[PATCH v12 00/11] perf: Support the deferred unwinding infrastructure

[PATCH v12 00/11] perf: Support the deferred unwinding infrastructure

[Steven Rostedt]

This is based on top of the deferred unwind core patch series:

 https://lore.kernel.org/linux-trace-kernel/20250701005321.942306427@goodmis.org/
 git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace.git unwind/core


This series implements the perf interface to use deferred user space stack
tracing.

The code can be found here:

  git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace.git unwind/perf

  Head SHA1: e00f51618c68b5b6fd6054488212504592c9e392

Changes since v11: https://lore.kernel.org/linux-trace-kernel/20250625231541.584226205@goodmis.org/

- Also check against PF_USER_WORKER as io workers do not have PF_KTHREAD
  set.

- Removed deferred_request_nmi() and have NMIs just use the normal
  deferred_request() function. As Peter Zijlstra has stated, in_nmi() can
  nest because some exceptions set in_nmi() and a real NMI could come in.


Josh Poimboeuf (5):
      perf: Remove get_perf_callchain() init_nr argument
      perf: Have get_perf_callchain() return NULL if crosstask and user are set
      perf: Simplify get_perf_callchain() user logic
      perf: Skip user unwind if the task is a kernel thread
      perf: Support deferred user callchains

Namhyung Kim (4):
      perf tools: Minimal CALLCHAIN_DEFERRED support
      perf record: Enable defer_callchain for user callchains
      perf script: Display PERF_RECORD_CALLCHAIN_DEFERRED
      perf tools: Merge deferred user callchains

Steven Rostedt (2):
      perf: Use current->flags & PF_KTHREAD|PF_USER_WORKER instead of current->mm == NULL
      perf: Support deferred user callchains for per CPU events

----
 include/linux/perf_event.h                |  13 +-
 include/uapi/linux/perf_event.h           |  19 +-
 kernel/bpf/stackmap.c                     |   8 +-
 kernel/events/callchain.c                 |  49 ++--
 kernel/events/core.c                      | 410 +++++++++++++++++++++++++++++-
 tools/include/uapi/linux/perf_event.h     |  19 +-
 tools/lib/perf/include/perf/event.h       |   7 +
 tools/perf/Documentation/perf-script.txt  |   5 +
 tools/perf/builtin-script.c               |  92 +++++++
 tools/perf/util/callchain.c               |  24 ++
 tools/perf/util/callchain.h               |   3 +
 tools/perf/util/event.c                   |   1 +
 tools/perf/util/evlist.c                  |   1 +
 tools/perf/util/evlist.h                  |   1 +
 tools/perf/util/evsel.c                   |  39 +++
 tools/perf/util/evsel.h                   |   1 +
 tools/perf/util/machine.c                 |   1 +
 tools/perf/util/perf_event_attr_fprintf.c |   1 +
 tools/perf/util/sample.h                  |   3 +-
 tools/perf/util/session.c                 |  78 ++++++
 tools/perf/util/tool.c                    |   2 +
 tools/perf/util/tool.h                    |   4 +-
 22 files changed, 745 insertions(+), 36 deletions(-)

[PATCH v12 01/11] perf: Remove get_perf_callchain() init_nr argument

[Steven Rostedt]

From: Josh Poimboeuf <jpoimboe@kernel.org>

The 'init_nr' argument has double duty: it's used to initialize both the
number of contexts and the number of stack entries.  That's confusing
and the callers always pass zero anyway.  Hard code the zero.

Acked-by: Namhyung Kim <Namhyung@kernel.org>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 include/linux/perf_event.h |  2 +-
 kernel/bpf/stackmap.c      |  4 ++--
 kernel/events/callchain.c  | 12 ++++++------
 kernel/events/core.c       |  2 +-
 4 files changed, 10 insertions(+), 10 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index ec9d96025683..54e0d31afcad 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -1719,7 +1719,7 @@ DECLARE_PER_CPU(struct perf_callchain_entry, perf_callchain_entry);
 extern void perf_callchain_user(struct perf_callchain_entry_ctx *entry, struct pt_regs *regs);
 extern void perf_callchain_kernel(struct perf_callchain_entry_ctx *entry, struct pt_regs *regs);
 extern struct perf_callchain_entry *
-get_perf_callchain(struct pt_regs *regs, u32 init_nr, bool kernel, bool user,
+get_perf_callchain(struct pt_regs *regs, bool kernel, bool user,
 		   u32 max_stack, bool crosstask, bool add_mark);
 extern int get_callchain_buffers(int max_stack);
 extern void put_callchain_buffers(void);
diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index 3615c06b7dfa..ec3a57a5fba1 100644
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -314,7 +314,7 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,
 	if (max_depth > sysctl_perf_event_max_stack)
 		max_depth = sysctl_perf_event_max_stack;
 
