && tracer_tracing_is_on(tr)) {
+		ret = -EBUSY;
+		goto out_unlock;
+	}
+
+	/*
+	 * synchronize_rcu() itself is a full barrier; no extra smp_mb()
+	 * is needed before it. It drains in-flight ftrace callbacks that
+	 * may have already passed the resetting check with the old value.
+	 */
+	synchronize_rcu();
+
+	/*
+	 * Take the reader_sem in exclusive mode. This serializes the
+	 * memset against any tracefs reader (seq_file iteration or
+	 * stack_map_bin snapshot) that may currently hold the rwsem
+	 * for read. synchronize_rcu() already drained the hot path;
+	 * this rwsem covers process-context readers that aren't
+	 * preempt-disabled.
+	 */
+	down_write(&smap->reader_sem);
+
+	/*
+	 * Clear the ring buffer(s) BEFORE the map, both under the write
+	 * lock. The ring buffer may still hold TRACE_STACK_ID events
+	 * whose stack_id points at slots we are about to free/reuse.
+	 * Resetting the buffer first guarantees an external observer
+	 * never sees the inconsistent "trace still has <stack_id N> but
+	 * the map is already empty" window: it sees either (old buffer,
+	 * old map) or (cleared buffer, old map) or (cleared buffer,
+	 * cleared map) -- never (old buffer, cleared map).
+	 *
+	 * Use tracing_reset_all_cpus() (not _online_cpus) so per-CPU
+	 * buffers belonging to currently offline CPUs are also cleared.
+	 * The ring buffer is allocated per-possible-CPU; an offline CPU's
+	 * buffer can still hold a TRACE_STACK_ID event written before
+	 * the CPU went offline. tracing_reset_online_cpus() iterates
+	 * for_each_online_buffer_cpu() and would leave that data behind
+	 * to be observed once the CPU comes back online (or by the
+	 * trace reader, which iterates all allocated CPU buffers),
+	 * recreating the stale-stack_id window we are trying to close.
+	 *
+	 * Since reset requires tracing to be stopped, this makes "reset"
+	 * an explicitly destructive operation on the owning trace_array,
+	 * keeping ring-buffer stack_ids and the map coherent.
+	 */
+	if (tr) {
+		tracing_reset_all_cpus(&tr->array_buffer);
+#ifdef CONFIG_TRACER_SNAPSHOT
+		if (tr->allocated_snapshot)
+			tracing_reset_all_cpus(&tr->snapshot_buffer);
+#endif
+	}
+
+	memset(smap->entries, 0, sizeof(*smap->entries) * smap->map_size);
+	memset(smap->elts, 0, sizeof(*smap->elts) * (size_t)smap->max_elts);
+
+	atomic_set(&smap->next_elt, 0);
+	{
+		int cpu;
+
+		for_each_possible_cpu(cpu) {
+			local_set(per_cpu_ptr(smap->successes, cpu), 0);
+			local_set(per_cpu_ptr(smap->drops, cpu), 0);
+		}
+	}
+
+	up_write(&smap->reader_sem);
+
+out_unlock:
+	mutex_unlock(&trace_types_lock);
+
+	/* Release resetting=0 so new get_id() observes a cleared map. */
+	atomic_set_release(&smap->resetting, 0);
+	return ret;
+}
+
+/* --- Core: get_id (lock-free, NMI-safe) --- */
+
+int ftrace_stackmap_get_id(struct ftrace_stackmap *smap,
+			   unsigned long *ips, unsigned int nr_entries)
+{
+	u32 key_hash, idx, test_key, trace_len;
+	struct stackmap_entry *entry;
+	struct stackmap_elt *val;
+	int probes = 0;
+
+	/*
+	 * atomic_read_acquire() pairs with atomic_set_release() in the
+	 * reset path. This ensures that subsequent reads of entry->key
+	 * and entry->val are ordered after this check; without acquire,
+	 * the CPU would only have a control dependency, which orders
+	 * subsequent stores but not loads (per LKMM).
+	 */
+	if (!smap || !nr_entries || atomic_read_acquire(&smap->resetting))
+		return -EINVAL;
+	/*
+	 * Never truncate: a stack deeper than the map can hold must not be
+	 * silently shortened, or two distinct traces sharing their first
+	 * FTRACE_STACKMAP_MAX_DEPTH frames would be merged into one
+	 * stack_id. The caller is expected to fall back to a full stack
+	 * trace for such events. Reject defensively in case of a future
+	 * caller that forgets this contract.
+	 */
+	if (nr_entries > FTRACE_STACKMAP_MAX_DEPTH)
+		return -E2BIG;
+
+	trace_len = nr_entries * sizeof(unsigned long);
+	/*
+	 * jhash2() requires the length in u32 units and the data to be
+	 * u32-aligned. On 64-bit kernels sizeof(unsigned long)==8, so
+	 * trace_len is always a multiple of 8 (hence of 4). Use jhash2
+	 * directly; the cast to u32* is safe because ips[] is naturally
+	 * aligned to sizeof(unsigned long) >= 4.
+	 */
+	key_hash = jhash2((const u32 *)ips, trace_len / sizeof(u32),
+			  smap->hash_seed);
+	if (key_hash == 0)
+		key_hash = 1;	/* 0 means free slot */
+
+	idx = key_hash >> (32 - (smap->map_bits + 1));
+
+	while (probes < FTRACE_STACKMAP_MAX_PROBE) {
+		idx &= (smap->map_size - 1);
+		entry = &smap->entries[idx];
+		/*
+		 * READ_ONCE() to avoid LKMM data race with concurrent
+		 * cmpxchg(&entry->key, 0, key_hash) on this slot.
+		 */
+		test_key = READ_ONCE(entry->key);
+
+		if (test_key == key_hash) {
+			val = stackmap_load_elt(entry);
+			/*
+			 * READ_ONCE(val->nr) keeps style consistent with
+			 * the seq_show / bin_open readers. nr is write-once
+			 * (set before publish, never modified afterwards),
+			 * so the load is data-race-free, but READ_ONCE
+			 * silences any analysis tool that flags a plain
+			 * read of a field that is also read under acquire
+			 * elsewhere.
+			 */
+			if (val && READ_ONCE(val->nr) == nr_entries &&
+			    memcmp(val->ips, ips, trace_len) == 0) {
+				/*
+				 * ref_count is a best-effort popularity
+				 * counter. On a long (from-boot, multi-hour)
+				 * trace a hot stack can be hit billions of
+				 * times. atomic_add_unless() gives true
+				 * saturation at INT_MAX even under concurrent
+				 * hits on multiple CPUs (a plain
+				 * check-then-inc could let several CPUs past
+				 * the check near the cap and still wrap).
+				 */
+				atomic_add_unless(&val->ref_count, 1, INT_MAX);
+				/*
+				 * successes/drops are best-effort throughput
+				 * counters. Saturate at LONG_MAX so they do
+				 * not wrap on long runs (notably where local_t
+				 * is 32-bit), matching ref_count's behaviour.
+				 */
+				local_add_unless(this_cpu_ptr(smap->successes),
+						 1, LONG_MAX);
+				return (int)idx;
+			}
+			/*
+			 * val == NULL: another CPU is mid-insert, or this
+			 * slot is "claimed but empty" (pool exhausted).
+			 * val != NULL but mismatch: 32-bit hash collision
+			 * with a different stack. In both cases, advance.
+			 */
+		} else if (!test_key) {
+			/*
+			 * Free slot: try to claim it.
+			 *
+			 * If two CPUs race here with the same key_hash
+			 * (same stack), one loses the cmpxchg, advances,
+			 * and may insert the same stack at a later slot.
+			 * This can produce a small number of duplicate
+			 * entries under heavy contention. The trade-off
+			 * is accepted to keep the hot path lock-free;
+			 * ref_count is split across the duplicates and
+			 * total memory cost is bounded by the element
+			 * pool size.
+			 */
+			if (cmpxchg(&entry->key, 0, key_hash) == 0) {
+				struct stackmap_elt *elt;
+
+				elt = stackmap_get_elt(smap);
+				if (!elt) {
+					/*
+					 * Pool exhausted. We claimed this
+					 * slot with cmpxchg but cannot fill
+					 * it. Leave key set so the slot
+					 * stays "claimed but empty" — future
+					 * lookups treat val==NULL as a miss
+					 * and probe past it. Cannot revert
+					 * key=0 without racing other CPUs.
+					 */
+					local_add_unless(this_cpu_ptr(smap->drops),
+							 1, LONG_MAX);
+					return -ENOSPC;
+				}
+
+				elt->nr = nr_entries;
+				atomic_set(&elt->ref_count, 1);
+				memcpy(elt->ips, ips, trace_len);
+
+				/*
+				 * Publish elt with release semantics so the
+				 * reader's smp_load_acquire can safely
+				 * dereference val->nr / val->ips.
+				 */
+				smp_store_release(&entry->val, elt);
+				local_add_unless(this_cpu_ptr(smap->successes),
+						 1, LONG_MAX);
+				return (int)idx;
+			}
+			/* cmpxchg failed; another CPU claimed this slot. */
+		}
+
+		idx++;
+		probes++;
+	}
+
+	local_add_unless(this_cpu_ptr(smap->drops), 1, LONG_MAX);
+	return -ENOSPC;
+}
+
+/* --- Text export: /sys/kernel/debug/tracing/stack_map --- */
+
+struct stackmap_seq_private {
+	struct ftrace_stackmap	*smap;
+};
+
+static void *stackmap_seq_start(struct seq_file *m, loff_t *pos)
+{
+	struct stackmap_seq_private *priv = m->private;
+	struct ftrace_stackmap *smap = priv->smap;
+	u32 i;
+
+	if (!smap)
+		return NULL;
+	/*
+	 * Take the reader_sem to serialize against ftrace_stackmap_reset(),
+	 * which holds it for write while clearing the table. Released in
+	 * stackmap_seq_stop(), which seq_file calls regardless of whether
+	 * start() returned an element or NULL (per Documentation/filesystems
+	 * /seq_file.rst: "the iterator value returned by start() or next()
+	 * is guaranteed to be passed to a subsequent next() or stop()").
+	 */
+	down_read(&smap->reader_sem);
+	for (i = *pos; i < smap->map_size; i++) {
+		if (READ_ONCE(smap->entries[i].key) &&
+		    stackmap_load_elt(&smap->entries[i])) {
+			*pos = i;
+			return &smap->entries[i];
+		}
+	}
+	return NULL;
+}
+
+static void *stackmap_seq_next(struct seq_file *m, void *v, loff_t *pos)
+{
+	struct stackmap_seq_private *priv = m->private;
+	struct ftrace_stackmap *smap = priv->smap;
+	u32 i;
+
+	if (!smap)
+		return NULL;
+	for (i = *pos + 1; i < smap->map_size; i++) {
+		if (READ_ONCE(smap->entries[i].key) &&
+		    stackmap_load_elt(&smap->entries[i])) {
+			*pos = i;
+			return &smap->entries[i];
+		}
+	}
+	/*
+	 * Advance *pos past the end so that on the next read() the
+	 * subsequent stackmap_seq_start() call returns NULL and the
+	 * iteration terminates. Without this, seq_read() would loop
+	 * on the last element.
+	 */
+	*pos = smap->map_size;
+	return NULL;
+}
+
+static void stackmap_seq_stop(struct seq_file *m, void *v)
+{
+	struct stackmap_seq_private *priv = m->private;
+	struct ftrace_stackmap *smap = priv->smap;
+
+	/*
+	 * seq_file invokes stop() unconditionally after each iteration
+	 * pass (see seq_read_iter / traverse), even when start() returned
+	 * NULL. Always release here, balanced against the down_read in
+	 * stackmap_seq_start().
+	 */
+	if (smap)
+		up_read(&smap->reader_sem);
+}
+
+static int stackmap_seq_show(struct seq_file *m, void *v)
+{
+	struct stackmap_entry *entry = v;
+	struct stackmap_seq_private *priv = m->private;
+	struct stackmap_elt *elt;
+	u32 idx = entry - priv->smap->entries;
+	u32 i, nr;
+
+	elt = stackmap_load_elt(entry);
+	if (!elt)
+		return 0;
+
+	nr = READ_ONCE(elt->nr);
+	if (nr > FTRACE_STACKMAP_MAX_DEPTH)
+		nr = FTRACE_STACKMAP_MAX_DEPTH;
+
+	seq_printf(m, "stack_id %u [ref %u, depth %u]\n",
+		   idx, atomic_read(&elt->ref_count), nr);
+	for (i = 0; i < nr; i++) {
+		unsigned long ip = elt->ips[i];
+
+		/*
+		 * Mirror trace_stack_print(): __ftrace_trace_stack()
+		 * may replace trampoline addresses with
+		 * FTRACE_TRAMPOLINE_MARKER before the stack reaches the
+		 * map, and normal addresses must go through
+		 * trace_adjust_address() (KASLR / module text delta)
+		 * before symbolization. Without this the export would
+		 * print a bogus symbol for the marker and unadjusted
+		 * addresses for everything else.
+		 */
+		if (ip == FTRACE_TRAMPOLINE_MARKER) {
+			seq_printf(m, "  [%u] [FTRACE TRAMPOLINE]\n", i);
+			continue;
+		}
+		seq_printf(m, "  [%u] %pS\n", i,
+			   (void *)trace_adjust_address(priv->smap->tr, ip));
+	}
+	seq_putc(m, '\n');
+	return 0;
+}
+
+static const struct seq_operations stackmap_seq_ops = {
+	.start	= stackmap_seq_start,
+	.next	= stackmap_seq_next,
+	.stop	= stackmap_seq_stop,
+	.show	= stackmap_seq_show,
+};
+
+static int stackmap_open(struct inode *inode, struct file *file)
+{
+	struct stackmap_seq_private *priv;
+	struct seq_file *m;
+	int ret;
+
+	ret = seq_open_private(file, &stackmap_seq_ops,
+			       sizeof(struct stackmap_seq_private));
+	if (ret)
+		return ret;
+	m = file->private_data;
+	priv = m->private;
+	priv->smap = inode->i_private;
+	return 0;
+}
+
+/*
+ * Accept exactly "0" or "reset" (optionally followed by a single newline).
+ */
+static bool stackmap_write_is_reset(const char *buf, size_t n)
+{
+	if (n > 0 && buf[n - 1] == '\n')
+		n--;
+	return (n == 1 && buf[0] == '0') ||
+	       (n == 5 && memcmp(buf, "reset", 5) == 0);
+}
+
+static ssize_t stackmap_write(struct file *file, const char __user *ubuf,
+			      size_t count, loff_t *ppos)
+{
+	struct seq_file *m = file->private_data;
+	struct stackmap_seq_private *priv = m->private;
+	char buf[8];
+	size_t n = min(count, sizeof(buf) - 1);
+	int ret;
+
+	if (n == 0)
+		return -EINVAL;
+	if (copy_from_user(buf, ubuf, n))
+		return -EFAULT;
+	buf[n] = '\0';
+
+	if (!stackmap_write_is_reset(buf, n))
+		return -EINVAL;
+
+	/*
+	 * ftrace_stackmap_reset() atomically claims reset rights via
+	 * cmpxchg and returns -EBUSY if another reset is in progress
+	 * or if tracing is active.
+	 */
+	ret = ftrace_stackmap_reset(priv->smap);
+	if (ret)
+		return ret;
+	return count;
+}
+
+const struct file_operations ftrace_stackmap_fops = {
+	.open		= stackmap_open,
+	.read		= seq_read,
+	.write		= stackmap_write,
+	.llseek		= seq_lseek,
+	.release	= seq_release_private,
+};
+
+/* --- Stats --- */
+
+static int stackmap_stat_show(struct seq_file *m, void *v)
+{
+	struct ftrace_stackmap *smap = m->private;
+	u64 successes = 0, drops = 0;
+	u32 entries;
+	int cpu;
+
+	if (!smap) {
+		seq_puts(m, "stackmap not initialized\n");
+		return 0;
+	}
+
+	entries = atomic_read(&smap->next_elt);
+	for_each_possible_cpu(cpu) {
+		successes += local_read(per_cpu_ptr(smap->successes, cpu));
+		drops += local_read(per_cpu_ptr(smap->drops, cpu));
+	}
+
+	seq_printf(m, "entries:      %u / %u\n", entries, smap->max_elts);
+	seq_printf(m, "table_size:   %u\n", smap->map_size);
+	seq_printf(m, "successes:    %llu\n", successes);
+	seq_printf(m, "drops:        %llu\n", drops);
+	if (successes + drops > 0)
+		seq_printf(m, "success_rate: %llu%%\n",
+			   successes * 100 / (successes + drops));
+	return 0;
+}
+
+static int stackmap_stat_open(struct inode *inode, struct file *file)
+{
+	return single_open(file, stackmap_stat_show, inode->i_private);
+}
+
+const struct file_operations ftrace_stackmap_stat_fops = {
+	.open		= stackmap_stat_open,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= single_release,
+};
+
+/* --- Binary export --- */
+
+struct stackmap_bin_snapshot {
+	/*
+	 * Use u64 (not size_t) so data[] is 8-byte aligned on both
+	 * 32-bit and 64-bit architectures. The IP array within data[]
+	 * is accessed as u64*, which would alignment-fault on strict
+	 * architectures (e.g. older ARM, SPARC) if data[] started at
+	 * a 4-byte boundary.
+	 */
+	u64	size;
+	char	data[];
+};
+
+static int stackmap_bin_open(struct inode *inode, struct file *file)
+{
+	struct ftrace_stackmap *smap = inode->i_private;
+	struct stackmap_bin_snapshot *snap;
+	struct ftrace_stackmap_bin_header *hdr;
+	size_t alloc_size, off;
+	u32 nr_entries, i, nr_stacks;
+
+	if (!smap)
+		return -ENODEV;
+
+	/*
+	 * Worst-case allocation size: every populated entry uses a
+	 * full-depth stack. The (+1) gives one slack slot in case a
+	 * concurrent insert lands between this snapshot and iteration.
+	 * The loop below performs an explicit bounds check anyway.
+	 *
+	 * At bits=18 this caps at ~135 MB. The file is mode 0440
+	 * (TRACE_MODE_READ), so only privileged users can open it.
+	 */
+	nr_entries = atomic_read(&smap->next_elt);
+	alloc_size = sizeof(*hdr) + (nr_entries + 1) *
+		     (sizeof(struct ftrace_stackmap_bin_entry) +
+		      FTRACE_STACKMAP_MAX_DEPTH * sizeof(u64));
+
+	snap = vmalloc(sizeof(*snap) + alloc_size);
+	if (!snap)
+		return -ENOMEM;
+
+	hdr = (struct ftrace_stackmap_bin_header *)snap->data;
+	hdr->magic = FTRACE_STACKMAP_BIN_MAGIC;
+	hdr->version = FTRACE_STACKMAP_BIN_VERSION;
+	hdr->reserved = 0;
+	off = sizeof(*hdr);
+	nr_stacks = 0;
+
+	/*
+	 * Take reader_sem to serialize against ftrace_stackmap_reset(),
+	 * which clears the table and elt pool under the write lock.
+	 */
+	down_read(&smap->reader_sem);
+
+	for (i = 0; i < smap->map_size; i++) {
+		struct stackmap_entry *entry = &smap->entries[i];
+		struct stackmap_elt *elt;
+		struct ftrace_stackmap_bin_entry *e;
+		u64 *ips_out;
+		u32 k, nr;
+
+		if (!READ_ONCE(entry->key))
+			continue;
+		elt = stackmap_load_elt(entry);
+		if (!elt)
+			continue;
+
+		nr = READ_ONCE(elt->nr);
+		if (nr > FTRACE_STACKMAP_MAX_DEPTH)
+			nr = FTRACE_STACKMAP_MAX_DEPTH;
+
+		/* Bounds check: stop if we would overflow the allocation. */
+		if (off + sizeof(*e) + nr * sizeof(u64) > alloc_size)
+			break;
+
+		e = (struct ftrace_stackmap_bin_entry *)(snap->data + off);
+		e->stack_id = i;
+		e->nr = nr;
+		e->ref_count = atomic_read(&elt->ref_count);
+		e->reserved = 0;
+		off += sizeof(*e);
+
+		ips_out = (u64 *)(snap->data + off);
+		for (k = 0; k < nr; k++) {
+			unsigned long ip = elt->ips[k];
+
+			/*
+			 * Emit the trampoline marker verbatim so userspace
+			 * can render it as [FTRACE TRAMPOLINE]; pass every
+			 * other address through trace_adjust_address() so the
+			 * binary export follows the same address-adjustment
+			 * rules as the text export.
+			 */
+			if (ip == FTRACE_TRAMPOLINE_MARKER)
+				ips_out[k] = (u64)FTRACE_TRAMPOLINE_MARKER;
+			else
+				ips_out[k] = (u64)trace_adjust_address(smap->tr, ip);
+		}
+		off += nr * sizeof(u64);
+		nr_stacks++;
+	}
+
+	up_read(&smap->reader_sem);
+
+	hdr->nr_stacks = nr_stacks;
+	snap->size = off;
+	file->private_data = snap;
+	return 0;
+}
+
+static ssize_t stackmap_bin_read(struct file *file, char __user *ubuf,
+				 size_t count, loff_t *ppos)
+{
+	struct stackmap_bin_snapshot *snap = file->private_data;
+
+	if (!snap)
+		return -EINVAL;
+	return simple_read_from_buffer(ubuf, count, ppos, snap->data, snap->size);
+}
+
+static int stackmap_bin_release(struct inode *inode, struct file *file)
+{
+	vfree(file->private_data);
+	return 0;
+}
+
+const struct file_operations ftrace_stackmap_bin_fops = {
+	.open		= stackmap_bin_open,
+	.read		= stackmap_bin_read,
+	.llseek		= default_llseek,
+	.release	= stackmap_bin_release,
+};
diff --git a/kernel/trace/trace_stackmap.h b/kernel/trace/trace_stackmap.h
new file mode 100644
index 000000000000..7c2e5ab9d36d
--- /dev/null
+++ b/kernel/trace/trace_stackmap.h
@@ -0,0 +1,57 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _TRACE_STACKMAP_H
+#define _TRACE_STACKMAP_H
+
+#include <linux/types.h>
+#include <linux/atomic.h>
+
+#define FTRACE_STACKMAP_MAX_DEPTH	64
+
+/* Binary export format */
+#define FTRACE_STACKMAP_BIN_MAGIC	0x46534D42	/* 'FSMB' */
+#define FTRACE_STACKMAP_BIN_VERSION	1
+
+struct ftrace_stackmap_bin_header {
+	u32 magic;
+	u32 version;
+	u32 nr_stacks;
+	u32 reserved;
+};
+
+struct ftrace_stackmap_bin_entry {
+	u32 stack_id;
+	u32 nr;
+	u32 ref_count;
+	u32 reserved;
+	/* followed by u64 ips[nr] */
+};
+
+struct trace_array;
+
+#ifdef CONFIG_FTRACE_STACKMAP
+
+struct ftrace_stackmap;
+
+struct ftrace_stackmap *ftrace_stackmap_create(struct trace_array *tr);
+void ftrace_stackmap_destroy(struct ftrace_stackmap *smap);
+int ftrace_stackmap_get_id(struct ftrace_stackmap *smap,
+			   unsigned long *ips, unsigned int nr_entries);
+int ftrace_stackmap_reset(struct ftrace_stackmap *smap);
+
+extern const struct file_operations ftrace_stackmap_fops;
+extern const struct file_operations ftrace_stackmap_stat_fops;
+extern const struct file_operations ftrace_stackmap_bin_fops;
+
+#else
+
+struct ftrace_stackmap;
+static inline struct ftrace_stackmap *
+ftrace_stackmap_create(struct trace_array *tr) { return NULL; }
+static inline void ftrace_stackmap_destroy(struct ftrace_stackmap *s) { }
+static inline int ftrace_stackmap_get_id(struct ftrace_stackmap *s,
+					 unsigned long *ips, unsigned int n)
+{ return -EOPNOTSUPP; }
+static inline int ftrace_stackmap_reset(struct ftrace_stackmap *s) { return 0; }
+
+#endif
+#endif /* _TRACE_STACKMAP_H */
-- 
2.34.1

