* [RFC PATCH v5 1/3] trace: add lock-free stackmap for stack trace deduplication
2026-09-02 6:42 [RFC PATCH v5 0/3] trace: stack trace deduplication for ftrace ring buffer Li Pengfei
@ 2026-09-02 6:42 ` Li Pengfei
2026-09-02 6:42 ` [RFC PATCH v5 2/3] trace: integrate stackmap into ftrace stack recording path Li Pengfei
2026-09-02 6:42 ` [RFC PATCH v5 3/3] trace: add documentation, selftest and tooling for stackmap Li Pengfei
2 siblings, 0 replies; 4+ messages in thread
From: Li Pengfei @ 2026-09-02 6:42 UTC (permalink / raw)
To: rostedt, mhiramat
Cc: mathieu.desnoyers, mark.rutland, corbet, skhan,
linux-trace-kernel, linux-kernel, linux-doc, linux-kselftest,
zhangbo56, lipengfei28
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 | 869 ++++++++++++++++++++++++++++++++++
kernel/trace/trace_stackmap.h | 57 +++
4 files changed, 949 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 084f34dc6c9f..2447fc59f7c7 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 f934ff586bd4..7710ec2659e9 100644
--- a/kernel/trace/Makefile
+++ b/kernel/trace/Makefile
@@ -86,6 +86,7 @@ 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..6d938aa414dc
--- /dev/null
+++ b/kernel/trace/trace_stackmap.c
@@ -0,0 +1,869 @@
+// 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 that clears the map only. It
+ * does not touch the ring buffer and does not require tracing to be
+ * stopped. 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).
+ * - synchronize_rcu() drains in-flight get_id() callers from the
+ * ftrace callback path. That path runs with preemption disabled,
+ * and a preempt-disabled region is itself an RCU read-side
+ * critical section (see the synchronize_rcu() kerneldoc: since
+ * v5.0, regions across which interrupts, preemption or softirqs
+ * are disabled also serve as RCU read-side critical sections), so
+ * the grace period covers it with no explicit rcu_read_lock().
+ *
+ * A trace collected before a reset can therefore still contain
+ * TRACE_STACK_ID records whose id no longer resolves, or resolves to
+ * a slot that has since been reused. That is misleading userspace
+ * output, not corruption; see ftrace_stackmap_reset().
+ *
+ * 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 <linux/math64.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;
+ /*
+ * Single-opener guard for stack_map_bin. Each open builds a
+ * full vmalloc snapshot (up to ~135 MB at bits=18); serialize
+ * opens so concurrent readers cannot pin several at once.
+ */
+ atomic_t bin_open;
+ /*
+ * 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 = vcalloc(smap->map_size, sizeof(*smap->entries));
+ 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 = vcalloc(smap->max_elts, sizeof(*smap->elts));
+ if (!smap->elts)
+ goto fail;
+
+ smap->successes = alloc_percpu(local_t);
+ if (!smap->successes)
+ goto fail;
+ smap->drops = alloc_percpu(local_t);
+ if (!smap->drops)
+ goto fail;
+
+ smap->hash_seed = get_random_u32();
+ atomic_set(&smap->next_elt, 0);
+ atomic_set(&smap->resetting, 0);
+ atomic_set(&smap->bin_open, 0);
+ init_rwsem(&smap->reader_sem);
+
+ return smap;
+
+fail:
+ /*
+ * free_percpu()/vfree()/kfree() all handle NULL, and smap was
+ * zero-initialized, so unwind in reverse allocation order.
+ */
+ free_percpu(smap->drops);
+ free_percpu(smap->successes);
+ vfree(smap->elts);
+ vfree(smap->entries);
+ kfree(smap);
+ 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, or -EBUSY if another reset is already in
+ * progress.
+ *
+ * Clears the map only. The ring buffer is left alone and tracing does
+ * not need to be stopped, so a trace can still contain TRACE_STACK_ID
+ * records after the map has been cleared. Such an id either has no
+ * entry in stack_map, or -- once tracing continues and the slot is
+ * reused -- resolves to an unrelated stack. Both are misleading
+ * userspace output rather than corruption: reset frees nothing, it
+ * only memsets storage the map still owns.
+ *
+ * Caller is process context (typically sysfs write handler).
+ *
+ * Protocol:
+ * 1. Atomically claim reset rights via cmpxchg on @resetting, which
+ * also blocks subsequent get_id() callers.
+ * 2. synchronize_rcu() drains in-flight get_id() callers from the
+ * ftrace callback path. That path is preempt-disabled, which is
+ * itself an RCU read-side critical section, so the grace period
+ * covers it without an explicit rcu_read_lock().
+ * 3. Take @reader_sem exclusively to exclude tracefs readers, then
+ * memset entries, elts, and counters.
+ * 4. 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)
+{
+ int cpu;
+
+ if (!smap)
+ return 0;
+
+ if (atomic_cmpxchg(&smap->resetting, 0, 1) != 0)
+ return -EBUSY;
+
+ /*
+ * 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);
+
+ 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);
+ 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);
+
+ /* Release resetting=0 so new get_id() observes a cleared map. */
+ atomic_set_release(&smap->resetting, 0);
+ return 0;
+}
+
+/* --- 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;
+ loff_t 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;
+ loff_t 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 already in
+ * progress.