-	trace = get_perf_callchain(regs, 0, kernel, user, max_depth,
+	trace = get_perf_callchain(regs, kernel, user, max_depth,
 				   false, false);
 
 	if (unlikely(!trace))
@@ -451,7 +451,7 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
 	else if (kernel && task)
 		trace = get_callchain_entry_for_task(task, max_depth);
 	else
-		trace = get_perf_callchain(regs, 0, kernel, user, max_depth,
+		trace = get_perf_callchain(regs, kernel, user, max_depth,
 					   crosstask, false);
 
 	if (unlikely(!trace) || trace->nr < skip) {
diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
index 6c83ad674d01..b0f5bd228cd8 100644
--- a/kernel/events/callchain.c
+++ b/kernel/events/callchain.c
@@ -217,7 +217,7 @@ static void fixup_uretprobe_trampoline_entries(struct perf_callchain_entry *entr
 }
 
 struct perf_callchain_entry *
-get_perf_callchain(struct pt_regs *regs, u32 init_nr, bool kernel, bool user,
+get_perf_callchain(struct pt_regs *regs, bool kernel, bool user,
 		   u32 max_stack, bool crosstask, bool add_mark)
 {
 	struct perf_callchain_entry *entry;
@@ -228,11 +228,11 @@ get_perf_callchain(struct pt_regs *regs, u32 init_nr, bool kernel, bool user,
 	if (!entry)
 		return NULL;
 
-	ctx.entry     = entry;
-	ctx.max_stack = max_stack;
-	ctx.nr	      = entry->nr = init_nr;
-	ctx.contexts       = 0;
-	ctx.contexts_maxed = false;
+	ctx.entry		= entry;
+	ctx.max_stack		= max_stack;
+	ctx.nr			= entry->nr = 0;
+	ctx.contexts		= 0;
+	ctx.contexts_maxed	= false;
 
 	if (kernel && !user_mode(regs)) {
 		if (add_mark)
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 7281230044d0..f2f7cff826e0 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -8178,7 +8178,7 @@ perf_callchain(struct perf_event *event, struct pt_regs *regs)
 	if (!kernel && !user)
 		return &__empty_callchain;
 
-	callchain = get_perf_callchain(regs, 0, kernel, user,
+	callchain = get_perf_callchain(regs, kernel, user,
 				       max_stack, crosstask, true);
 	return callchain ?: &__empty_callchain;
 }
-- 
2.47.2

[PATCH v12 02/11] perf: Have get_perf_callchain() return NULL if crosstask and user are set

[Steven Rostedt]

From: Josh Poimboeuf <jpoimboe@kernel.org>

get_perf_callchain() doesn't support cross-task unwinding for user space
stacks, have it return NULL if both the crosstask and user arguments are
set.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 kernel/events/callchain.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
index b0f5bd228cd8..cd0e3fc7ed05 100644
--- a/kernel/events/callchain.c
+++ b/kernel/events/callchain.c
@@ -224,6 +224,10 @@ get_perf_callchain(struct pt_regs *regs, bool kernel, bool user,
 	struct perf_callchain_entry_ctx ctx;
 	int rctx, start_entry_idx;
 
+	/* crosstask is not supported for user stacks */
+	if (crosstask && user && !kernel)
+		return NULL;
+
 	entry = get_callchain_entry(&rctx);
 	if (!entry)
 		return NULL;
@@ -240,7 +244,7 @@ get_perf_callchain(struct pt_regs *regs, bool kernel, bool user,
 		perf_callchain_kernel(&ctx, regs);
 	}
 
-	if (user) {
+	if (user && !crosstask) {
 		if (!user_mode(regs)) {
 			if  (current->mm)
 				regs = task_pt_regs(current);
@@ -249,9 +253,6 @@ get_perf_callchain(struct pt_regs *regs, bool kernel, bool user,
 		}
 
 		if (regs) {
-			if (crosstask)
-				goto exit_put;
-
 			if (add_mark)
 				perf_callchain_store_context(&ctx, PERF_CONTEXT_USER);
 
@@ -261,7 +262,6 @@ get_perf_callchain(struct pt_regs *regs, bool kernel, bool user,
 		}
 	}
 
-exit_put:
 	put_callchain_entry(rctx);
 
 	return entry;
-- 
2.47.2

[PATCH v12 03/11] perf: Use current->flags & PF_KTHREAD|PF_USER_WORKER instead of current->mm == NULL

[Steven Rostedt]

From: Steven Rostedt <rostedt@goodmis.org>

To determine if a task is a kernel thread or not, it is more reliable to
use (current->flags & (PF_KTHREAD|PF_USER_WORKERi)) than to rely on
current->mm being NULL.  That is because some kernel tasks (io_uring
helpers) may have a mm field.