[RFC PATCH v4 2/3] trace: integrate stackmap into ftrace stack recording path

[Li Pengfei]

From: Pengfei Li <lipengfei28@xiaomi.com>

Add TRACE_STACK_ID event type and integrate ftrace_stackmap into
__ftrace_trace_stack(). When the 'stackmap' trace option is enabled,
the stack recording path stores a 4-byte stack_id in the ring buffer
instead of the full stack trace.

Changes:

- New TRACE_STACK_ID in trace_type enum and stack_id_entry in
  trace_entries.h.

- New TRACE_ITER(STACKMAP) trace option flag; when CONFIG_FTRACE_STACKMAP
  is disabled, TRACE_ITER_STACKMAP_BIT is defined as -1 so that
  TRACE_ITER(STACKMAP) evaluates to 0 (following the existing pattern
  used by TRACE_ITER_PROF_TEXT_OFFSET).

- 'stackmap' is added to TOP_LEVEL_TRACE_FLAGS and ZEROED_TRACE_FLAGS
  so it is only exposed under the top-level trace instance, matching
  the convention already used for global-only options such as 'printk'
  and 'record-cmd'. Secondary instances under tracing/instances/*/
  do not see the option in their options/ directory.

- set_tracer_flag() additionally rejects enabling STACKMAP on a
  secondary instance. The per-option file is hidden on secondary
  instances, but a write to the aggregate trace_options file still
  reaches set_tracer_flag(); without this check the bit could be
  accepted and then become a silent no-op in the hot path (where
  tr->stackmap is NULL). This closes the global-instance-only gate
  at the write path, not just in the tracefs layout.

- __ftrace_trace_stack() reserves the TRACE_STACK_ID ring-buffer slot
  BEFORE calling ftrace_stackmap_get_id(), so the map (and its
  ref_count / success counters) is only mutated when a ring-buffer
  event will actually reference the entry. If the reservation fails
  it falls back to a full stack; if get_id() fails it discards the
  reserved slot and falls back. A stack deeper than
  FTRACE_STACKMAP_MAX_DEPTH skips the map entirely (get_id() would
  return -E2BIG) and records a full stack, so deep traces are never
  truncated or merged.

- Stackmap pointer read with smp_load_acquire(), published with
  smp_store_release() to ensure proper initialization ordering. The
  hot path falls back to a full stack whenever tr->stackmap is NULL.

- ftrace_stackmap_create() takes the owning trace_array so the
  stackmap can later clear that trace_array's buffers during reset.

- Added stack_id print handler in trace_output.c and TRACE_STACK_ID
  to trace_valid_entry() in trace_selftest.c so ftrace startup
  selftests accept the new entry type when the stackmap option is
  enabled.

Failure-atomic init and boot-time activation:

- The global stackmap and its tracefs files are created during
  tracer_init_tracefs(). stack_map is the single required file (it is
  both the resolver and the reset interface); it is created BEFORE the
  map pointer is published with smp_store_release(), so an observed
  non-NULL tr->stackmap implies the resolver/reset file exists. If
  stack_map cannot be created the map is destroyed and never published.

- A small init-state (PENDING / DONE / FAILED) lets set_tracer_flag()
  distinguish "not initialized yet" from "init failed". Boot-time
  options (trace_options=stackmap,stacktrace) are applied before the
  tracefs init work runs; the flag is allowed to be set while init is
  PENDING (the hot path falls back until the map is published, then the
  boot-set option takes effect), and is only rejected once init has
  permanently FAILED. On failure the STACKMAP flag is also cleared from
  the global instance so options/stackmap never reports an enabled
  no-op.

Fallback behavior: if stackmap returns an error (pool exhausted,
resetting, NULL pointer, or a too-deep stack), the full stack trace is
recorded as before -- no new failure modes introduced.

Per-instance stackmap support is left as a follow-up; gating the
option to the global instance (both in the tracefs layout and at the
set_tracer_flag() write path) makes the global-only scope explicit.

Usage:
  echo 1 > /sys/kernel/debug/tracing/options/stackmap
  echo 1 > /sys/kernel/debug/tracing/options/stacktrace

Signed-off-by: Pengfei Li <lipengfei28@xiaomi.com>
---
 kernel/trace/trace.c          | 216 +++++++++++++++++++++++++++++++++-
 kernel/trace/trace.h          |  17 +++
 kernel/trace/trace_entries.h  |  15 +++
 kernel/trace/trace_output.c   |  23 ++++
 kernel/trace/trace_selftest.c |   1 +
 5 files changed, 269 insertions(+), 3 deletions(-)

diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 6eb4d3097a4d..e00bee5d0e01 100644
--- a/kernel/trace/trace.c
+++ b/kernel/trace/trace.c
@@ -57,6 +57,7 @@
 
 #include "trace.h"
 #include "trace_output.h"
+#include "trace_stackmap.h"
 
 #ifdef CONFIG_FTRACE_STARTUP_TEST
 /*
@@ -509,12 +510,13 @@ EXPORT_SYMBOL_GPL(unregister_ftrace_export);
 /* trace_options that are only supported by global_trace */
 #define TOP_LEVEL_TRACE_FLAGS (TRACE_ITER(PRINTK) |			\
 	       TRACE_ITER(PRINTK_MSGONLY) | TRACE_ITER(RECORD_CMD) |	\
-	       TRACE_ITER(PROF_TEXT_OFFSET) | FPROFILE_DEFAULT_FLAGS)
+	       TRACE_ITER(PROF_TEXT_OFFSET) | TRACE_ITER(STACKMAP) |	\
+	       FPROFILE_DEFAULT_FLAGS)
 
 /* trace_flags that are default zero for instances */
 #define ZEROED_TRACE_FLAGS \
 	(TRACE_ITER(EVENT_FORK) | TRACE_ITER(FUNC_FORK) | TRACE_ITER(TRACE_PRINTK) | \
-	 TRACE_ITER(COPY_MARKER))
+	 TRACE_ITER(COPY_MARKER) | TRACE_ITER(STACKMAP))
 
 /*
  * The global_trace is the descriptor that holds the top-level tracing
@@ -1562,7 +1564,7 @@ void tracing_reset_online_cpus(struct array_buffer *buf)
 	ring_buffer_record_enable(buffer);
 }
 
-static void tracing_reset_all_cpus(struct array_buffer *buf)
+void tracing_reset_all_cpus(struct array_buffer *buf)
 {
 	struct trace_buffer *buffer = buf->buffer;
 
@@ -2184,6 +2186,75 @@ void __ftrace_trace_stack(struct trace_array *tr,
 	}
 #endif
 