+ */
+ 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)
+ /*
+ * mul_u64_u64_div_u64() uses a 128-bit intermediate, so
+ * (successes * 100) cannot overflow even when successes
+ * approaches U64_MAX on a very long trace.
+ */
+ seq_printf(m, "success_rate: %llu%%\n",
+ mul_u64_u64_div_u64(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;
+ struct ftrace_stackmap_bin_entry *e;
+ size_t alloc_size, off;
+ u32 nr_entries, i, nr_stacks;
+
+ if (!smap)
+ return -ENODEV;
+
+ /*
+ * Serialize opens: only one snapshot may be pinned at a time
+ * (see @bin_open). Released in stackmap_bin_release().
+ */
+ if (atomic_cmpxchg(&smap->bin_open, 0, 1) != 0)
+ return -EBUSY;
+
+ /*
+ * 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) * struct_size(e, ips, FTRACE_STACKMAP_MAX_DEPTH);
+
+ snap = vmalloc(sizeof(*snap) + alloc_size);
+ if (!snap) {
+ atomic_set(&smap->bin_open, 0);
+ 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;
+ 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 + 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;
+
+ 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)
+ e->ips[k] = (u64)FTRACE_TRAMPOLINE_MARKER;
+ else
+ e->ips[k] = (u64)trace_adjust_address(smap->tr, ip);
+ }
+ 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)
+{
+ struct ftrace_stackmap *smap = inode->i_private;
+
+ vfree(file->private_data);
+ if (smap)
+ atomic_set(&smap->bin_open, 0);
+ 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..2176cbe47572
--- /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;
+ u64 ips[]; /* nr entries follow the header */
+};
+
+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
^ permalink raw reply related [flat|nested] 4+ messages in thread* [RFC PATCH v5 2/3] trace: integrate stackmap into ftrace stack recording path
2026-09-02 6:42 [RFC PATCH v5 0/3] trace: stack trace deduplication for ftrace ring buffer Li Pengfei
2026-09-02 6:42 ` [RFC PATCH v5 1/3] trace: add lock-free stackmap for stack trace deduplication Li Pengfei
@ 2026-09-02 6:42 ` Li Pengfei
2026-09-02 6:42 ` [RFC PATCH v5 3/3] trace: add documentation, selftest and tooling for stackmap Li Pengfei
2 siblings, 0 replies; 4+ messages in thread
From: Li Pengfei @ 2026-09-02 6:42 UTC (permalink / raw)
To: rostedt, mhiramat
Cc: mathieu.desnoyers, mark.rutland, corbet, skhan,
linux-trace-kernel, linux-kernel, linux-doc, linux-kselftest,
zhangbo56, lipengfei28
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 | 228 ++++++++++++++++++++++++++-
kernel/trace/trace.h | 17 ++
kernel/trace/trace_entries.h | 15 ++
kernel/trace/trace_functions_graph.c | 1 +
kernel/trace/trace_output.c | 23 +++
kernel/trace/trace_selftest.c | 1 +
6 files changed, 282 insertions(+), 3 deletions(-)
diff --git a/kernel/trace/trace.c b/kernel/trace/trace.c
index 18710c190c92..0210dc597f77 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);
@@ -3976,6 +4047,50 @@ 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;
+
+/*
+ * Mark the global stackmap init as permanently failed.
+ *
+ * Clears any boot-time STACKMAP flag (trace_options=stackmap applied before
+ * the map was created) so options/stackmap does not report an enabled no-op
+ * and later userspace enables return -EINVAL. The flag is cleared under
+ * trace_types_lock because set_tracer_flag() updates trace_flags under that
+ * lock; the tracefs init runs in an unlocked workqueue context that can race
+ * with a concurrent trace_options write.
+ */
+static void __init stackmap_mark_init_failed(void)
+{
+ guard(mutex)(&trace_types_lock);
+ WRITE_ONCE(stackmap_init_state, STACKMAP_INIT_FAILED);
+ global_trace.trace_flags &= ~TRACE_ITER(STACKMAP);
+}
+#endif
+
int set_tracer_flag(struct trace_array *tr, u64 mask, int enabled)
{
switch (mask) {
@@ -3990,6 +4105,33 @@ 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 +9364,86 @@ 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
+ * boot-time STACKMAP flag (under
+ * trace_types_lock) so options/stackmap does
+ * not report an enabled no-op and later
+ * userspace enables return -EINVAL.
+ */
+ stackmap_mark_init_failed();
+ } 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. Use the helper so the flag
+ * clear happens under trace_types_lock, matching the
+ * stack_map-create failure path above.