Link: https://lore.kernel.org/linux-trace-kernel/20250424163607.GE18306@noisy.programming.kicks-ass.net/
Link: https://lore.kernel.org/all/20250624130744.602c5b5f@batman.local.home/

Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
Changes since v11: https://lore.kernel.org/20250625231622.172100822@goodmis.org

- Also check against PF_USER_WORKER as io workers do not have PF_KTHREAD
  set.

 kernel/events/callchain.c | 6 +++---
 kernel/events/core.c      | 4 ++--
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
index cd0e3fc7ed05..5982d18f169b 100644
--- a/kernel/events/callchain.c
+++ b/kernel/events/callchain.c
@@ -246,10 +246,10 @@ get_perf_callchain(struct pt_regs *regs, bool kernel, bool user,
 
 	if (user && !crosstask) {
 		if (!user_mode(regs)) {
-			if  (current->mm)
-				regs = task_pt_regs(current);
-			else
+			if (current->flags & (PF_KTHREAD | PF_USER_WORKER))
 				regs = NULL;
+			else
+				regs = task_pt_regs(current);
 		}
 
 		if (regs) {
diff --git a/kernel/events/core.c b/kernel/events/core.c
index f2f7cff826e0..2c524fe4bd19 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -7414,7 +7414,7 @@ static void perf_sample_regs_user(struct perf_regs *regs_user,
 	if (user_mode(regs)) {
 		regs_user->abi = perf_reg_abi(current);
 		regs_user->regs = regs;
-	} else if (!(current->flags & PF_KTHREAD)) {
+	} else if (!(current->flags & (PF_KTHREAD | PF_USER_WORKER))) {
 		perf_get_regs_user(regs_user, regs);
 	} else {
 		regs_user->abi = PERF_SAMPLE_REGS_ABI_NONE;
@@ -8054,7 +8054,7 @@ static u64 perf_virt_to_phys(u64 virt)
 		 * Try IRQ-safe get_user_page_fast_only first.
 		 * If failed, leave phys_addr as 0.
 		 */
-		if (current->mm != NULL) {
+		if (!(current->flags & PF_KTHREAD)) {
 			struct page *p;
 
 			pagefault_disable();
-- 
2.47.2

[PATCH v12 04/11] perf: Simplify get_perf_callchain() user logic

[Steven Rostedt]

From: Josh Poimboeuf <jpoimboe@kernel.org>

Simplify the get_perf_callchain() user logic a bit.  task_pt_regs()
should never be NULL.

Acked-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 kernel/events/callchain.c | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
index 5982d18f169b..808c0d7a31fa 100644
--- a/kernel/events/callchain.c
+++ b/kernel/events/callchain.c
@@ -247,21 +247,19 @@ get_perf_callchain(struct pt_regs *regs, bool kernel, bool user,
 	if (user && !crosstask) {
 		if (!user_mode(regs)) {
 			if (current->flags & (PF_KTHREAD | PF_USER_WORKER))
-				regs = NULL;
-			else
-				regs = task_pt_regs(current);
+				goto exit_put;
+			regs = task_pt_regs(current);
 		}
 
-		if (regs) {
-			if (add_mark)
-				perf_callchain_store_context(&ctx, PERF_CONTEXT_USER);
+		if (add_mark)
+			perf_callchain_store_context(&ctx, PERF_CONTEXT_USER);
 
-			start_entry_idx = entry->nr;
-			perf_callchain_user(&ctx, regs);
-			fixup_uretprobe_trampoline_entries(entry, start_entry_idx);
-		}
+		start_entry_idx = entry->nr;
+		perf_callchain_user(&ctx, regs);
+		fixup_uretprobe_trampoline_entries(entry, start_entry_idx);
 	}
 
+exit_put:
 	put_callchain_entry(rctx);
 
 	return entry;
-- 
2.47.2

[PATCH v12 05/11] perf: Skip user unwind if the task is a kernel thread

[Steven Rostedt]