+#ifdef CONFIG_FTRACE_STACKMAP
+	/*
+	 * If stackmap dedup is enabled, try to store only the stack_id
+	 * in the ring buffer instead of the full stack trace.
+	 *
+	 * Reserve the TRACE_STACK_ID ring-buffer slot BEFORE inserting
+	 * into the stackmap. This guarantees the map is only mutated
+	 * (and its ref_count / success counters bumped) when a
+	 * ring-buffer event will actually reference the entry:
+	 *   - reservation fails  -> fall back to full stack, map untouched
+	 *   - get_id() fails      -> discard the reserved slot, fall back
+	 * so stack_map_stat counters stay consistent with what the ring
+	 * buffer holds, and a failed reservation never consumes a map
+	 * slot for an event that records a full stack anyway.
+	 */
+	if (tr->trace_flags & TRACE_ITER(STACKMAP)) {
+		struct ftrace_stackmap *smap;
+		struct stack_id_entry *sid_entry;
+		int sid;
+
+		/*
+		 * Pairs with the smp_store_release() that publishes the
+		 * fully initialized global stackmap at tracefs init.
+		 */
+		smap = smp_load_acquire(&tr->stackmap);
+		if (!smap)
+			goto full_stack;
+
+		/*
+		 * The stackmap stores at most FTRACE_STACKMAP_MAX_DEPTH
+		 * frames per entry. A deeper trace would be truncated, and
+		 * two distinct stacks that share the first MAX_DEPTH frames
+		 * would hash and compare equal, silently merging into one
+		 * stack_id. Keep the conservative full-stack path for deep
+		 * traces so no information is lost or misattributed.
+		 */
+		if (nr_entries > FTRACE_STACKMAP_MAX_DEPTH)
+			goto full_stack;
+
+		event = __trace_buffer_lock_reserve(buffer, TRACE_STACK_ID,
+						    sizeof(*sid_entry), trace_ctx);
+		if (!event)
+			goto full_stack;
+
+		sid = ftrace_stackmap_get_id(smap, fstack->calls, nr_entries);
+		if (sid < 0) {
+			/*
+			 * Pool exhausted or a reset is in progress. Discard
+			 * the reserved stack_id slot and record the full
+			 * stack instead, so the event still gets a trace.
+			 */
+			__trace_event_discard_commit(buffer, event);
+			goto full_stack;
+		}
+
+		sid_entry = ring_buffer_event_data(event);
+		sid_entry->stack_id = sid;
+		/*
+		 * stack_id is a synthetic side-event attached to a
+		 * primary trace event that was already subject to
+		 * filtering. No per-event filter is defined for
+		 * TRACE_STACK_ID, so commit unconditionally.
+		 */
+		__buffer_unlock_commit(buffer, event);
+		goto out;
+	}
+full_stack:
+#endif
+
 	event = __trace_buffer_lock_reserve(buffer, TRACE_STACK,
 				    struct_size(entry, caller, nr_entries),
 				    trace_ctx);
@@ -3979,6 +4050,33 @@ int trace_keep_overwrite(struct tracer *tracer, u64 mask, int set)
 	return 0;
 }
 
+#ifdef CONFIG_FTRACE_STACKMAP
+/*
+ * Tracks tracefs-time initialization of the global stackmap so that
+ * set_tracer_flag() can distinguish "not initialized yet" from
+ * "initialization permanently failed".
+ *
+ * Boot-time options (trace_options=stackmap,stacktrace) are applied
+ * very early, before tracer_init_tracefs() creates and publishes the
+ * map. We must allow the STACKMAP flag to be set during that window
+ * (the hot path falls back to a full stack while tr->stackmap is NULL,
+ * then starts using the map once it is published). We must, however,
+ * reject the enable once init has *failed*, so options/stackmap never
+ * reports an enabled no-op.
+ *
+ * Written once from the tracefs init work before any concurrent
+ * userspace writer to trace_options can run, then only read; a plain
+ * int is therefore sufficient.
+ */
+enum {
+	STACKMAP_INIT_PENDING,	/* tracer_init_tracefs() not run yet */
+	STACKMAP_INIT_DONE,	/* map published, stack_map file created */
+	STACKMAP_INIT_FAILED,	/* permanent failure, never available */
+};
+
+static int stackmap_init_state = STACKMAP_INIT_PENDING;
+#endif
+
 int set_tracer_flag(struct trace_array *tr, u64 mask, int enabled)
 {
 	switch (mask) {
@@ -3993,6 +4091,33 @@ int set_tracer_flag(struct trace_array *tr, u64 mask, int enabled)
 	if (!!(tr->trace_flags & mask) == !!enabled)
 		return 0;
 
+#ifdef CONFIG_FTRACE_STACKMAP
+	/*
+	 * STACKMAP is intentionally global-instance-only: the dedup map,
+	 * its tracefs files (stack_map / stack_map_stat / stack_map_bin)
+	 * and the lifetime/reset semantics are tied to the global trace
+	 * array. options/stackmap is hidden on secondary instances via
+	 * TOP_LEVEL_TRACE_FLAGS, but writes still reach set_tracer_flag()
+	 * through the aggregate trace_options file. Reject the enable on
+	 * a secondary instance so it cannot be silently accepted and then
+	 * become a no-op in the hot path (where tr->stackmap is NULL and
+	 * the code falls back to a full stack trace).
+	 *
+	 * On the global instance, allow the enable while init is still
+	 * pending (boot-time trace_options=stackmap is applied before the
+	 * tracefs init work creates the map; the hot path falls back
+	 * until the map is published). Only reject once init has
+	 * permanently failed, so options/stackmap never reports an
+	 * enabled no-op. READ_ONCE() suffices: this only inspects the
+	 * init state, it does not dereference the map (the hot path uses
+	 * smp_load_acquire(&tr->stackmap) for that).
+	 */
+	if (mask == TRACE_ITER(STACKMAP) && enabled &&
+	    (tr != &global_trace ||
+	     READ_ONCE(stackmap_init_state) == STACKMAP_INIT_FAILED))
+		return -EINVAL;
+#endif
+
 	/* Give the tracer a chance to approve the change */
 	if (tr->current_trace->flag_changed)
 		if (tr->current_trace->flag_changed(tr, mask, !!enabled))
@@ -9222,6 +9347,91 @@ static __init void tracer_init_tracefs_work_func(struct work_struct *work)
 			NULL, &tracing_dyn_info_fops);
 #endif
 
+#ifdef CONFIG_FTRACE_STACKMAP
+	{
+		struct ftrace_stackmap *smap;
+		struct dentry *map_file;
+
+		smap = ftrace_stackmap_create(&global_trace);
+		if (!IS_ERR(smap)) {
+			/*
+			 * Failure-atomic init: stack_map is the single
+			 * required tracefs file (it doubles as the reset
+			 * interface and the human-readable resolver). If
+			 * we cannot create it, the hot path must not be
+			 * able to emit <stack_id N> events that no one can
+			 * resolve or clear, so refuse to publish the map
+			 * and tear it down.
+			 *
+			 * Create stack_map BEFORE smp_store_release() so an
+			 * observed non-NULL global_trace.stackmap implies
+			 * its resolver/reset file exists.
+			 */
+			map_file = trace_create_file("stack_map",
+						     TRACE_MODE_WRITE, NULL,
+						     smap,
+						     &ftrace_stackmap_fops);
+			if (!map_file) {
+				pr_warn("ftrace stackmap init: stack_map create failed, dedup disabled\n");
+				ftrace_stackmap_destroy(smap);
+				/*
+				 * Permanent failure. Record it and clear a
+				 * STACKMAP flag that a boot-time
+				 * trace_options=stackmap may have set, so
+				 * options/stackmap does not report an
+				 * enabled no-op and later userspace enables
+				 * return -EINVAL.
+				 */
+				WRITE_ONCE(stackmap_init_state,
+					   STACKMAP_INIT_FAILED);
+				global_trace.trace_flags &=
+					~TRACE_ITER(STACKMAP);
+			} else {
+				/*
+				 * smp_store_release pairs with the
+				 * smp_load_acquire() in
+				 * __ftrace_trace_stack(). Publishing only
+				 * after the required file exists keeps
+				 * "smap visible" => "resolver/reset
+				 * available".
+				 */
+				smp_store_release(&global_trace.stackmap,
+						  smap);
+				WRITE_ONCE(stackmap_init_state,
+					   STACKMAP_INIT_DONE);
+				/*
+				 * stat and bin are auxiliary observability
+				 * surfaces. If they fail to be created we
+				 * keep dedup enabled (the kernel side still
+				 * works, and stack_map alone is enough to
+				 * resolve and reset); trace_create_file()
+				 * already pr_warn()s on failure.
+				 */
+				trace_create_file("stack_map_stat",
+						  TRACE_MODE_READ, NULL,
+						  smap,
+						  &ftrace_stackmap_stat_fops);
+				trace_create_file("stack_map_bin",
+						  TRACE_MODE_READ, NULL,
+						  smap,
+						  &ftrace_stackmap_bin_fops);
+			}
+		} else {
+			pr_warn("ftrace stackmap init failed, dedup disabled\n");
+			/*
+			 * global_trace is statically defined; its stackmap
+			 * field is zero-initialized via BSS, so leaving it
+			 * NULL ensures the smp_load_acquire() in
+			 * __ftrace_trace_stack() falls back to full stack.
+			 * Mark init failed and clear any boot-time STACKMAP
+			 * flag so userspace enables are rejected rather than
+			 * becoming silent no-ops.
+			 */
+			WRITE_ONCE(stackmap_init_state, STACKMAP_INIT_FAILED);
+			global_trace.trace_flags &= ~TRACE_ITER(STACKMAP);
+		}
+	}
+#endif
 	create_trace_instances(NULL);
 
 	update_tracer_options();
diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 80fe152af1dd..95db43bfc747 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -57,6 +57,7 @@ enum trace_type {
 	TRACE_TIMERLAT,
 	TRACE_RAW_DATA,
 	TRACE_FUNC_REPEATS,
+	TRACE_STACK_ID,
 
 	__TRACE_LAST_TYPE,
 };
@@ -453,6 +454,9 @@ struct trace_array {
 	struct cond_snapshot	*cond_snapshot;
 #endif
 	struct trace_func_repeats	__percpu *last_func_repeats;
+#ifdef CONFIG_FTRACE_STACKMAP
+	struct ftrace_stackmap		*stackmap;
+#endif
 	/*
 	 * On boot up, the ring buffer is set to the minimum size, so that
 	 * we do not waste memory on systems that are not using tracing.
@@ -579,6 +583,8 @@ extern void __ftrace_bad_type(void);
 			  TRACE_GRAPH_RET);		\
 		IF_ASSIGN(var, ent, struct func_repeats_entry,		\
 			  TRACE_FUNC_REPEATS);				\
+		IF_ASSIGN(var, ent, struct stack_id_entry,		\
+			  TRACE_STACK_ID);				\
 		__ftrace_bad_type();					\
 	} while (0)
 
@@ -689,6 +695,7 @@ extern int tracing_disabled;
 int tracer_init(struct tracer *t, struct trace_array *tr);
 int tracing_is_enabled(void);
 void tracing_reset_online_cpus(struct array_buffer *buf);
+void tracing_reset_all_cpus(struct array_buffer *buf);
 void tracing_reset_all_online_cpus(void);
 void tracing_reset_all_online_cpus_unlocked(void);
 int tracing_open_generic(struct inode *inode, struct file *filp);
@@ -1449,7 +1456,16 @@ extern int trace_get_user(struct trace_parser *parser, const char __user *ubuf,
 # define STACK_FLAGS
 #endif
 
+#ifdef CONFIG_FTRACE_STACKMAP
+# define STACKMAP_FLAGS				\
+			C(STACKMAP,		"stackmap"),
+#else
+# define STACKMAP_FLAGS
+# define TRACE_ITER_STACKMAP_BIT	-1
+#endif
+
 #ifdef CONFIG_FUNCTION_PROFILER
+
 # define PROFILER_FLAGS					\
 		C(PROF_TEXT_OFFSET,	"prof-text-offset"),
 # ifdef CONFIG_FUNCTION_GRAPH_TRACER
@@ -1506,6 +1522,7 @@ extern int trace_get_user(struct trace_parser *parser, const char __user *ubuf,
 		FUNCTION_FLAGS					\
 		FGRAPH_FLAGS					\
 		STACK_FLAGS					\
+		STACKMAP_FLAGS					\
 		BRANCH_FLAGS					\
 		PROFILER_FLAGS					\
 		FPROFILE_FLAGS
diff --git a/kernel/trace/trace_entries.h b/kernel/trace/trace_entries.h
index 54417468fdeb..89ed14b7e5fd 100644
--- a/kernel/trace/trace_entries.h
+++ b/kernel/trace/trace_entries.h
@@ -250,6 +250,21 @@ FTRACE_ENTRY(user_stack, userstack_entry,
 		 (void *)__entry->caller[6], (void *)__entry->caller[7])
 );
 
+/*
+ * Stack ID entry - stores only a stack_id referencing the stackmap.
+ * Used when CONFIG_FTRACE_STACKMAP is enabled to deduplicate stacks.
+ */
+FTRACE_ENTRY(stack_id, stack_id_entry,
+
+	TRACE_STACK_ID,
+
+	F_STRUCT(
+		__field(	int,		stack_id	)
+	),
+
+	F_printk("<stack_id %d>", __entry->stack_id)
+);
+
 /*
  * trace_printk entry:
  */
diff --git a/kernel/trace/trace_output.c b/kernel/trace/trace_output.c
index a5ad76175d10..68678ea88159 100644
--- a/kernel/trace/trace_output.c
+++ b/kernel/trace/trace_output.c
@@ -1517,6 +1517,28 @@ static struct trace_event trace_user_stack_event = {
 	.funcs		= &trace_user_stack_funcs,
 };
 
+/* TRACE_STACK_ID */
+static enum print_line_t trace_stack_id_print(struct trace_iterator *iter,
+					      int flags, struct trace_event *event)
+{
+	struct stack_id_entry *field;
+	struct trace_seq *s = &iter->seq;
+
+	trace_assign_type(field, iter->ent);
+	trace_seq_printf(s, "<stack_id %d>\n", field->stack_id);
+
+	return trace_handle_return(s);
+}
+
+static struct trace_event_functions trace_stack_id_funcs = {
+	.trace		= trace_stack_id_print,
+};
+
+static struct trace_event trace_stack_id_event = {
+	.type		= TRACE_STACK_ID,
+	.funcs		= &trace_stack_id_funcs,
+};
+
 /* TRACE_HWLAT */
 static enum print_line_t
 trace_hwlat_print(struct trace_iterator *iter, int flags,
@@ -1908,6 +1930,7 @@ static struct trace_event *events[] __initdata = {
 	&trace_wake_event,
 	&trace_stack_event,
 	&trace_user_stack_event,
+	&trace_stack_id_event,
 	&trace_bputs_event,
 	&trace_bprint_event,
 	&trace_print_event,
diff --git a/kernel/trace/trace_selftest.c b/kernel/trace/trace_selftest.c
index 929c84075315..0c97065b0d68 100644
--- a/kernel/trace/trace_selftest.c
+++ b/kernel/trace/trace_selftest.c
@@ -14,6 +14,7 @@ static inline int trace_valid_entry(struct trace_entry *entry)
 	case TRACE_CTX:
 	case TRACE_WAKE:
 	case TRACE_STACK:
+	case TRACE_STACK_ID:
 	case TRACE_PRINT:
 	case TRACE_BRANCH:
 	case TRACE_GRAPH_ENT:
-- 
2.34.1

[RFC PATCH v4 3/3] trace: add documentation, selftest and tooling for stackmap

[Li Pengfei]

From: Pengfei Li <lipengfei28@xiaomi.com>

Add supporting files for the ftrace stackmap feature:

Documentation/trace/ftrace-stackmap.rst:
  Documentation covering design, usage, tracefs interface, binary
  format, and performance characteristics. Added to the 'Core Tracing
  Frameworks' toctree in Documentation/trace/index.rst. Documents:
  - Reset is destructive: it requires tracing to be stopped and also
    clears the ring buffer so no stale <stack_id N> survives
  - Boot-time activation via trace_options=stackmap
  - bits parameter range [10, 18] and worst-case memory usage
  - tracefs file modes (0640 / 0440)
  - Best-effort snapshot semantics for stack_map_bin, serialized
    against reset via the reader_sem
  - Counter naming: successes (events served), drops, success_rate;
    successes/drops are best-effort and saturate on long runs
  - Gravestone amplification when the pool is exhausted

tools/testing/selftests/ftrace/test.d/ftrace/stackmap-basic.tc:
  Functional selftest verifying:
  - stackmap tracefs nodes exist
  - enabling stackmap + stacktrace produces stack_id events
  - stack_map_stat shows non-zero successes (a nonzero drops count is
    a legitimate by-design fallback and is not treated as failure;
    only zero successes alongside nonzero drops is fatal)
  - reset clears entries when tracing is stopped
  - reset is rejected (-EBUSY) while tracing is active
  Test reads trace contents BEFORE switching back to the nop tracer
  (tracer_init() unconditionally resets the ring buffer). The
  function:tracer dependency is declared in '# requires:' so
  ftracetest skips on kernels without CONFIG_FUNCTION_TRACER instead
  of failing spuriously.

tools/testing/selftests/ftrace/test.d/ftrace/stackmap-reset.tc:
  Verifies the destructive-reset semantics and the binary ABI header:
  - after 'echo 0 > stack_map', the trace buffer no longer contains
    any stale <stack_id N>
  - stack_map_bin begins with the expected magic and version

tools/testing/selftests/ftrace/test.d/ftrace/stackmap-instance-gate.tc:
  Verifies the option is gated to the top-level instance: a secondary
  instance neither exposes options/stackmap nor the stack_map* nodes,
  and writing 'stackmap' to its aggregate trace_options file is
  rejected rather than accepted as a no-op.

tools/tracing/stackmap_dump.py:
  Python script to parse the binary stack_map_bin export.
  Features:
  - Automatic endianness detection via magic number
  - Batched addr2line via stdin (avoids ARG_MAX with large stacks)
  - JSON output mode (ips are always hex addresses; the ftrace
    trampoline marker is shown only in the resolved symbols)
  - Top-N filtering by ref_count

Binary format: all fields are native-endian. The parser detects
byte order by reading the magic value (0x46534D42 = 'FSMB').