+ */
+ stackmap_mark_init_failed();
+ }
+ }
+#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_functions_graph.c b/kernel/trace/trace_functions_graph.c
index 0d2d3a2ea7dd..0e1a390a6130 100644
--- a/kernel/trace/trace_functions_graph.c
+++ b/kernel/trace/trace_functions_graph.c
@@ -1492,6 +1492,7 @@ print_graph_function_flags(struct trace_iterator *iter, u32 flags)
return print_graph_return(field, s, entry, iter, flags);
}
case TRACE_STACK:
+ case TRACE_STACK_ID:
case TRACE_FN:
/* dont trace stack and functions as comments */
return TRACE_TYPE_UNHANDLED;
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
^ permalink raw reply related [flat|nested] 4+ messages in thread* [RFC PATCH v5 3/3] trace: add documentation, selftest and tooling for stackmap
2026-09-02 6:42 [RFC PATCH v5 0/3] trace: stack trace deduplication for ftrace ring buffer Li Pengfei
2026-09-02 6:42 ` [RFC PATCH v5 1/3] trace: add lock-free stackmap for stack trace deduplication Li Pengfei
2026-09-02 6:42 ` [RFC PATCH v5 2/3] trace: integrate stackmap into ftrace stack recording path Li Pengfei
@ 2026-09-02 6:42 ` Li Pengfei
2 siblings, 0 replies; 4+ messages in thread
From: Li Pengfei @ 2026-09-02 6:42 UTC (permalink / raw)
To: rostedt, mhiramat
Cc: mathieu.desnoyers, mark.rutland, corbet, skhan,
linux-trace-kernel, linux-kernel, linux-doc, linux-kselftest,
zhangbo56, lipengfei28
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 | 175 ++++++++++++++++++
Documentation/trace/index.rst | 1 +
.../ftrace/test.d/ftrace/stackmap-basic.tc | 110 +++++++++++
.../test.d/ftrace/stackmap-instance-gate.tc | 54 ++++++
.../ftrace/test.d/ftrace/stackmap-reset.tc | 77 ++++++++
tools/tracing/stackmap_dump.py | 164 ++++++++++++++++
6 files changed, 581 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..e056865bf6f1
--- /dev/null
+++ b/Documentation/trace/ftrace-stackmap.rst
@@ -0,0 +1,175 @@
+.. 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::
+
+ echo 0 > /sys/kernel/debug/tracing/stack_map
+
+Reset returns ``-EBUSY`` only if another reset is already in progress.
+
+Reset clears the map and nothing else: the trace buffer is left
+untouched and tracing does not have to be stopped. As a result a trace
+can still contain ``<stack_id N>`` records after a reset. Such an id
+either has no entry in ``stack_map``, or -- once tracing continues and
+the slot is reused -- resolves to an unrelated stack. That is
+misleading output, not corruption. If you need the ids in an existing
+trace to stay meaningful, read the trace out 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.
+
+``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 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..ce3ac893a071
--- /dev/null
+++ b/tools/testing/selftests/ftrace/test.d/ftrace/stackmap-basic.tc
@@ -0,0 +1,110 @@
+#!/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 succeeds while tracing is active (it clears the map
+# only and leaves the ring buffer alone)
+# 6. Verify reset also 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
+
+# Reset does not require tracing to be stopped: it clears the map only
+# and leaves the ring buffer alone, so it must succeed with tracing on.
+enable_tracing
+echo 0 > stack_map || fail "stackmap reset failed while tracing is active"
+disable_tracing
+
+# Test reset works when tracing is stopped as well
+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..e2e0f2adc403
--- /dev/null
+++ b/tools/testing/selftests/ftrace/test.d/ftrace/stackmap-reset.tc
@@ -0,0 +1,77 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0
+# description: ftrace - stackmap reset clears the map but not the trace buffer
+# 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) clears the map and
+# leaves the trace buffer alone. Existing <stack_id N> records
+# therefore survive a reset; they may no longer resolve, which is
+# documented as misleading-but-harmless userspace output.
+# 2. The stack_map_bin header carries the expected magic ('FSMB' =
+# 0x46534D42) and version (1).
+
+fail() {
+ echo "FAIL: $1"
+ exit_fail
+}
+
+cleanup() {
+ disable_tracing 2>/dev/null
+ echo nop > current_tracer 2>/dev/null
+ echo 0 > options/stackmap 2>/dev/null
+ echo 0 > options/stacktrace 2>/dev/null
+}
+trap cleanup EXIT
+
+disable_tracing
+clear_trace
+
+echo 1 > options/stackmap
+echo 1 > options/stacktrace
+echo function > current_tracer
+enable_tracing
+sleep 1
+disable_tracing
+
+# Sanity: the buffer must contain stack_id events before reset, otherwise
+# the buffer-untouched check below would be meaningless.
+before=$(grep -c "<stack_id" trace || true)
+: "${before:=0}"
+if [ "$before" -eq 0 ]; then
+ fail "no <stack_id> events captured before reset"
+fi
+
+# Reset clears the map only. It must succeed and must not disturb the
+# trace buffer.
+echo 0 > stack_map || fail "reset failed"
+
+after=$(grep -c "<stack_id" trace || true)
+: "${after:=0}"
+if [ "$after" -ne "$before" ]; then
+ fail "reset changed the trace buffer: $before -> $after <stack_id> events"
+fi
+
+entries=$(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: map cleared, $before stack_id events kept, 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
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