From: Josh Poimboeuf <jpoimboe@kernel.org>

If the task is not a user thread, there's no user stack to unwind.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
Changes since v11: https://lore.kernel.org/20250625231622.509379386@goodmis.org

- Added test against PF_USER_WORKER as well as PF_KTHREAD

 kernel/events/core.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 2c524fe4bd19..43d76ed882da 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -8166,7 +8166,8 @@ struct perf_callchain_entry *
 perf_callchain(struct perf_event *event, struct pt_regs *regs)
 {
 	bool kernel = !event->attr.exclude_callchain_kernel;
-	bool user   = !event->attr.exclude_callchain_user;
+	bool user   = !event->attr.exclude_callchain_user &&
+		!(current->flags & (PF_KTHREAD | PF_USER_WORKER));
 	/* Disallow cross-task user callchains. */
 	bool crosstask = event->ctx->task && event->ctx->task != current;
 	const u32 max_stack = event->attr.sample_max_stack;
-- 
2.47.2

[PATCH v12 06/11] perf: Support deferred user callchains

[Steven Rostedt]

From: Josh Poimboeuf <jpoimboe@kernel.org>

Use the new unwind_deferred_trace() interface (if available) to defer
unwinds to task context.  This will allow the use of .sframe (when it
becomes available) and also prevents duplicate userspace unwinds.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Co-developed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
Changes since v11: https://lore.kernel.org/20250625231622.678033124@goodmis.org

- Removed deferred_request_nmi() and have NMIs just use the normal
  deferred_request() function. As Peter Zijlstra has stated, in_nmi() can
  nest because some exceptions set in_nmi() and another NMI could come in.

- Added test against PF_USER_WORKER as well as PF_KTHREAD

 include/linux/perf_event.h            |   7 +-
 include/uapi/linux/perf_event.h       |  19 +++-
 kernel/bpf/stackmap.c                 |   4 +-
 kernel/events/callchain.c             |  11 +-
 kernel/events/core.c                  | 149 +++++++++++++++++++++++++-
 tools/include/uapi/linux/perf_event.h |  19 +++-
 6 files changed, 201 insertions(+), 8 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 54e0d31afcad..c7d474391e51 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -53,6 +53,7 @@
 #include <linux/security.h>
 #include <linux/static_call.h>
 #include <linux/lockdep.h>
+#include <linux/unwind_deferred.h>
 
 #include <asm/local.h>
 
@@ -880,6 +881,10 @@ struct perf_event {
 	struct callback_head		pending_task;
 	unsigned int			pending_work;
 
+	unsigned int			pending_unwind_callback;
+	struct callback_head		pending_unwind_work;
+	struct rcuwait			pending_unwind_wait;
+
 	atomic_t			event_limit;
 
 	/* address range filters */
@@ -1720,7 +1725,7 @@ extern void perf_callchain_user(struct perf_callchain_entry_ctx *entry, struct p
 extern void perf_callchain_kernel(struct perf_callchain_entry_ctx *entry, struct pt_regs *regs);
 extern struct perf_callchain_entry *
 get_perf_callchain(struct pt_regs *regs, bool kernel, bool user,
-		   u32 max_stack, bool crosstask, bool add_mark);
+		   u32 max_stack, bool crosstask, bool add_mark, bool defer_user);
 extern int get_callchain_buffers(int max_stack);
 extern void put_callchain_buffers(void);
 extern struct perf_callchain_entry *get_callchain_entry(int *rctx);
diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index 78a362b80027..184740d1e79d 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -463,7 +463,8 @@ struct perf_event_attr {
 				inherit_thread :  1, /* children only inherit if cloned with CLONE_THREAD */
 				remove_on_exec :  1, /* event is removed from task on exec */
 				sigtrap        :  1, /* send synchronous SIGTRAP on event */
-				__reserved_1   : 26;
+				defer_callchain:  1, /* generate PERF_RECORD_CALLCHAIN_DEFERRED records */
+				__reserved_1   : 25;
 
 	union {
 		__u32		wakeup_events;	  /* wake up every n events */
@@ -1239,6 +1240,21 @@ enum perf_event_type {
 	 */
 	PERF_RECORD_AUX_OUTPUT_HW_ID		= 21,
 