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202605160010.fakzGVVq-lkp@intel.com/
Signed-off-by: Pengfei Li <lipengfei28@xiaomi.com>
---
 Documentation/trace/ftrace-stackmap.rst       | 177 ++++++++++++++++++
 Documentation/trace/index.rst                 |   1 +
 .../ftrace/test.d/ftrace/stackmap-basic.tc    | 111 +++++++++++
 .../test.d/ftrace/stackmap-instance-gate.tc   |  54 ++++++
 .../ftrace/test.d/ftrace/stackmap-reset.tc    |  76 ++++++++
 tools/tracing/stackmap_dump.py                | 164 ++++++++++++++++
 6 files changed, 583 insertions(+)
 create mode 100644 Documentation/trace/ftrace-stackmap.rst
 create mode 100644 tools/testing/selftests/ftrace/test.d/ftrace/stackmap-basic.tc
 create mode 100644 tools/testing/selftests/ftrace/test.d/ftrace/stackmap-instance-gate.tc
 create mode 100644 tools/testing/selftests/ftrace/test.d/ftrace/stackmap-reset.tc
 create mode 100755 tools/tracing/stackmap_dump.py

diff --git a/Documentation/trace/ftrace-stackmap.rst b/Documentation/trace/ftrace-stackmap.rst
new file mode 100644
index 000000000000..8d0b5c389862
--- /dev/null
+++ b/Documentation/trace/ftrace-stackmap.rst
@@ -0,0 +1,177 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+======================
+Ftrace Stack Map
+======================
+
+:Author: Pengfei Li <lipengfei28@xiaomi.com>
+
+Overview
+========
+
+The ftrace stack map provides stack trace deduplication for the ftrace
+ring buffer. When enabled, instead of storing full kernel stack traces
+(typically 80-160 bytes each) in the ring buffer for every event, ftrace
+stores only a 4-byte ``stack_id``. The full stacks are maintained in a
+separate hash table and exported via tracefs for userspace to resolve.
+
+This is inspired by eBPF's ``BPF_MAP_TYPE_STACK_TRACE`` but integrated
+into ftrace's infrastructure, requiring no userspace daemon.
+
+Configuration
+=============
+
+Enable ``CONFIG_FTRACE_STACKMAP=y`` in the kernel config.
+
+Kernel command line parameters:
+
+- ``ftrace_stackmap.bits=N`` - Set map capacity to 2^N unique stacks
+  (default: 14 → 16384 stacks; valid range: 10-18).
+
+  At ``bits=18`` the kernel reserves roughly 130 MB of vmalloc memory
+  for the element pool. Each ``open()`` of ``stack_map_bin`` may
+  briefly allocate a similar amount for a snapshot. The cap is set
+  intentionally to bound memory usage.
+
+Usage
+=====
+
+Enable stack deduplication::
+
+    echo 1 > /sys/kernel/debug/tracing/options/stackmap
+    echo 1 > /sys/kernel/debug/tracing/options/stacktrace
+    echo function > /sys/kernel/debug/tracing/current_tracer
+
+The trace output will show ``<stack_id N>`` instead of full stack traces::
+
+    sh-1234 [006] d.h.. 123.456789: <stack_id 42>
+
+To view the actual stacks::
+
+    cat /sys/kernel/debug/tracing/stack_map
+
+Output format::
+
+    stack_id 42 [ref 1337, depth 8]
+      [0] schedule+0x48/0xc0
+      [1] schedule_timeout+0x1c/0x30
+      ...
+
+To view statistics::
+
+    cat /sys/kernel/debug/tracing/stack_map_stat
+
+Output::
+
+    entries:      2500 / 16384
+    table_size:   32768
+    successes:    148923
+    drops:        0
+    success_rate: 100%
+
+To reset the stack map (tracing must be stopped first)::
+
+    echo 0 > /sys/kernel/debug/tracing/tracing_on
+    echo 0 > /sys/kernel/debug/tracing/stack_map
+
+Reset returns ``-EBUSY`` if tracing is currently active, or if another
+reset is already in progress.
+
+Reset is destructive to the trace buffer: because the ring buffer may
+still hold ``<stack_id N>`` events that reference soon-to-be-reused
+slots, resetting the map also resets the owning trace buffer (and its
+snapshot, if allocated). This keeps ring-buffer stack_ids and the map
+coherent. Read out any trace data you need before resetting.
+
+Boot-time activation
+====================
+
+The stackmap option can be enabled from the kernel command line::
+
+    trace_options=stackmap,stacktrace
+
+Trace events that fire before the tracefs filesystem is initialized
+(``fs_initcall`` time) fall back to recording full stack traces; once
+``ftrace_stackmap_create()`` runs, subsequent events are deduplicated.
+The crossover is automatic and lossless — no events are dropped, but
+early-boot stacks recorded before the crossover are not deduplicated.
+
+Tracefs Nodes
+=============
+
+The stack_map files are owned by root and not world-readable
+(``stack_map``: 0640; ``stack_map_stat`` and ``stack_map_bin``: 0440).
+
+``stack_map``
+    Text export of all deduplicated stacks with symbol resolution.
+    Writing ``0`` or ``reset`` clears all entries (only when tracing
+    is stopped).
+
+``stack_map_stat``
+    Statistics: entries (allocated unique stacks), table_size,
+    successes (events served), drops (events that fell back to
+    full-stack recording), and success_rate. Drops accumulate when
+    the element pool is exhausted; once that happens, slots that
+    won the cmpxchg but failed to allocate an element remain
+    "claimed but empty" and increase probe pressure for any future
+    insert hashing to the same bucket. Reset (when tracing is
+    stopped) clears these gravestones.
+
+``stack_map_bin``
+    Binary export for efficient userspace consumption. Format:
+
+    - Header (16 bytes): magic(u32) + version(u32) + nr_stacks(u32) + reserved(u32)
+    - Per stack: stack_id(u32) + nr(u32) + ref_count(u32) + reserved(u32) + ips(u64 × nr)
+
+    All fields are written in the kernel's native byte order.
+    Userspace tools detect endianness by reading the magic value.
+    Magic: ``0x46534D42`` ('FSMB'), Version: 1.
+
+    Trampoline frames are exported as the sentinel value
+    ``0x7fffffff`` (FTRACE_TRAMPOLINE_MARKER); all other addresses are
+    passed through ``trace_adjust_address()`` so they match the
+    ``stack_map`` text output's address-adjustment rules. Note this is
+    the same adjustment ftrace applies to its own trace output (mainly
+    relevant for persistent / last-boot buffers), not a general KASLR
+    un-offset: resolving these addresses offline still requires the
+    matching kernel's symbol information.
+
+    The export is a best-effort snapshot allocated at ``open()``;
+    concurrent inserts during the snapshot may be truncated. A
+    bounds check ensures no overflow.
+
+Design
+======
+
+The stack map is modeled after ``tracing_map.c`` (used by hist triggers),
+using a lock-free design based on Dr. Cliff Click's non-blocking hash table
+algorithm:
+
+- **Lookup/Insert**: Lock-free via ``cmpxchg``, safe in NMI/IRQ/any context
+- **Memory**: Pre-allocated element pool, zero allocation on the hot path
+  (no GFP_ATOMIC failures under memory pressure)
+- **Collision**: Linear probing with a 2x over-provisioned table; probe
+  length is bounded so worst-case insert/lookup is O(1)
+- **Scope**: Currently supports the global trace instance
+- **Hash**: 32-bit jhash with a per-instance random seed; full ``memcmp``
+  confirms matches
+
+Deduplication is best-effort, not strict: if two CPUs race in the
+insert path with the same ``key_hash`` (i.e. the same stack), the
+``cmpxchg`` loser advances by one slot and may insert the same stack
+again. Under heavy contention this can produce a small number of
+duplicate entries for the same stack; ``ref_count`` is then split
+across the duplicates. Total memory is still bounded by the element
+pool size, and lookup correctness is unaffected (each duplicate is
+a self-consistent entry with its own ``stack_id``). The trade-off is
+intentional and keeps the hot path lock-free.
+
+Performance
+===========
+
+Typical results on an aarch64 SMP system (function tracer, 2 seconds):
+
+- Unique stacks: ~3000
+- Dedup rate: 84-98% (depends on workload diversity)
+- Ring buffer savings: ~80% for stack data
+- Overhead per event: ~50ns (one jhash + hash table lookup)
diff --git a/Documentation/trace/index.rst b/Documentation/trace/index.rst
index 5d9bf4694d5d..ac8b1141c23a 100644
--- a/Documentation/trace/index.rst
+++ b/Documentation/trace/index.rst
@@ -33,6 +33,7 @@ the Linux kernel.
    ftrace
    ftrace-design
    ftrace-uses
+   ftrace-stackmap
    kprobes
    kprobetrace
    fprobetrace
diff --git a/tools/testing/selftests/ftrace/test.d/ftrace/stackmap-basic.tc b/tools/testing/selftests/ftrace/test.d/ftrace/stackmap-basic.tc
new file mode 100644
index 000000000000..64dfe7cc66bd
--- /dev/null
+++ b/tools/testing/selftests/ftrace/test.d/ftrace/stackmap-basic.tc
@@ -0,0 +1,111 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+# description: ftrace - stackmap basic functionality
+# requires: stack_map options/stackmap function:tracer
+
+# Test that ftrace stackmap deduplication works:
+# 1. Enable stackmap + stacktrace options
+# 2. Run function tracer briefly
+# 3. Verify trace contains <stack_id> events (read BEFORE switching
+#    tracer back to nop, since tracer_init() resets the ring buffer)
+# 4. Verify stack_map has entries and at least some successes (drops is
+#    a legitimate by-design fallback counter and is allowed to be nonzero;
+#    only zero successes alongside nonzero drops indicates breakage)
+# 5. Verify reset is rejected (-EBUSY) while tracing is active
+# 6. Verify reset clears the map when tracing is stopped
+
+fail() {
+    echo "FAIL: $1"
+    exit_fail
+}
+
+# Restore state on any exit (success, fail, or interrupt) so a
+# half-finished test does not leave stacktrace/stackmap enabled.
+cleanup() {
+    disable_tracing 2>/dev/null
+    echo nop > current_tracer 2>/dev/null
+    echo 0 > options/stackmap 2>/dev/null
+    echo 0 > options/stacktrace 2>/dev/null
+}
+trap cleanup EXIT
+
+disable_tracing
+clear_trace
+
+# Verify stackmap files exist
+test -f stack_map      || fail "stack_map file missing"
+test -f stack_map_stat || fail "stack_map_stat file missing"
+test -f stack_map_bin  || fail "stack_map_bin file missing"
+
+# Enable stackmap dedup
+echo 1 > options/stackmap
+echo 1 > options/stacktrace
+
+# Run function tracer briefly
+echo function > current_tracer
+enable_tracing
+sleep 1
+disable_tracing
+
+# Read trace contents NOW, before switching tracer back to nop.
+# tracer_init() unconditionally calls tracing_reset_online_cpus(),
+# so the ring buffer would be empty after 'echo nop > current_tracer'.
+count=$(grep -c "<stack_id" trace || true)
+: "${count:=0}"
+if [ "$count" -eq 0 ]; then
+    fail "trace has no <stack_id> events"
+fi
+
+# Now safe to switch back and disable options
+echo nop > current_tracer
+echo 0 > options/stackmap
+
+# Check stack_map_stat
+entries=$(cat stack_map_stat | grep "^entries:" | awk '{print $2}')
+: "${entries:=0}"
+if [ "$entries" -eq 0 ]; then
+    fail "stackmap has zero entries after tracing"
+fi
+
+successes=$(cat stack_map_stat | grep "^successes:" | awk '{print $2}')
+: "${successes:=0}"
+if [ "$successes" -eq 0 ]; then
+    fail "stackmap has zero successes"
+fi
+
+drops=$(cat stack_map_stat | grep "^drops:" | awk '{print $2}')
+: "${drops:=0}"
+# drops is a legitimate by-design fallback counter: when the map is full
+# or under heavy probe pressure, stackmap falls back to recording a full
+# stack instead of a stack_id. A nonzero drops count is therefore not a
+# failure. Only treat it as fatal if dedup never worked at all (no
+# successes), which would indicate the feature is genuinely broken rather
+# than merely under pressure.
+if [ "$successes" -eq 0 ] && [ "$drops" -ne 0 ]; then
+    fail "stackmap had $drops drops and zero successes (feature broken?)"
+fi
+
+# Check stack_map text output is parseable
+first_id=$(cat stack_map | grep "^stack_id" | head -1 | awk '{print $2}')
+if [ -z "$first_id" ]; then
+    fail "stack_map output has no stack_id entries"
+fi
+
+# Test that reset is rejected while tracing is active
+enable_tracing
+if echo 0 > stack_map 2>/dev/null; then
+    disable_tracing
+    fail "stackmap reset should fail while tracing is active"
+fi
+disable_tracing
+
+# Test reset works when tracing is stopped
+echo 0 > stack_map
+entries_after=$(cat stack_map_stat | grep "^entries:" | awk '{print $2}')
+: "${entries_after:=-1}"
+if [ "$entries_after" -ne 0 ]; then
+    fail "stackmap reset did not clear entries (got $entries_after)"
+fi
+
+echo "stackmap basic test passed: $entries unique stacks, $successes successes, $drops drops"
+exit 0
diff --git a/tools/testing/selftests/ftrace/test.d/ftrace/stackmap-instance-gate.tc b/tools/testing/selftests/ftrace/test.d/ftrace/stackmap-instance-gate.tc
new file mode 100644
index 000000000000..28810ba20432
--- /dev/null
+++ b/tools/testing/selftests/ftrace/test.d/ftrace/stackmap-instance-gate.tc
@@ -0,0 +1,54 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+# description: ftrace - stackmap option is gated to the top-level trace instance
+# requires: stack_map options/stackmap instances
+
+# The 'stackmap' option is added to TOP_LEVEL_TRACE_FLAGS, matching the
+# convention used for global-only options like 'printk' and 'record-cmd'.
+# Verify that:
+# 1. The global instance exposes options/stackmap and the stack_map* nodes.
+# 2. A newly created secondary instance under instances/ does NOT expose
+#    options/stackmap or stack_map* nodes.
+
+fail() {
+    echo "FAIL: $1"
+    rmdir instances/test_stackmap_gate 2>/dev/null
+    exit_fail
+}
+
+# 1. Global instance must expose the option and the nodes
+test -e options/stackmap || fail "options/stackmap missing on global instance"
+test -e stack_map        || fail "stack_map missing on global instance"
+test -e stack_map_stat   || fail "stack_map_stat missing on global instance"
+test -e stack_map_bin    || fail "stack_map_bin missing on global instance"
+
+# 2. Create a secondary instance and verify it does NOT see the option
+#    or the stack_map* nodes.
+mkdir instances/test_stackmap_gate || fail "could not create secondary instance"
+
+if [ -e instances/test_stackmap_gate/options/stackmap ]; then
+    fail "secondary instance unexpectedly exposes options/stackmap"
+fi
+
+for f in stack_map stack_map_stat stack_map_bin; do
+    if [ -e instances/test_stackmap_gate/$f ]; then
+        fail "secondary instance unexpectedly has $f"
+    fi
+done
+
+# 3. The aggregate trace_options file still reaches set_tracer_flag(),
+#    so writing 'stackmap' there must be rejected on a secondary
+#    instance. Otherwise the bit could appear set in trace_options
+#    while the hot path silently falls back to a full stack trace
+#    (tr->stackmap == NULL).
+if echo stackmap > instances/test_stackmap_gate/trace_options 2>/dev/null; then
+    fail "secondary instance accepted 'echo stackmap > trace_options'"
+fi
+if grep -qw stackmap instances/test_stackmap_gate/trace_options; then
+    fail "secondary instance trace_options reports stackmap as set"
+fi
+
+rmdir instances/test_stackmap_gate || fail "could not remove secondary instance"
+
+echo "stackmap option gating to top-level instance works"
+exit 0
diff --git a/tools/testing/selftests/ftrace/test.d/ftrace/stackmap-reset.tc b/tools/testing/selftests/ftrace/test.d/ftrace/stackmap-reset.tc
new file mode 100644
index 000000000000..803cc282f9ab
--- /dev/null
+++ b/tools/testing/selftests/ftrace/test.d/ftrace/stackmap-reset.tc
@@ -0,0 +1,76 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+# description: ftrace - stackmap reset clears the trace buffer and ABI header
+# requires: stack_map options/stackmap function:tracer
+
+# Lock in the two things most likely to regress in the stackmap ABI /
+# lifetime:
+#   1. Resetting the stackmap (echo 0 > stack_map, tracing stopped) also
+#      clears the trace buffer, so no stale <stack_id N> can be left
+#      dangling against an emptied map.
+#   2. The stack_map_bin header carries the expected magic ('FSMB' =
+#      0x46534D42) and version (1).
+
+fail() {
+    echo "FAIL: $1"
+    exit_fail
+}
+
+cleanup() {
+    disable_tracing 2>/dev/null
+    echo nop > current_tracer 2>/dev/null
+    echo 0 > options/stackmap 2>/dev/null
+    echo 0 > options/stacktrace 2>/dev/null
+}
+trap cleanup EXIT
+
+disable_tracing
+clear_trace
+
+echo 1 > options/stackmap
+echo 1 > options/stacktrace
+echo function > current_tracer
+enable_tracing
+sleep 1
+disable_tracing
+
+# Sanity: the buffer must contain stack_id events before reset, otherwise
+# the post-reset emptiness check below would be meaningless.
+before=$(grep -c "<stack_id" trace || true)
+: "${before:=0}"
+if [ "$before" -eq 0 ]; then
+    fail "no <stack_id> events captured before reset"
+fi
+
+# Reset while tracing is stopped. This must succeed AND clear the trace
+# buffer (destructive reset semantics).
+echo 0 > stack_map || fail "reset rejected while tracing stopped"
+
+after=$(grep -c "<stack_id" trace || true)
+: "${after:=0}"
+if [ "$after" -ne 0 ]; then
+    fail "trace still has $after stale <stack_id> events after reset"
+fi
+
+entries=$(cat stack_map_stat | grep "^entries:" | awk '{print $2}')
+: "${entries:=-1}"
+if [ "$entries" -ne 0 ]; then
+    fail "stackmap still has $entries entries after reset"
+fi
+
+# Binary export header: magic 'FSMB' (0x46534D42) + version 1.
+# od -tx4 renders the 32-bit words in the target's native byte order,
+# which matches what the kernel wrote, so the comparison is endian-safe.
+if command -v od >/dev/null 2>&1; then
+    magic=$(od -An -tx4 -N4 stack_map_bin | tr -d ' \n')
+    if [ "$magic" != "46534d42" ]; then
+        fail "stack_map_bin bad magic: 0x$magic (expected 46534d42)"
+    fi
+    ver=$(od -An -tx4 -j4 -N4 stack_map_bin | tr -d ' \n')
+    if [ "$ver" != "00000001" ]; then
+        fail "stack_map_bin bad version: 0x$ver (expected 00000001)"
+    fi
+fi
+
+echo "stackmap reset test passed: cleared $before stack_id events, ABI header ok"
+exit 0
diff --git a/tools/tracing/stackmap_dump.py b/tools/tracing/stackmap_dump.py
new file mode 100755
index 000000000000..2d9c49b776e6
--- /dev/null
+++ b/tools/tracing/stackmap_dump.py
@@ -0,0 +1,164 @@
+#!/usr/bin/env python3
+# SPDX-License-Identifier: GPL-2.0
+"""
+stackmap_dump.py - Parse and display ftrace stack_map_bin binary export.
+
+Usage:
+    # Pull from device and parse
+    adb pull /sys/kernel/debug/tracing/stack_map_bin /tmp/stack_map.bin
+    python3 stackmap_dump.py /tmp/stack_map.bin
+
+    # With vmlinux for offline symbol resolution
+    python3 stackmap_dump.py /tmp/stack_map.bin --vmlinux vmlinux
+
+    # JSON output for tooling
+    python3 stackmap_dump.py /tmp/stack_map.bin --json
+"""
+
+import struct
+import sys
+import argparse
+import json
+import subprocess
+
+MAGIC = 0x46534D42  # 'FSMB'
+HEADER_SIZE = 16  # 4 x u32
+ENTRY_SIZE = 16   # 4 x u32
+
+# __ftrace_trace_stack() replaces trampoline addresses with this marker
+# (FTRACE_TRAMPOLINE_MARKER == (unsigned long)INT_MAX) before the stack
+# is stored, so the binary export carries it verbatim.
+FTRACE_TRAMPOLINE_MARKER = 0x7fffffff
+TRAMPOLINE_LABEL = '[FTRACE TRAMPOLINE]'
+
+
+def detect_endianness(data):
+    """Detect byte order from magic number in header."""
+    if len(data) < 4:
+        raise ValueError("File too small")
+    magic_le = struct.unpack_from('<I', data, 0)[0]
+    if magic_le == MAGIC:
+        return '<'
+    magic_be = struct.unpack_from('>I', data, 0)[0]
+    if magic_be == MAGIC:
+        return '>'
+    raise ValueError(f"Bad magic: 0x{magic_le:08x} (neither LE nor BE)")
+
+
+def batch_addr2line(vmlinux, addrs):
+    """Resolve multiple addresses in one addr2line invocation."""
+    if not addrs:
+        return {}
+    try:
+        # Feed addresses on stdin to avoid ARG_MAX limits with large
+        # numbers of addresses (one stack can have 30+ frames; a
+        # snapshot can have thousands of unique stacks).
+        stdin = '\n'.join(hex(a) for a in addrs) + '\n'
+        result = subprocess.run(
+            ['addr2line', '-f', '-e', vmlinux],
+            input=stdin, capture_output=True, text=True, timeout=60
+        )
+        lines = result.stdout.split('\n')
+        # addr2line outputs 2 lines per address: function name + source location
+        symbols = {}
+        for i, addr in enumerate(addrs):
+            idx = i * 2
+            if idx < len(lines) and lines[idx] and lines[idx] != '??':
+                symbols[addr] = lines[idx]
+        return symbols
+    except (subprocess.TimeoutExpired, FileNotFoundError) as e:
+        print(f"warning: addr2line failed: {e}", file=sys.stderr)
+        return {}
+
+
+def parse_stackmap_bin(data):
+    """Parse binary stackmap data, yield (stack_id, ref_count, [ips])."""
+    if len(data) < HEADER_SIZE:
+        raise ValueError("File too small for header")
+
+    endian = detect_endianness(data)
+    header_fmt = f'{endian}IIII'
+    entry_fmt = f'{endian}IIII'
+
+    magic, version, nr_stacks, _ = struct.unpack_from(header_fmt, data, 0)
+    if version != 1:
+        raise ValueError(f"Unsupported version: {version}")
+
+    offset = HEADER_SIZE
+    for _ in range(nr_stacks):
+        if offset + ENTRY_SIZE > len(data):
+            break
+        stack_id, nr, ref_count, _ = struct.unpack_from(entry_fmt, data, offset)
+        offset += ENTRY_SIZE
+
+        ips_size = nr * 8
+        if offset + ips_size > len(data):
+            break
+        ips = struct.unpack_from(f'{endian}{nr}Q', data, offset)
+        offset += ips_size
+
+        yield stack_id, ref_count, list(ips)
+
+
+def main():
+    parser = argparse.ArgumentParser(description='Parse ftrace stack_map_bin')
+    parser.add_argument('file', help='Path to stack_map_bin file')
+    parser.add_argument('--vmlinux', help='Path to vmlinux for symbol resolution')
+    parser.add_argument('--json', action='store_true', help='JSON output')
+    parser.add_argument('--top', type=int, default=0,
+                        help='Show only top N stacks by ref_count')
+    args = parser.parse_args()
+
+    with open(args.file, 'rb') as f:
+        data = f.read()
+
+    stacks = list(parse_stackmap_bin(data))
+
+    if args.top > 0:
+        stacks.sort(key=lambda x: x[1], reverse=True)
+        stacks = stacks[:args.top]
+
+    # Batch symbol resolution
+    symbols = {}
+    if args.vmlinux:
+        all_addrs = set()
+        for _, _, ips in stacks:
+            all_addrs.update(ip for ip in ips
+                             if ip != FTRACE_TRAMPOLINE_MARKER)
+        symbols = batch_addr2line(args.vmlinux, list(all_addrs))
+
+    def render(ip):
+        if ip == FTRACE_TRAMPOLINE_MARKER:
+            return TRAMPOLINE_LABEL
+        return symbols.get(ip, f'0x{ip:x}')
+
+    if args.json:
+        output = []
+        for stack_id, ref_count, ips in stacks:
+            entry = {
+                'stack_id': stack_id,
+                'ref_count': ref_count,
+                'ips': [f'0x{ip:x}' for ip in ips]
+            }
+            if args.vmlinux:
+                entry['symbols'] = [render(ip) for ip in ips]
+            output.append(entry)
+        print(json.dumps(output, indent=2))
+    else:
+        for stack_id, ref_count, ips in stacks:
+            print(f"stack_id {stack_id} [ref {ref_count}, depth {len(ips)}]")
+            for i, ip in enumerate(ips):
+                if ip == FTRACE_TRAMPOLINE_MARKER:
+                    print(f"  [{i}] {TRAMPOLINE_LABEL}")
+                    continue
+                sym = symbols.get(ip, '')
+                if sym:
+                    sym = f' {sym}'
+                print(f"  [{i}] 0x{ip:x}{sym}")
+            print()
+
+    print(f"Total: {len(stacks)} unique stacks", file=sys.stderr)
+
+
+if __name__ == '__main__':
+    main()
-- 
2.34.1