+	/*
+	 * This user callchain capture was deferred until shortly before
+	 * returning to user space.  Previous samples would have kernel
+	 * callchains only and they need to be stitched with this to make full
+	 * callchains.
+	 *
+	 * struct {
+	 *	struct perf_event_header	header;
+	 *	u64				nr;
+	 *	u64				ips[nr];
+	 *	struct sample_id		sample_id;
+	 * };
+	 */
+	PERF_RECORD_CALLCHAIN_DEFERRED		= 22,
+
 	PERF_RECORD_MAX,			/* non-ABI */
 };
 
@@ -1269,6 +1285,7 @@ enum perf_callchain_context {
 	PERF_CONTEXT_HV				= (__u64)-32,
 	PERF_CONTEXT_KERNEL			= (__u64)-128,
 	PERF_CONTEXT_USER			= (__u64)-512,
+	PERF_CONTEXT_USER_DEFERRED		= (__u64)-640,
 
 	PERF_CONTEXT_GUEST			= (__u64)-2048,
 	PERF_CONTEXT_GUEST_KERNEL		= (__u64)-2176,
diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index ec3a57a5fba1..339f7cbbcf36 100644
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -315,7 +315,7 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,
 		max_depth = sysctl_perf_event_max_stack;
 
 	trace = get_perf_callchain(regs, kernel, user, max_depth,
-				   false, false);
+				   false, false, false);
 
 	if (unlikely(!trace))
 		/* couldn't fetch the stack trace */
@@ -452,7 +452,7 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
 		trace = get_callchain_entry_for_task(task, max_depth);
 	else
 		trace = get_perf_callchain(regs, kernel, user, max_depth,
-					   crosstask, false);
+					   crosstask, false, false);
 
 	if (unlikely(!trace) || trace->nr < skip) {
 		if (may_fault)
diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
index 808c0d7a31fa..d0e0da66a164 100644
--- a/kernel/events/callchain.c
+++ b/kernel/events/callchain.c
@@ -218,7 +218,7 @@ static void fixup_uretprobe_trampoline_entries(struct perf_callchain_entry *entr
 
 struct perf_callchain_entry *
 get_perf_callchain(struct pt_regs *regs, bool kernel, bool user,
-		   u32 max_stack, bool crosstask, bool add_mark)
+		   u32 max_stack, bool crosstask, bool add_mark, bool defer_user)
 {
 	struct perf_callchain_entry *entry;
 	struct perf_callchain_entry_ctx ctx;
@@ -251,6 +251,15 @@ get_perf_callchain(struct pt_regs *regs, bool kernel, bool user,
 			regs = task_pt_regs(current);
 		}
 
+		if (defer_user) {
+			/*
+			 * Foretell the coming of PERF_RECORD_CALLCHAIN_DEFERRED
+			 * which can be stitched to this one.
+			 */
+			perf_callchain_store_context(&ctx, PERF_CONTEXT_USER_DEFERRED);
+			goto exit_put;
+		}
+
 		if (add_mark)
 			perf_callchain_store_context(&ctx, PERF_CONTEXT_USER);
 
diff --git a/kernel/events/core.c b/kernel/events/core.c
index 43d76ed882da..b427360447c9 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5582,6 +5582,89 @@ static bool exclusive_event_installable(struct perf_event *event,
 	return true;
 }
 
+static void perf_pending_unwind_sync(struct perf_event *event)
+{
+	might_sleep();
+
+	if (!event->pending_unwind_callback)
+		return;
+
+	/*
+	 * If the task is queued to the current task's queue, we
+	 * obviously can't wait for it to complete. Simply cancel it.
+	 */
+	if (task_work_cancel(current, &event->pending_unwind_work)) {
+		event->pending_unwind_callback = 0;
+		local_dec(&event->ctx->nr_no_switch_fast);
+		return;
+	}
+
+	/*
+	 * All accesses related to the event are within the same RCU section in
+	 * perf_event_callchain_deferred(). The RCU grace period before the
+	 * event is freed will make sure all those accesses are complete by then.
+	 */
+	rcuwait_wait_event(&event->pending_unwind_wait, !event->pending_unwind_callback, TASK_UNINTERRUPTIBLE);
+}
+
+struct perf_callchain_deferred_event {
+	struct perf_event_header	header;
+	u64				nr;
+	u64				ips[];
+};
+
+static void perf_event_callchain_deferred(struct callback_head *work)
+{
+	struct perf_event *event = container_of(work, struct perf_event, pending_unwind_work);
+	struct perf_callchain_deferred_event deferred_event;
+	u64 callchain_context = PERF_CONTEXT_USER;
+	struct unwind_stacktrace trace;
+	struct perf_output_handle handle;
+	struct perf_sample_data data;
+	u64 nr;
+
+	if (!event->pending_unwind_callback)
+		return;
+
+	if (unwind_deferred_trace(&trace) < 0)
+		goto out;
+
+	/*
+	 * All accesses to the event must belong to the same implicit RCU
+	 * read-side critical section as the ->pending_unwind_callback reset.
+	 * See comment in perf_pending_unwind_sync().
+	 */
+	guard(rcu)();
+
+	if (current->flags & (PF_KTHREAD | PF_USER_WORKER))
+		goto out;
+
+	nr = trace.nr + 1 ; /* '+1' == callchain_context */
+
+	deferred_event.header.type = PERF_RECORD_CALLCHAIN_DEFERRED;
+	deferred_event.header.misc = PERF_RECORD_MISC_USER;
+	deferred_event.header.size = sizeof(deferred_event) + (nr * sizeof(u64));
+
+	deferred_event.nr = nr;
+
+	perf_event_header__init_id(&deferred_event.header, &data, event);
+
+	if (perf_output_begin(&handle, &data, event, deferred_event.header.size))
+		goto out;
+
+	perf_output_put(&handle, deferred_event);
+	perf_output_put(&handle, callchain_context);
+	perf_output_copy(&handle, trace.entries, trace.nr * sizeof(u64));
+	perf_event__output_id_sample(event, &handle, &data);
+
+	perf_output_end(&handle);
+
+out:
+	event->pending_unwind_callback = 0;
+	local_dec(&event->ctx->nr_no_switch_fast);
+	rcuwait_wake_up(&event->pending_unwind_wait);
+}
+
 static void perf_free_addr_filters(struct perf_event *event);
 
 /* vs perf_event_alloc() error */
@@ -5649,6 +5732,7 @@ static void _free_event(struct perf_event *event)
 {
 	irq_work_sync(&event->pending_irq);
 	irq_work_sync(&event->pending_disable_irq);
+	perf_pending_unwind_sync(event);
 
 	unaccount_event(event);
 
@@ -8162,6 +8246,46 @@ static u64 perf_get_page_size(unsigned long addr)
 
 static struct perf_callchain_entry __empty_callchain = { .nr = 0, };
 
+/*
+ * Returns:
+*     > 0 : if already queued.
+ *      0 : if it performed the queuing
+ *    < 0 : if it did not get queued.
+ */
+static int deferred_request(struct perf_event *event)
+{
+	struct callback_head *work = &event->pending_unwind_work;
+	int pending;
+	int ret;
+
+	/* Only defer for task events */
+	if (!event->ctx->task)
+		return -EINVAL;
+
+	if ((current->flags & (PF_KTHREAD | PF_USER_WORKER)) ||
+	    !user_mode(task_pt_regs(current)))
+		return -EINVAL;
+
+	guard(irqsave)();
+
+	/* callback already pending? */
+	pending = READ_ONCE(event->pending_unwind_callback);
+	if (pending)
+		return 1;
+
+	/* Claim the work unless an NMI just now swooped in to do so. */
+	if (!try_cmpxchg(&event->pending_unwind_callback, &pending, 1))
+		return 1;
+
+	/* The work has been claimed, now schedule it. */
+	ret = task_work_add(current, work, TWA_RESUME);
+	if (WARN_ON_ONCE(ret)) {
+		WRITE_ONCE(event->pending_unwind_callback, 0);
+		return ret;
+	}
+	return 0;
+}
+
 struct perf_callchain_entry *
 perf_callchain(struct perf_event *event, struct pt_regs *regs)
 {
@@ -8172,6 +8296,8 @@ perf_callchain(struct perf_event *event, struct pt_regs *regs)
 	bool crosstask = event->ctx->task && event->ctx->task != current;
 	const u32 max_stack = event->attr.sample_max_stack;
 	struct perf_callchain_entry *callchain;
+	bool defer_user = IS_ENABLED(CONFIG_UNWIND_USER) && user &&
+			  event->attr.defer_callchain;
 
 	if (!current->mm)
 		user = false;
@@ -8179,8 +8305,21 @@ perf_callchain(struct perf_event *event, struct pt_regs *regs)
 	if (!kernel && !user)
 		return &__empty_callchain;
 