Re: [RFC PATCH v4 0/3] trace: stack trace deduplication for ftrace ring buffer

[Li Pengfei]

From: Pengfei Li <lipengfei28@xiaomi.com>

Hi Steven, Masami,

Gentle ping on this v4 series now that the 7.2 merge window has settled.

I'd especially value a quick steer on the two design questions from the
cover letter, since they shape how I respin:

  1. Eager vs lazy allocation of the element pool (~8 MB at the default
     bits=14 when CONFIG_FTRACE_STACKMAP=y, whether or not userspace ever
     enables the option). Is eager allocation acceptable, or should v5
     switch to lazy allocation on the first 'echo 1 > options/stackmap'?

  2. Whether to introduce the stack_map_bin binary interface now, or ship
     the text stack_map interface first and add the binary export once
     trace-cmd / libtraceevent integration is designed.

Happy to respin in either direction. Full series for reference:

  https://lore.kernel.org/all/20260616064119.438063-1-lipengfei28@xiaomi.com/

Thanks,
Pengfei

Re: [RFC PATCH v4 0/3] trace: stack trace deduplication for ftrace ring buffer

[Steven Rostedt]

On Mon,  6 Jul 2026 14:31:54 +0800
Li Pengfei <ljdlns1987@gmail.com> wrote:

> From: Pengfei Li <lipengfei28@xiaomi.com>
> 
> Hi Steven, Masami,
> 
> Gentle ping on this v4 series now that the 7.2 merge window has settled.
> 
> I'd especially value a quick steer on the two design questions from the
> cover letter, since they shape how I respin:
> 
>   1. Eager vs lazy allocation of the element pool (~8 MB at the default
>      bits=14 when CONFIG_FTRACE_STACKMAP=y, whether or not userspace ever
>      enables the option). Is eager allocation acceptable, or should v5
>      switch to lazy allocation on the first 'echo 1 > options/stackmap'?
> 
>   2. Whether to introduce the stack_map_bin binary interface now, or ship
>      the text stack_map interface first and add the binary export once
>      trace-cmd / libtraceevent integration is designed.
> 
> Happy to respin in either direction. Full series for reference:
> 
>   https://lore.kernel.org/all/20260616064119.438063-1-lipengfei28@xiaomi.com/

Sorry for the late reply. I just started a new job (on 6/15) and spent the
first two weeks focused on that. I'm just catching up now.

Note, I tried out this patch set with function tracing on lock functions:

  trace-cmd start -p function -O stackmap --func-stack -l '*lock*'

And wow! it grows pretty quickly

  # cat /sys/kernel/tracing/stack_map | wc
 326593  708742 10979376

Anyway, I do like the idea and I may even use this to help do user space
stack traces. But I will have to take a deeper dive into it, which I will
hopefully have some time next week to do so. But if not next week, perhaps
within the next month.

I just letting you know that I'm not ignoring you. I'm just a bit behind.

Thanks,

-- Steve

Re: [RFC PATCH v4 1/3] trace: add lock-free stackmap for stack trace deduplication

[Steven Rostedt]

On Tue, 16 Jun 2026 14:41:17 +0800
Li Pengfei <ljdlns1987@gmail.com> wrote:

> --- /dev/null
> +++ b/kernel/trace/trace_stackmap.c
> @@ -0,0 +1,889 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * Ftrace Stack Map - Lock-free stack trace deduplication for ftrace
> + *
> + * Modeled after tracing_map.c (used by hist triggers), this provides
> + * a lock-free hash map optimized for the ftrace hot path. The design
> + * is based on Dr. Cliff Click's non-blocking hash table algorithm.
> + *
> + * Key properties:
> + * - Lock-free insert via cmpxchg, safe in NMI/IRQ/any context
> + * - Pre-allocated element pool (zero allocation on hot path)
> + * - Linear probing with 2x over-provisioned table; probe length
> + *   bounded by FTRACE_STACKMAP_MAX_PROBE to keep worst-case lookup
> + *   cost constant even when the table is heavily loaded
> + * - Single global instance (initialized for the global trace array)
> + *
> + * Reset is a control-path operation, only allowed when tracing is
> + * stopped on the owning trace_array. The protocol is:
> + *
> + *   - atomic_cmpxchg(&resetting, 0, 1) atomically claims reset rights
> + *     and blocks new get_id() callers (they observe resetting=1 and
> + *     return -EINVAL).
> + *   - trace_types_lock serializes the tracer_tracing_is_on() check and
> + *     the destructive ring-buffer reset against tracefs writes to
> + *     tracing_on.
> + *   - synchronize_rcu() drains in-flight get_id() callers from the
> + *     ftrace callback path, which runs with preemption disabled.
> + *
> + * Online reset (with tracing active) is intentionally not supported
> + * to keep the design simple and the proof obligations small.
> + *
> + * The 32-bit jhash of the stack IPs is the hash table key. On hash
> + * collision, linear probing finds the next slot and full memcmp
> + * confirms the match.
> + *
> + * Concurrent userspace readers (cat stack_map / stack_map_bin) get
> + * a best-effort snapshot. They are coherent with the hot path
> + * (smp_load_acquire on entry->val); they are also serialized
> + * against reset via smap->reader_sem (readers take it in shared
> + * mode, reset in exclusive mode), so a reset cannot tear an
> + * iteration in progress -- it waits for active readers to drop
> + * the rwsem before clearing the map. The hot path is coordinated
> + * with reset separately, via acquire/release on smap->resetting.
> + */
> +
> +#include <linux/kernel.h>
> +#include <linux/slab.h>
> +#include <linux/jhash.h>
> +#include <linux/seq_file.h>
> +#include <linux/kallsyms.h>
> +#include <linux/vmalloc.h>
> +#include <linux/atomic.h>
> +#include <linux/local_lock.h>
> +#include <linux/percpu.h>
> +#include <linux/random.h>
> +#include <linux/rcupdate.h>
> +#include <linux/log2.h>
> +#include <asm/local.h>
> +
> +#include "trace.h"
> +#include "trace_stackmap.h"
> +
> +/*
> + * Bound the linear-probe scan length. With a 2x over-provisioned table,
> + * a well-distributed hash gives very short probe chains. Capping at 64
> + * keeps worst-case lookup O(1) even when the table is heavily loaded
> + * with claimed-but-empty slots from pool exhaustion.
> + */
> +#define FTRACE_STACKMAP_MAX_PROBE	64
> +
> +/*
> + * Memory ordering of entry->val: published with smp_store_release()
> + * by the inserter; consumed with smp_load_acquire() by every reader
> + * that dereferences the elt (get_id, seq_show, bin_open). This pairs
> + * the writes to elt->{nr,ips,ref_count} (initialized BEFORE the
> + * publish) with the reads of those fields (which happen AFTER the
> + * load). seq_start / seq_next only test val for NULL and use the
> + * acquire load purely to keep memory ordering symmetric.
> + */
> +
> +/*
> + * Each pre-allocated element holds one unique stack trace.
> + * Fixed size: MAX_DEPTH entries regardless of actual depth.
> + */
> +struct stackmap_elt {
> +	u32		nr;		/* actual number of IPs */
> +	atomic_t	ref_count;
> +	unsigned long	ips[FTRACE_STACKMAP_MAX_DEPTH];
> +};
> +
> +/*
> + * Hash table entry: a 32-bit key (jhash of stack) + pointer to elt.
> + * key == 0 means the slot is free.
> + */
> +struct stackmap_entry {
> +	u32			key;	/* 0 = free, non-zero = jhash */
> +	struct stackmap_elt	*val;	/* NULL until fully published */
> +};
> +
> +static struct stackmap_elt *stackmap_load_elt(struct stackmap_entry *entry)
> +{
> +	/*
> +	 * Pairs with the smp_store_release() that publishes entry->val
> +	 * after fully initializing the element payload.
> +	 */
> +	return smp_load_acquire(&entry->val);
> +}
> +
> +struct ftrace_stackmap {
> +	struct trace_array	*tr;		/* owning trace_array */
> +	unsigned int		map_bits;
> +	unsigned int		map_size;	/* 1 << (map_bits + 1) */
> +	unsigned int		max_elts;	/* 1 << map_bits */
> +	u32			hash_seed;	/* per-instance jhash seed */
> +	atomic_t		next_elt;	/* index into elts pool */
> +	struct stackmap_entry	*entries;	/* hash table */
> +	struct stackmap_elt	*elts;		/* flat element pool */
> +	atomic_t		resetting;
> +	/*
> +	 * Reader/reset serialization. Held in shared mode (read lock)
> +	 * across seq_file iteration and binary snapshot construction;
> +	 * held in exclusive mode (write lock) by reset's clearing
> +	 * phase. The hot path (get_id) does not take this lock — it
> +	 * uses smp_load_acquire/smp_store_release on entry->val and
> +	 * the resetting flag for the lock-free protocol.
> +	 */
> +	struct rw_semaphore	reader_sem;
> +	/*
> +	 * Per-CPU counters using local_t. local_t increments are NMI-
> +	 * safe on all architectures (single-instruction or interrupt-
> +	 * masked) and avoid the raw_spinlock_t fallback that
> +	 * atomic64_t uses on 32-bit GENERIC_ATOMIC64 — which would
> +	 * deadlock if an NMI hit while the spinlock was held.
> +	 */
> +	local_t __percpu	*successes;	/* events served (hits + new inserts) */
> +	local_t __percpu	*drops;
> +};
> +
> +/*
> + * Cap the bits parameter to keep worst-case allocations bounded:
> + *   bits=18 → 256K elts, 512K slots, ~130 MB elt pool, ~130 MB bin
> + *             export.
> + * Smaller workloads should use the default (14) which gives 16K elts
> + * (~8 MB pool); bump bits via the ftrace_stackmap.bits= kernel
> + * parameter for higher unique-stack capacity.
> + */
> +#define FTRACE_STACKMAP_BITS_MIN	10
> +#define FTRACE_STACKMAP_BITS_MAX	18
> +#define FTRACE_STACKMAP_BITS_DEFAULT	14
> +
> +static unsigned int stackmap_map_bits = FTRACE_STACKMAP_BITS_DEFAULT;
> +static int __init stackmap_bits_setup(char *str)
> +{
> +	unsigned long val;
> +
> +	if (kstrtoul(str, 0, &val))
> +		return -EINVAL;
> +	val = clamp_val(val, FTRACE_STACKMAP_BITS_MIN, FTRACE_STACKMAP_BITS_MAX);
> +	stackmap_map_bits = val;
> +	return 0;
> +}
> +early_param("ftrace_stackmap.bits", stackmap_bits_setup);
> +
> +/* --- Element pool --- */
> +
> +static struct stackmap_elt *stackmap_get_elt(struct ftrace_stackmap *smap)
> +{
> +	int idx;
> +
> +	/*
> +	 * Fast-path early-out once the pool is fully consumed. Avoids
> +	 * the contended atomic RMW on next_elt for every traced event
> +	 * after the pool is exhausted.
> +	 */
> +	if (atomic_read(&smap->next_elt) >= smap->max_elts)
> +		return NULL;
> +
> +	idx = atomic_fetch_add_unless(&smap->next_elt, 1, smap->max_elts);
> +	if (idx < smap->max_elts)
> +		return &smap->elts[idx];
> +	return NULL;
> +}
> +
> +/* --- Create / Destroy / Reset --- */
> +
> +struct ftrace_stackmap *ftrace_stackmap_create(struct trace_array *tr)
> +{
> +	struct ftrace_stackmap *smap;
> +	unsigned int bits;
> +
> +	smap = kzalloc_obj(*smap, GFP_KERNEL);
> +	if (!smap)
> +		return ERR_PTR(-ENOMEM);
> +
> +	/* Defensive clamp: reject bogus bits even if early_param is bypassed. */
> +	bits = clamp_val(stackmap_map_bits,
> +			 FTRACE_STACKMAP_BITS_MIN,
> +			 FTRACE_STACKMAP_BITS_MAX);
> +
> +	smap->tr = tr;
> +	smap->map_bits = bits;
> +	smap->max_elts = 1U << bits;
> +	smap->map_size = 1U << (bits + 1);	/* 2x over-provision */
> +
> +	smap->entries = vzalloc(sizeof(*smap->entries) * smap->map_size);

Why not:

	smap->entries = vcalloc(smap->map_size, sizeof(*smap->entries));

?

> +	if (!smap->entries) {
> +		kfree(smap);
> +		return ERR_PTR(-ENOMEM);
> +	}

Make the error paths have:

	if (!smap->entries)
		goto fail;

> +
> +	/*
> +	 * Single large vmalloc of the element pool, indexed flat.
> +	 * At bits=18 this is 256K * sizeof(struct stackmap_elt). The
> +	 * struct is ~520 B (8 + 4 + 4 + 64*8), so total ~135 MB.
> +	 */
> +	smap->elts = vzalloc(sizeof(*smap->elts) * (size_t)smap->max_elts);

vcalloc()?

> +	if (!smap->elts) {

		goto fail;

> +		vfree(smap->entries);
> +		kfree(smap);
> +		return ERR_PTR(-ENOMEM);
> +	}
> +
> +	smap->successes = alloc_percpu(local_t);
> +	if (!smap->successes) {

		goto fail;

> +		vfree(smap->elts);
> +		vfree(smap->entries);
> +		kfree(smap);
> +		return ERR_PTR(-ENOMEM);
> +	}
> +	smap->drops = alloc_percpu(local_t);
> +	if (!smap->drops) {

		goto fail;

> +		free_percpu(smap->successes);
> +		vfree(smap->elts);
> +		vfree(smap->entries);
> +		kfree(smap);
> +		return ERR_PTR(-ENOMEM);
> +	}
> +
> +	smap->hash_seed = get_random_u32();
> +	atomic_set(&smap->next_elt, 0);
> +	atomic_set(&smap->resetting, 0);
> +	init_rwsem(&smap->reader_sem);
> +
> +	return smap;

fail:

	if (smap) {
		free_percpu(smap->successes);
		vfree(smap->elts);
		vfree(smap->entries);
		kfree(smap);

// As all the above handle passing in NULL just fine.

	}

	return ERR_PTR(-ENOMEM);



> +}
> +
> +void ftrace_stackmap_destroy(struct ftrace_stackmap *smap)
> +{
> +	if (!smap || IS_ERR(smap))
> +		return;
> +	free_percpu(smap->drops);
> +	free_percpu(smap->successes);
> +	vfree(smap->elts);
> +	vfree(smap->entries);
> +	kfree(smap);
> +}
> +
> +/**
> + * ftrace_stackmap_reset - clear all entries in the stackmap
> + * @smap: the stackmap to reset
> + *
> + * Returns 0 on success, -EBUSY if another reset is already in
> + * progress, or if tracing is currently active on the owning
> + * trace_array.
> + *
> + * Online reset (with tracing active) is not supported. Caller must
> + * stop tracing first (echo 0 > tracing_on).
> + *
> + * Caller is process context (typically sysfs write handler).
> + *
> + * Protocol:
> + *   1. Atomically claim reset rights via cmpxchg on @resetting.
> + *   2. Take trace_types_lock to serialize against tracefs writes to
> + *      tracing_on.
> + *   3. Verify tracing is stopped on @smap->tr; if not, release the
> + *      claim and return -EBUSY. The resetting flag itself blocks
> + *      any subsequent get_id() callers.
> + *   4. synchronize_rcu() drains in-flight get_id() callers from the
> + *      ftrace callback path (which runs preempt-disabled).
> + *   5. Reset the ring buffer(s), then memset entries, elts, and
> + *      counters.
> + *   6. Release the resetting flag with release semantics so any new
> + *      get_id() observes a fully cleared map.
> + */
> +int ftrace_stackmap_reset(struct ftrace_stackmap *smap)
> +{
> +	struct trace_array *tr;
> +	int ret = 0;
> +
> +	if (!smap)
> +		return 0;
> +
> +	if (atomic_cmpxchg(&smap->resetting, 0, 1) != 0)
> +		return -EBUSY;
> +
> +	mutex_lock(&trace_types_lock);
> +
> +	tr = smap->tr;
> +	if (tr && tracer_tracing_is_on(tr)) {
> +		ret = -EBUSY;
> +		goto out_unlock;
> +	}
> +
> +	/*
> +	 * synchronize_rcu() itself is a full barrier; no extra smp_mb()
> +	 * is needed before it. It drains in-flight ftrace callbacks that
> +	 * may have already passed the resetting check with the old value.
> +	 */
> +	synchronize_rcu();
> +
> +	/*
> +	 * Take the reader_sem in exclusive mode. This serializes the
> +	 * memset against any tracefs reader (seq_file iteration or
> +	 * stack_map_bin snapshot) that may currently hold the rwsem
> +	 * for read. synchronize_rcu() already drained the hot path;
> +	 * this rwsem covers process-context readers that aren't
> +	 * preempt-disabled.
> +	 */
> +	down_write(&smap->reader_sem);
> +
> +	/*
> +	 * Clear the ring buffer(s) BEFORE the map, both under the write
> +	 * lock. The ring buffer may still hold TRACE_STACK_ID events
> +	 * whose stack_id points at slots we are about to free/reuse.
> +	 * Resetting the buffer first guarantees an external observer
> +	 * never sees the inconsistent "trace still has <stack_id N> but
> +	 * the map is already empty" window: it sees either (old buffer,
> +	 * old map) or (cleared buffer, old map) or (cleared buffer,
> +	 * cleared map) -- never (old buffer, cleared map).
> +	 *
> +	 * Use tracing_reset_all_cpus() (not _online_cpus) so per-CPU
> +	 * buffers belonging to currently offline CPUs are also cleared.
> +	 * The ring buffer is allocated per-possible-CPU; an offline CPU's
> +	 * buffer can still hold a TRACE_STACK_ID event written before
> +	 * the CPU went offline. tracing_reset_online_cpus() iterates
> +	 * for_each_online_buffer_cpu() and would leave that data behind
> +	 * to be observed once the CPU comes back online (or by the
> +	 * trace reader, which iterates all allocated CPU buffers),
> +	 * recreating the stale-stack_id window we are trying to close.
> +	 *
> +	 * Since reset requires tracing to be stopped, this makes "reset"
> +	 * an explicitly destructive operation on the owning trace_array,
> +	 * keeping ring-buffer stack_ids and the map coherent.
> +	 */
> +	if (tr) {
> +		tracing_reset_all_cpus(&tr->array_buffer);
> +#ifdef CONFIG_TRACER_SNAPSHOT
> +		if (tr->allocated_snapshot)
> +			tracing_reset_all_cpus(&tr->snapshot_buffer);
> +#endif
> +	}
> +
> +	memset(smap->entries, 0, sizeof(*smap->entries) * smap->map_size);
> +	memset(smap->elts, 0, sizeof(*smap->elts) * (size_t)smap->max_elts);
> +
> +	atomic_set(&smap->next_elt, 0);
> +	{

Do not add anonymous blocks in functions.

> +		int cpu;

Just declare cpu at the beginning of the function.