-	callchain = get_perf_callchain(regs, kernel, user,
-				       max_stack, crosstask, true);
+	/* Disallow cross-task callchains. */
+	if (event->ctx->task && event->ctx->task != current)
+		return &__empty_callchain;
+
+	if (defer_user) {
+		int ret = deferred_request(event);
+		if (!ret)
+			local_inc(&event->ctx->nr_no_switch_fast);
+		else if (ret < 0)
+			defer_user = false;
+	}
+
+	callchain = get_perf_callchain(regs, kernel, user, max_stack,
+				       crosstask, true, defer_user);
+
 	return callchain ?: &__empty_callchain;
 }
 
@@ -12850,6 +12989,8 @@ perf_event_alloc(struct perf_event_attr *attr, int cpu,
 	event->pending_disable_irq = IRQ_WORK_INIT_HARD(perf_pending_disable);
 	init_task_work(&event->pending_task, perf_pending_task);
 
+	rcuwait_init(&event->pending_unwind_wait);
+
 	mutex_init(&event->mmap_mutex);
 	raw_spin_lock_init(&event->addr_filters.lock);
 
@@ -13018,6 +13159,10 @@ perf_event_alloc(struct perf_event_attr *attr, int cpu,
 	if (err)
 		return ERR_PTR(err);
 
+	if (event->attr.defer_callchain)
+		init_task_work(&event->pending_unwind_work,
+			       perf_event_callchain_deferred);
+
 	/* symmetric to unaccount_event() in _free_event() */
 	account_event(event);
 
diff --git a/tools/include/uapi/linux/perf_event.h b/tools/include/uapi/linux/perf_event.h
index 78a362b80027..184740d1e79d 100644
--- a/tools/include/uapi/linux/perf_event.h
+++ b/tools/include/uapi/linux/perf_event.h
@@ -463,7 +463,8 @@ struct perf_event_attr {
 				inherit_thread :  1, /* children only inherit if cloned with CLONE_THREAD */
 				remove_on_exec :  1, /* event is removed from task on exec */
 				sigtrap        :  1, /* send synchronous SIGTRAP on event */
-				__reserved_1   : 26;
+				defer_callchain:  1, /* generate PERF_RECORD_CALLCHAIN_DEFERRED records */
+				__reserved_1   : 25;
 
 	union {
 		__u32		wakeup_events;	  /* wake up every n events */
@@ -1239,6 +1240,21 @@ enum perf_event_type {
 	 */
 	PERF_RECORD_AUX_OUTPUT_HW_ID		= 21,
 
+	/*
+	 * This user callchain capture was deferred until shortly before
+	 * returning to user space.  Previous samples would have kernel
+	 * callchains only and they need to be stitched with this to make full
+	 * callchains.
+	 *
+	 * struct {
+	 *	struct perf_event_header	header;
+	 *	u64				nr;
+	 *	u64				ips[nr];
+	 *	struct sample_id		sample_id;
+	 * };
+	 */
+	PERF_RECORD_CALLCHAIN_DEFERRED		= 22,
+
 	PERF_RECORD_MAX,			/* non-ABI */
 };
 
@@ -1269,6 +1285,7 @@ enum perf_callchain_context {
 	PERF_CONTEXT_HV				= (__u64)-32,
 	PERF_CONTEXT_KERNEL			= (__u64)-128,
 	PERF_CONTEXT_USER			= (__u64)-512,
+	PERF_CONTEXT_USER_DEFERRED		= (__u64)-640,
 
 	PERF_CONTEXT_GUEST			= (__u64)-2048,
 	PERF_CONTEXT_GUEST_KERNEL		= (__u64)-2176,
-- 
2.47.2

[PATCH v12 07/11] perf: Support deferred user callchains for per CPU events

[Steven Rostedt]

From: Steven Rostedt <rostedt@goodmis.org>

The deferred unwinder works fine for task events (events that trace only a
specific task), as it can use a task_work from an interrupt or NMI and
when the task goes back to user space it will call the event's callback to
do the deferred unwinding.

But for per CPU events things are not so simple. When a per CPU event
wants a deferred unwinding to occur, it cannot simply use a task_work as
there's a many to many relationship. If the task migrates and another task
is scheduled in where the per CPU event wants a deferred unwinding to
occur on that task as well, and the task that migrated to another CPU has
that CPU's event want to unwind it too, each CPU may need unwinding from
more than one task, and each task may have requests from many CPUs.

To solve this, when a per CPU event is created that has defer_callchain
attribute set, it will do a lookup from a global list
(unwind_deferred_list), for a perf_unwind_deferred descriptor that has the
id that matches the PID of the current task's group_leader.