> +
> +		for_each_possible_cpu(cpu) {
> +			local_set(per_cpu_ptr(smap->successes, cpu), 0);
> +			local_set(per_cpu_ptr(smap->drops, cpu), 0);
> +		}
> +	}
> +
> +	up_write(&smap->reader_sem);
> +
> +out_unlock:
> +	mutex_unlock(&trace_types_lock);
> +
> +	/* Release resetting=0 so new get_id() observes a cleared map. */
> +	atomic_set_release(&smap->resetting, 0);
> +	return ret;
> +}
> +
> +/* --- Core: get_id (lock-free, NMI-safe) --- */
> +
> +int ftrace_stackmap_get_id(struct ftrace_stackmap *smap,
> +			   unsigned long *ips, unsigned int nr_entries)
> +{
> +	u32 key_hash, idx, test_key, trace_len;
> +	struct stackmap_entry *entry;
> +	struct stackmap_elt *val;
> +	int probes = 0;
> +
> +	/*
> +	 * atomic_read_acquire() pairs with atomic_set_release() in the
> +	 * reset path. This ensures that subsequent reads of entry->key
> +	 * and entry->val are ordered after this check; without acquire,
> +	 * the CPU would only have a control dependency, which orders
> +	 * subsequent stores but not loads (per LKMM).
> +	 */
> +	if (!smap || !nr_entries || atomic_read_acquire(&smap->resetting))
> +		return -EINVAL;
> +	/*
> +	 * Never truncate: a stack deeper than the map can hold must not be
> +	 * silently shortened, or two distinct traces sharing their first
> +	 * FTRACE_STACKMAP_MAX_DEPTH frames would be merged into one
> +	 * stack_id. The caller is expected to fall back to a full stack
> +	 * trace for such events. Reject defensively in case of a future
> +	 * caller that forgets this contract.
> +	 */
> +	if (nr_entries > FTRACE_STACKMAP_MAX_DEPTH)
> +		return -E2BIG;
> +
> +	trace_len = nr_entries * sizeof(unsigned long);
> +	/*
> +	 * jhash2() requires the length in u32 units and the data to be
> +	 * u32-aligned. On 64-bit kernels sizeof(unsigned long)==8, so
> +	 * trace_len is always a multiple of 8 (hence of 4). Use jhash2
> +	 * directly; the cast to u32* is safe because ips[] is naturally
> +	 * aligned to sizeof(unsigned long) >= 4.
> +	 */
> +	key_hash = jhash2((const u32 *)ips, trace_len / sizeof(u32),
> +			  smap->hash_seed);
> +	if (key_hash == 0)
> +		key_hash = 1;	/* 0 means free slot */
> +
> +	idx = key_hash >> (32 - (smap->map_bits + 1));
> +
> +	while (probes < FTRACE_STACKMAP_MAX_PROBE) {
> +		idx &= (smap->map_size - 1);
> +		entry = &smap->entries[idx];
> +		/*
> +		 * READ_ONCE() to avoid LKMM data race with concurrent
> +		 * cmpxchg(&entry->key, 0, key_hash) on this slot.
> +		 */
> +		test_key = READ_ONCE(entry->key);
> +
> +		if (test_key == key_hash) {
> +			val = stackmap_load_elt(entry);
> +			/*
> +			 * READ_ONCE(val->nr) keeps style consistent with
> +			 * the seq_show / bin_open readers. nr is write-once
> +			 * (set before publish, never modified afterwards),
> +			 * so the load is data-race-free, but READ_ONCE
> +			 * silences any analysis tool that flags a plain
> +			 * read of a field that is also read under acquire
> +			 * elsewhere.
> +			 */
> +			if (val && READ_ONCE(val->nr) == nr_entries &&
> +			    memcmp(val->ips, ips, trace_len) == 0) {
> +				/*
> +				 * ref_count is a best-effort popularity
> +				 * counter. On a long (from-boot, multi-hour)
> +				 * trace a hot stack can be hit billions of
> +				 * times. atomic_add_unless() gives true
> +				 * saturation at INT_MAX even under concurrent
> +				 * hits on multiple CPUs (a plain
> +				 * check-then-inc could let several CPUs past
> +				 * the check near the cap and still wrap).
> +				 */
> +				atomic_add_unless(&val->ref_count, 1, INT_MAX);
> +				/*
> +				 * successes/drops are best-effort throughput
> +				 * counters. Saturate at LONG_MAX so they do
> +				 * not wrap on long runs (notably where local_t
> +				 * is 32-bit), matching ref_count's behaviour.
> +				 */
> +				local_add_unless(this_cpu_ptr(smap->successes),
> +						 1, LONG_MAX);
> +				return (int)idx;
> +			}
> +			/*
> +			 * val == NULL: another CPU is mid-insert, or this
> +			 * slot is "claimed but empty" (pool exhausted).
> +			 * val != NULL but mismatch: 32-bit hash collision
> +			 * with a different stack. In both cases, advance.
> +			 */
> +		} else if (!test_key) {
> +			/*
> +			 * Free slot: try to claim it.
> +			 *
> +			 * If two CPUs race here with the same key_hash
> +			 * (same stack), one loses the cmpxchg, advances,
> +			 * and may insert the same stack at a later slot.
> +			 * This can produce a small number of duplicate
> +			 * entries under heavy contention. The trade-off
> +			 * is accepted to keep the hot path lock-free;
> +			 * ref_count is split across the duplicates and
> +			 * total memory cost is bounded by the element
> +			 * pool size.
> +			 */
> +			if (cmpxchg(&entry->key, 0, key_hash) == 0) {
> +				struct stackmap_elt *elt;
> +
> +				elt = stackmap_get_elt(smap);
> +				if (!elt) {
> +					/*
> +					 * Pool exhausted. We claimed this
> +					 * slot with cmpxchg but cannot fill
> +					 * it. Leave key set so the slot
> +					 * stays "claimed but empty" — future
> +					 * lookups treat val==NULL as a miss
> +					 * and probe past it. Cannot revert
> +					 * key=0 without racing other CPUs.
> +					 */
> +					local_add_unless(this_cpu_ptr(smap->drops),
> +							 1, LONG_MAX);
> +					return -ENOSPC;
> +				}
> +
> +				elt->nr = nr_entries;
> +				atomic_set(&elt->ref_count, 1);
> +				memcpy(elt->ips, ips, trace_len);
> +
> +				/*
> +				 * Publish elt with release semantics so the
> +				 * reader's smp_load_acquire can safely
> +				 * dereference val->nr / val->ips.
> +				 */
> +				smp_store_release(&entry->val, elt);
> +				local_add_unless(this_cpu_ptr(smap->successes),
> +						 1, LONG_MAX);
> +				return (int)idx;
> +			}
> +			/* cmpxchg failed; another CPU claimed this slot. */
> +		}
> +
> +		idx++;
> +		probes++;
> +	}
> +
> +	local_add_unless(this_cpu_ptr(smap->drops), 1, LONG_MAX);
> +	return -ENOSPC;
> +}
> +
> +/* --- Text export: /sys/kernel/debug/tracing/stack_map --- */
> +
> +struct stackmap_seq_private {
> +	struct ftrace_stackmap	*smap;
> +};
> +
> +static void *stackmap_seq_start(struct seq_file *m, loff_t *pos)
> +{
> +	struct stackmap_seq_private *priv = m->private;
> +	struct ftrace_stackmap *smap = priv->smap;
> +	u32 i;
> +
> +	if (!smap)
> +		return NULL;
> +	/*
> +	 * Take the reader_sem to serialize against ftrace_stackmap_reset(),
> +	 * which holds it for write while clearing the table. Released in
> +	 * stackmap_seq_stop(), which seq_file calls regardless of whether
> +	 * start() returned an element or NULL (per Documentation/filesystems
> +	 * /seq_file.rst: "the iterator value returned by start() or next()
> +	 * is guaranteed to be passed to a subsequent next() or stop()").
> +	 */
> +	down_read(&smap->reader_sem);
> +	for (i = *pos; i < smap->map_size; i++) {
> +		if (READ_ONCE(smap->entries[i].key) &&
> +		    stackmap_load_elt(&smap->entries[i])) {
> +			*pos = i;
> +			return &smap->entries[i];
> +		}
> +	}
> +	return NULL;
> +}
> +
> +static void *stackmap_seq_next(struct seq_file *m, void *v, loff_t *pos)
> +{
> +	struct stackmap_seq_private *priv = m->private;
> +	struct ftrace_stackmap *smap = priv->smap;
> +	u32 i;
> +
> +	if (!smap)
> +		return NULL;
> +	for (i = *pos + 1; i < smap->map_size; i++) {
> +		if (READ_ONCE(smap->entries[i].key) &&
> +		    stackmap_load_elt(&smap->entries[i])) {
> +			*pos = i;
> +			return &smap->entries[i];
> +		}
> +	}
> +	/*
> +	 * Advance *pos past the end so that on the next read() the
> +	 * subsequent stackmap_seq_start() call returns NULL and the
> +	 * iteration terminates. Without this, seq_read() would loop
> +	 * on the last element.
> +	 */
> +	*pos = smap->map_size;
> +	return NULL;
> +}
> +
> +static void stackmap_seq_stop(struct seq_file *m, void *v)
> +{
> +	struct stackmap_seq_private *priv = m->private;
> +	struct ftrace_stackmap *smap = priv->smap;
> +
> +	/*
> +	 * seq_file invokes stop() unconditionally after each iteration
> +	 * pass (see seq_read_iter / traverse), even when start() returned
> +	 * NULL. Always release here, balanced against the down_read in
> +	 * stackmap_seq_start().
> +	 */
> +	if (smap)
> +		up_read(&smap->reader_sem);
> +}
> +
> +static int stackmap_seq_show(struct seq_file *m, void *v)
> +{
> +	struct stackmap_entry *entry = v;
> +	struct stackmap_seq_private *priv = m->private;
> +	struct stackmap_elt *elt;
> +	u32 idx = entry - priv->smap->entries;
> +	u32 i, nr;
> +
> +	elt = stackmap_load_elt(entry);
> +	if (!elt)
> +		return 0;
> +
> +	nr = READ_ONCE(elt->nr);
> +	if (nr > FTRACE_STACKMAP_MAX_DEPTH)
> +		nr = FTRACE_STACKMAP_MAX_DEPTH;
> +
> +	seq_printf(m, "stack_id %u [ref %u, depth %u]\n",
> +		   idx, atomic_read(&elt->ref_count), nr);
> +	for (i = 0; i < nr; i++) {
> +		unsigned long ip = elt->ips[i];
> +
> +		/*
> +		 * Mirror trace_stack_print(): __ftrace_trace_stack()
> +		 * may replace trampoline addresses with
> +		 * FTRACE_TRAMPOLINE_MARKER before the stack reaches the
> +		 * map, and normal addresses must go through
> +		 * trace_adjust_address() (KASLR / module text delta)
> +		 * before symbolization. Without this the export would
> +		 * print a bogus symbol for the marker and unadjusted
> +		 * addresses for everything else.
> +		 */
> +		if (ip == FTRACE_TRAMPOLINE_MARKER) {
> +			seq_printf(m, "  [%u] [FTRACE TRAMPOLINE]\n", i);
> +			continue;
> +		}
> +		seq_printf(m, "  [%u] %pS\n", i,
> +			   (void *)trace_adjust_address(priv->smap->tr, ip));
> +	}
> +	seq_putc(m, '\n');
> +	return 0;
> +}
> +
> +static const struct seq_operations stackmap_seq_ops = {
> +	.start	= stackmap_seq_start,
> +	.next	= stackmap_seq_next,
> +	.stop	= stackmap_seq_stop,
> +	.show	= stackmap_seq_show,
> +};
> +
> +static int stackmap_open(struct inode *inode, struct file *file)
> +{
> +	struct stackmap_seq_private *priv;
> +	struct seq_file *m;
> +	int ret;
> +
> +	ret = seq_open_private(file, &stackmap_seq_ops,
> +			       sizeof(struct stackmap_seq_private));
> +	if (ret)
> +		return ret;
> +	m = file->private_data;
> +	priv = m->private;
> +	priv->smap = inode->i_private;
> +	return 0;
> +}
> +
> +/*
> + * Accept exactly "0" or "reset" (optionally followed by a single newline).
> + */
> +static bool stackmap_write_is_reset(const char *buf, size_t n)
> +{
> +	if (n > 0 && buf[n - 1] == '\n')
> +		n--;
> +	return (n == 1 && buf[0] == '0') ||
> +	       (n == 5 && memcmp(buf, "reset", 5) == 0);
> +}
> +
> +static ssize_t stackmap_write(struct file *file, const char __user *ubuf,
> +			      size_t count, loff_t *ppos)
> +{
> +	struct seq_file *m = file->private_data;
> +	struct stackmap_seq_private *priv = m->private;
> +	char buf[8];
> +	size_t n = min(count, sizeof(buf) - 1);
> +	int ret;
> +
> +	if (n == 0)
> +		return -EINVAL;
> +	if (copy_from_user(buf, ubuf, n))
> +		return -EFAULT;
> +	buf[n] = '\0';
> +
> +	if (!stackmap_write_is_reset(buf, n))
> +		return -EINVAL;
> +
> +	/*
> +	 * ftrace_stackmap_reset() atomically claims reset rights via
> +	 * cmpxchg and returns -EBUSY if another reset is in progress
> +	 * or if tracing is active.
> +	 */
> +	ret = ftrace_stackmap_reset(priv->smap);
> +	if (ret)
> +		return ret;
> +	return count;
> +}
> +
> +const struct file_operations ftrace_stackmap_fops = {
> +	.open		= stackmap_open,
> +	.read		= seq_read,
> +	.write		= stackmap_write,
> +	.llseek		= seq_lseek,
> +	.release	= seq_release_private,
> +};
> +
> +/* --- Stats --- */
> +
> +static int stackmap_stat_show(struct seq_file *m, void *v)
> +{
> +	struct ftrace_stackmap *smap = m->private;
> +	u64 successes = 0, drops = 0;
> +	u32 entries;
> +	int cpu;
> +
> +	if (!smap) {
> +		seq_puts(m, "stackmap not initialized\n");
> +		return 0;
> +	}
> +
> +	entries = atomic_read(&smap->next_elt);
> +	for_each_possible_cpu(cpu) {
> +		successes += local_read(per_cpu_ptr(smap->successes, cpu));
> +		drops += local_read(per_cpu_ptr(smap->drops, cpu));
> +	}
> +
> +	seq_printf(m, "entries:      %u / %u\n", entries, smap->max_elts);
> +	seq_printf(m, "table_size:   %u\n", smap->map_size);
> +	seq_printf(m, "successes:    %llu\n", successes);
> +	seq_printf(m, "drops:        %llu\n", drops);
> +	if (successes + drops > 0)
> +		seq_printf(m, "success_rate: %llu%%\n",
> +			   successes * 100 / (successes + drops));
> +	return 0;
> +}
> +
> +static int stackmap_stat_open(struct inode *inode, struct file *file)
> +{
> +	return single_open(file, stackmap_stat_show, inode->i_private);
> +}
> +
> +const struct file_operations ftrace_stackmap_stat_fops = {
> +	.open		= stackmap_stat_open,
> +	.read		= seq_read,
> +	.llseek		= seq_lseek,
> +	.release	= single_release,
> +};
> +
> +/* --- Binary export --- */
> +
> +struct stackmap_bin_snapshot {
> +	/*
> +	 * Use u64 (not size_t) so data[] is 8-byte aligned on both
> +	 * 32-bit and 64-bit architectures. The IP array within data[]
> +	 * is accessed as u64*, which would alignment-fault on strict
> +	 * architectures (e.g. older ARM, SPARC) if data[] started at
> +	 * a 4-byte boundary.
> +	 */
> +	u64	size;
> +	char	data[];
> +};
> +
> +static int stackmap_bin_open(struct inode *inode, struct file *file)
> +{
> +	struct ftrace_stackmap *smap = inode->i_private;
> +	struct stackmap_bin_snapshot *snap;
> +	struct ftrace_stackmap_bin_header *hdr;
> +	size_t alloc_size, off;
> +	u32 nr_entries, i, nr_stacks;
> +
> +	if (!smap)
> +		return -ENODEV;
> +
> +	/*
> +	 * Worst-case allocation size: every populated entry uses a
> +	 * full-depth stack. The (+1) gives one slack slot in case a
> +	 * concurrent insert lands between this snapshot and iteration.
> +	 * The loop below performs an explicit bounds check anyway.
> +	 *
> +	 * At bits=18 this caps at ~135 MB. The file is mode 0440
> +	 * (TRACE_MODE_READ), so only privileged users can open it.
> +	 */
> +	nr_entries = atomic_read(&smap->next_elt);
> +	alloc_size = sizeof(*hdr) + (nr_entries + 1) *
> +		     (sizeof(struct ftrace_stackmap_bin_entry) +
> +		      FTRACE_STACKMAP_MAX_DEPTH * sizeof(u64));

Really should have ftrace_stackmap_bin_entry have a flexible array:

(move struct ftrace_stackmap_bin_entry *e to top)

	alloc_size = sizeof(*hdr) + (nr_entries + 1) *
		struct_size(e, ips, FTRACE_STACKMAP_MAX_DEPTH);

> +
> +	snap = vmalloc(sizeof(*snap) + alloc_size);
> +	if (!snap)
> +		return -ENOMEM;
> +
> +	hdr = (struct ftrace_stackmap_bin_header *)snap->data;
> +	hdr->magic = FTRACE_STACKMAP_BIN_MAGIC;
> +	hdr->version = FTRACE_STACKMAP_BIN_VERSION;
> +	hdr->reserved = 0;
> +	off = sizeof(*hdr);
> +	nr_stacks = 0;
> +
> +	/*
> +	 * Take reader_sem to serialize against ftrace_stackmap_reset(),
> +	 * which clears the table and elt pool under the write lock.
> +	 */
> +	down_read(&smap->reader_sem);
> +
> +	for (i = 0; i < smap->map_size; i++) {
> +		struct stackmap_entry *entry = &smap->entries[i];
> +		struct stackmap_elt *elt;

> +		struct ftrace_stackmap_bin_entry *e;

move to top of function.

> +		u64 *ips_out;
> +		u32 k, nr;
> +
> +		if (!READ_ONCE(entry->key))
> +			continue;
> +		elt = stackmap_load_elt(entry);
> +		if (!elt)
> +			continue;
> +
> +		nr = READ_ONCE(elt->nr);
> +		if (nr > FTRACE_STACKMAP_MAX_DEPTH)
> +			nr = FTRACE_STACKMAP_MAX_DEPTH;
> +
> +		/* Bounds check: stop if we would overflow the allocation. */
> +		if (off + sizeof(*e) + nr * sizeof(u64) > alloc_size)

		if (off + struct_size(e, ips, nr) > alloc_size)

> +			break;
> +
> +		e = (struct ftrace_stackmap_bin_entry *)(snap->data + off);
> +		e->stack_id = i;
> +		e->nr = nr;
> +		e->ref_count = atomic_read(&elt->ref_count);
> +		e->reserved = 0;

> +		off += sizeof(*e);

delete the above.

> +
> +		ips_out = (u64 *)(snap->data + off);

		ips_out = e->ips;

> +		for (k = 0; k < nr; k++) {
> +			unsigned long ip = elt->ips[k];
> +
> +			/*
> +			 * Emit the trampoline marker verbatim so userspace
> +			 * can render it as [FTRACE TRAMPOLINE]; pass every
> +			 * other address through trace_adjust_address() so the
> +			 * binary export follows the same address-adjustment
> +			 * rules as the text export.
> +			 */
> +			if (ip == FTRACE_TRAMPOLINE_MARKER)
> +				ips_out[k] = (u64)FTRACE_TRAMPOLINE_MARKER;
> +			else
> +				ips_out[k] = (u64)trace_adjust_address(smap->tr, ip);
> +		}
> +		off += nr * sizeof(u64);

		off += struct_size(e, ips, nr);

> +		nr_stacks++;
> +	}
> +
> +	up_read(&smap->reader_sem);
> +
> +	hdr->nr_stacks = nr_stacks;
> +	snap->size = off;
> +	file->private_data = snap;
> +	return 0;
> +}
> +
> +static ssize_t stackmap_bin_read(struct file *file, char __user *ubuf,
> +				 size_t count, loff_t *ppos)
> +{
> +	struct stackmap_bin_snapshot *snap = file->private_data;
> +
> +	if (!snap)
> +		return -EINVAL;
> +	return simple_read_from_buffer(ubuf, count, ppos, snap->data, snap->size);
> +}
> +
> +static int stackmap_bin_release(struct inode *inode, struct file *file)
> +{
> +	vfree(file->private_data);
> +	return 0;
> +}
> +
> +const struct file_operations ftrace_stackmap_bin_fops = {
> +	.open		= stackmap_bin_open,
> +	.read		= stackmap_bin_read,
> +	.llseek		= default_llseek,
> +	.release	= stackmap_bin_release,
> +};
> diff --git a/kernel/trace/trace_stackmap.h b/kernel/trace/trace_stackmap.h
> new file mode 100644
> index 000000000000..7c2e5ab9d36d
> --- /dev/null
> +++ b/kernel/trace/trace_stackmap.h
> @@ -0,0 +1,57 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _TRACE_STACKMAP_H
> +#define _TRACE_STACKMAP_H
> +
> +#include <linux/types.h>
> +#include <linux/atomic.h>
> +
> +#define FTRACE_STACKMAP_MAX_DEPTH	64
> +
> +/* Binary export format */
> +#define FTRACE_STACKMAP_BIN_MAGIC	0x46534D42	/* 'FSMB' */
> +#define FTRACE_STACKMAP_BIN_VERSION	1
> +
> +struct ftrace_stackmap_bin_header {
> +	u32 magic;
> +	u32 version;
> +	u32 nr_stacks;
> +	u32 reserved;
> +};
> +
> +struct ftrace_stackmap_bin_entry {
> +	u32 stack_id;
> +	u32 nr;
> +	u32 ref_count;
> +	u32 reserved;
> +	/* followed by u64 ips[nr] */

Why not make this a flexible array?

	u64 ips[];

Then the code can be simpler as described above.

-- Steve


> +};
> +
> +struct trace_array;
> +
> +#ifdef CONFIG_FTRACE_STACKMAP
> +
> +struct ftrace_stackmap;
> +
> +struct ftrace_stackmap *ftrace_stackmap_create(struct trace_array *tr);
> +void ftrace_stackmap_destroy(struct ftrace_stackmap *smap);
> +int ftrace_stackmap_get_id(struct ftrace_stackmap *smap,
> +			   unsigned long *ips, unsigned int nr_entries);
> +int ftrace_stackmap_reset(struct ftrace_stackmap *smap);
> +
> +extern const struct file_operations ftrace_stackmap_fops;
> +extern const struct file_operations ftrace_stackmap_stat_fops;
> +extern const struct file_operations ftrace_stackmap_bin_fops;
> +
> +#else
> +
> +struct ftrace_stackmap;
> +static inline struct ftrace_stackmap *
> +ftrace_stackmap_create(struct trace_array *tr) { return NULL; }
> +static inline void ftrace_stackmap_destroy(struct ftrace_stackmap *s) { }
> +static inline int ftrace_stackmap_get_id(struct ftrace_stackmap *s,
> +					 unsigned long *ips, unsigned int n)
> +{ return -EOPNOTSUPP; }
> +static inline int ftrace_stackmap_reset(struct ftrace_stackmap *s) { return 0; }
> +
> +#endif
> +#endif /* _TRACE_STACKMAP_H */

Re: [RFC PATCH v4 2/3] trace: integrate stackmap into ftrace stack recording path

[Steven Rostedt]

On Tue, 16 Jun 2026 14:41:18 +0800
Li Pengfei <ljdlns1987@gmail.com> wrote:

>  int set_tracer_flag(struct trace_array *tr, u64 mask, int enabled)
>  {
>  	switch (mask) {
> @@ -3993,6 +4091,33 @@ int set_tracer_flag(struct trace_array *tr, u64 mask, int enabled)
>  	if (!!(tr->trace_flags & mask) == !!enabled)
>  		return 0;
>  
> +#ifdef CONFIG_FTRACE_STACKMAP
> +	/*
> +	 * STACKMAP is intentionally global-instance-only: the dedup map,
> +	 * its tracefs files (stack_map / stack_map_stat / stack_map_bin)
> +	 * and the lifetime/reset semantics are tied to the global trace
> +	 * array. options/stackmap is hidden on secondary instances via
> +	 * TOP_LEVEL_TRACE_FLAGS, but writes still reach set_tracer_flag()
> +	 * through the aggregate trace_options file. Reject the enable on
> +	 * a secondary instance so it cannot be silently accepted and then
> +	 * become a no-op in the hot path (where tr->stackmap is NULL and
> +	 * the code falls back to a full stack trace).
> +	 *
> +	 * On the global instance, allow the enable while init is still
> +	 * pending (boot-time trace_options=stackmap is applied before the
> +	 * tracefs init work creates the map; the hot path falls back
> +	 * until the map is published). Only reject once init has
> +	 * permanently failed, so options/stackmap never reports an
> +	 * enabled no-op. READ_ONCE() suffices: this only inspects the
> +	 * init state, it does not dereference the map (the hot path uses
> +	 * smp_load_acquire(&tr->stackmap) for that).
> +	 */
> +	if (mask == TRACE_ITER(STACKMAP) && enabled &&
> +	    (tr != &global_trace ||
> +	     READ_ONCE(stackmap_init_state) == STACKMAP_INIT_FAILED))
> +		return -EINVAL;
> +#endif
> +
>  	/* Give the tracer a chance to approve the change */
>  	if (tr->current_trace->flag_changed)
>  		if (tr->current_trace->flag_changed(tr, mask, !!enabled))
> @@ -9222,6 +9347,91 @@ static __init void tracer_init_tracefs_work_func(struct work_struct *work)
>  			NULL, &tracing_dyn_info_fops);
>  #endif
>  
> +#ifdef CONFIG_FTRACE_STACKMAP
> +	{
> +		struct ftrace_stackmap *smap;
> +		struct dentry *map_file;
> +
> +		smap = ftrace_stackmap_create(&global_trace);
> +		if (!IS_ERR(smap)) {
> +			/*
> +			 * Failure-atomic init: stack_map is the single
> +			 * required tracefs file (it doubles as the reset
> +			 * interface and the human-readable resolver). If
> +			 * we cannot create it, the hot path must not be
> +			 * able to emit <stack_id N> events that no one can
> +			 * resolve or clear, so refuse to publish the map
> +			 * and tear it down.
> +			 *
> +			 * Create stack_map BEFORE smp_store_release() so an
> +			 * observed non-NULL global_trace.stackmap implies
> +			 * its resolver/reset file exists.
> +			 */
> +			map_file = trace_create_file("stack_map",
> +						     TRACE_MODE_WRITE, NULL,
> +						     smap,
> +						     &ftrace_stackmap_fops);
> +			if (!map_file) {
> +				pr_warn("ftrace stackmap init: stack_map create failed, dedup disabled\n");
> +				ftrace_stackmap_destroy(smap);
> +				/*
> +				 * Permanent failure. Record it and clear a
> +				 * STACKMAP flag that a boot-time
> +				 * trace_options=stackmap may have set, so
> +				 * options/stackmap does not report an
> +				 * enabled no-op and later userspace enables
> +				 * return -EINVAL.
> +				 */
> +				WRITE_ONCE(stackmap_init_state,
> +					   STACKMAP_INIT_FAILED);
> +				global_trace.trace_flags &=
> +					~TRACE_ITER(STACKMAP);

80 columns is no longer a hard requirement. 100 is more the default, so the
above should be:

				WRITE_ONCE(stackmap_init_state, STACKMAP_INIT_FAILED);
				global_trace.trace_flags &= ~TRACE_ITER(STACKMAP);



> +			} else {
> +				/*
> +				 * smp_store_release pairs with the
> +				 * smp_load_acquire() in
> +				 * __ftrace_trace_stack(). Publishing only
> +				 * after the required file exists keeps
> +				 * "smap visible" => "resolver/reset
> +				 * available".
> +				 */

> +				smp_store_release(&global_trace.stackmap,
> +						  smap);
> +				WRITE_ONCE(stackmap_init_state,
> +					   STACKMAP_INIT_DONE);

Same with the above two.

> +				/*
> +				 * stat and bin are auxiliary observability
> +				 * surfaces. If they fail to be created we
> +				 * keep dedup enabled (the kernel side still
> +				 * works, and stack_map alone is enough to
> +				 * resolve and reset); trace_create_file()
> +				 * already pr_warn()s on failure.
> +				 */
> +				trace_create_file("stack_map_stat",
> +						  TRACE_MODE_READ, NULL,
> +						  smap,
> +						  &ftrace_stackmap_stat_fops);
> +				trace_create_file("stack_map_bin",
> +						  TRACE_MODE_READ, NULL,
> +						  smap,
> +						  &ftrace_stackmap_bin_fops);
> +			}

-- Steve

Re: [RFC PATCH v4 1/3] trace: add lock-free stackmap for stack trace deduplication

[Li Pengfei]

From: Pengfei Li <lipengfei28@xiaomi.com>

On Tue, 14 Jul 2026 17:11:44 -0400 Steven Rostedt wrote:
> smap->entries = vcalloc(smap->map_size, sizeof(*smap->entries));
> Make the error paths have: ... goto fail; ...
Will switch both entries and elts to vcalloc(), and collapse the
error paths into a single goto-fail ladder (freeing in reverse
alloc order, including the drops percpu).

> Do not add anonymous blocks in functions.
Will move the cpu declaration to the top of ftrace_stackmap_reset().

> Really should have ftrace_stackmap_bin_entry have a flexible
> array: u64 ips[];
Agreed - will add the u64 ips[] flexible member and use
struct_size(e, ips, nr) in stackmap_bin_open(). On-disk layout is
unchanged, so the bin format stays compatible.

I'll also fold in two issues I found locally: a missing lock on one
init failure path, and TRACE_STACK_ID not being handled in the
function_graph output (it falls through to print_graph_comment()
instead of being punted like TRACE_STACK).

One design point I'd like your steer on before respinning: reset
checks tracer_tracing_is_on() once under trace_types_lock, but
traceon triggers (ftrace_traceon / traceon_trigger) can re-enable
tracing without that lock during reset's clear phase. As far as I
can tell this is a semantic-contract issue, not a memory-safety
one: the map is protected by the resetting flag (get_id bails with
-EINVAL and synchronize_rcu() drains in-flight callers), and the
ring buffer pages aren't freed by reset, so the worst case is a
non-empty / inconsistent buffer after reset rather than corruption.
So I'm leaning toward documenting reset as best-effort ("stop
tracing, including traceon triggers, before reset") rather than
adding machinery to block the window. Does that match your view, or
is there a ring-buffer-state hazard I'm missing that would justify
blocking it explicitly?

On the element pool: your `-l '*lock*'` run is a good illustration -
the ~326K-line stack_map dump is the 16K-entry pool (bits=14)
filling up quickly under broad function tracing. I'm leaning toward
keeping eager allocation for v5 (it keeps the hot path free of an
allocation-failure path), but I'm happy to switch to lazy allocation
on the first 'echo 1 > options/stackmap' if you'd rather not pay the
~8MB resident cost when the option is never enabled. I'd keep the
stack_map_bin interface as-is.

Thanks,
Pengfei

Re: [RFC PATCH v4 1/3] trace: add lock-free stackmap for stack trace deduplication

[Li Pengfei]

From: Pengfei Li <lipengfei28@xiaomi.com>

Hi Steve,

Just a gentle follow-up on the reset question below when you have a
chance.

I've addressed the review comments locally, but have held off on
respinning because this choice affects the reset semantics. Would you
prefer that the traceon race be documented as a best-effort limitation,
or that reset explicitly prevent or recheck tracing being re-enabled
during the clear phase?

I'm happy to implement either approach.

Thanks,
Pengfei

Re: [RFC PATCH v4 1/3] trace: add lock-free stackmap for stack trace deduplication

[Steven Rostedt]

On Wed, 15 Jul 2026 11:12:45 +0800
Li Pengfei <ljdlns1987@gmail.com> wrote:

Sorry for the late reply, I haven't had the time to look deeply at this
series.

> 
> One design point I'd like your steer on before respinning: reset
> checks tracer_tracing_is_on() once under trace_types_lock, but
> traceon triggers (ftrace_traceon / traceon_trigger) can re-enable
> tracing without that lock during reset's clear phase. As far as I
> can tell this is a semantic-contract issue, not a memory-safety
> one: the map is protected by the resetting flag (get_id bails with
> -EINVAL and synchronize_rcu() drains in-flight callers), and the
> ring buffer pages aren't freed by reset, so the worst case is a
> non-empty / inconsistent buffer after reset rather than corruption.
> So I'm leaning toward documenting reset as best-effort ("stop
> tracing, including traceon triggers, before reset") rather than
> adding machinery to block the window. Does that match your view, or
> is there a ring-buffer-state hazard I'm missing that would justify
> blocking it explicitly?

Honestly, I don't think we need to be as strict on reset as you are
trying to be. What is the worse thing that happens if a reset happens
while the buffer is not cleared and the trace still exists?

-- Steve

Re: [RFC PATCH v4 1/3] trace: add lock-free stackmap for stack trace deduplication

[Li Pengfei]

From: Pengfei Li <lipengfei28@xiaomi.com>

On Fri, 21 Aug 2026 23:51:29 -0400 Steven Rostedt wrote:
> Honestly, I don't think we need to be as strict on reset as you are
> trying to be. What is the worse thing that happens if a reset happens
> while the buffer is not cleared and the trace still exists?

Nothing that hurts the kernel. The map side stands on its own: the
resetting flag turns away new get_id() callers, synchronize_rcu()
drains the in-flight ones, and reset frees nothing - it only memsets
storage it still owns. So there is no use-after-free and no torn
read to worry about.

What is left is purely what userspace reads back. The buffer can
still hold TRACE_STACK_ID events after the map has been cleared, and
those ids resolve in one of two ways: either stack_map has no entry
for the id, or - once tracing continues and the slot gets reused -
the id now resolves to an unrelated stack. The second one is the
uglier of the two, since it is silent misattribution rather than an
obvious gap. Either way it is misleading output, not corruption.

Since that is the whole exposure, I agree it does not warrant
machinery to close the traceon window. v5 documents reset as
best-effort and leaves it at that.

If you would rather relax it further - drop the -EBUSY when tracing
is on, and stop clearing the ring buffer, leaving reset to only clear
the map - I am happy to do that. I would send it as a follow-up
rather than hold v5 on it, since the id-reuse case above is the only
thing the buffer clear was buying and it is cosmetic.

Thanks,
Pengfei