If it is not found, then it will create one and add it to the global list.
This descriptor contains an array of all possible CPUs, where each element
is a perf_unwind_cpu descriptor.

The perf_unwind_cpu descriptor has a list of all the per CPU events that
is tracing the matching CPU that corresponds to its index in the array,
where the events belong to a task that has the same group_leader.
It also has a processing bit and rcuwait to handle removal.

For each occupied perf_unwind_cpu descriptor in the array, the
perf_deferred_unwind descriptor increments its nr_cpu_events. When a
perf_unwind_cpu descriptor is empty, the nr_cpu_events is decremented.
This is used to know when to free the perf_deferred_unwind descriptor, as
when it becomes empty, it is no longer referenced.

Finally, the perf_deferred_unwind descriptor has an id that holds the PID
of the group_leader for the tasks that the events were created by.

When a second (or more) per CPU event is created where the
perf_deferred_unwind descriptor is already created, it just adds itself to
the perf_unwind_cpu array of that descriptor. Updating the necessary
counter. This is used to map different per CPU events to each other based
on their group leader PID.

Each of these perf_deferred_unwind descriptors have a unwind_work that
registers with the deferred unwind infrastructure via
unwind_deferred_init(), where it also registers a callback to
perf_event_deferred_cpu().

Now when a per CPU event requests a deferred unwinding, it calls
unwind_deferred_request() with the associated perf_deferred_unwind
descriptor. It is expected that the program that uses this has events on
all CPUs, as the deferred trace may not be called on the CPU event that
requested it. That is, the task may migrate and its user stack trace will
be recorded on the CPU event of the CPU that it exits back to user space
on.

Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 include/linux/perf_event.h |   4 +
 kernel/events/core.c       | 306 +++++++++++++++++++++++++++++++++----
 2 files changed, 284 insertions(+), 26 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index c7d474391e51..546a7f81be96 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -733,6 +733,7 @@ struct swevent_hlist {
 struct bpf_prog;
 struct perf_cgroup;
 struct perf_buffer;
+struct perf_unwind_deferred;
 
 struct pmu_event_list {
 	raw_spinlock_t			lock;
@@ -885,6 +886,9 @@ struct perf_event {
 	struct callback_head		pending_unwind_work;
 	struct rcuwait			pending_unwind_wait;
 
+	struct perf_unwind_deferred	*unwind_deferred;
+	struct list_head		unwind_list;
+
 	atomic_t			event_limit;
 
 	/* address range filters */
diff --git a/kernel/events/core.c b/kernel/events/core.c
index b427360447c9..b91dc0a100f6 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -5582,10 +5582,185 @@ static bool exclusive_event_installable(struct perf_event *event,
 	return true;
 }
 
+/* Holds a list of per CPU events that registered for deferred unwinding */
+struct perf_unwind_cpu {
+	struct list_head	list;
+	struct rcuwait		pending_unwind_wait;
+	int			processing;
+};
+
+struct perf_unwind_deferred {
+	struct list_head		list;
+	struct unwind_work		unwind_work;
+	struct perf_unwind_cpu __rcu	*cpu_events;
+	struct rcu_head			rcu_head;
+	int				nr_cpu_events;
+	int				id;
+};
+
+static DEFINE_MUTEX(unwind_deferred_mutex);
+static LIST_HEAD(unwind_deferred_list);
+
+static void perf_event_deferred_cpu(struct unwind_work *work,
+				    struct unwind_stacktrace *trace, u64 timestamp);
+
+/*
+ * Add a per CPU event.
+ *
+ * The deferred callstack can happen on a different CPU than what was
+ * requested. If one CPU event requests a deferred callstack, but the
+ * tasks migrates, it will execute on a different CPU and save the
+ * stack trace to that CPU event.
+ *
+ * In order to map all the CPU events with the same application,
+ * use the current->gorup_leader->pid as the identifier of what
+ * events share the same program.
+ *
+ * A perf_unwind_deferred descriptor is created for each unique
+ * group_leader pid, and all the events that have the same group_leader
+ * pid will be linked to the same deferred descriptor.
+ *
+ * If there's no descriptor that matches the current group_leader pid,
+ * one will be created.
+ */
+static int perf_add_unwind_deferred(struct perf_event *event)
+{
+	struct perf_unwind_deferred *defer;
+	struct perf_unwind_cpu *cpu_events;
+	int id = current->group_leader->pid;
+	bool found = false;
+	int ret = 0;
+
+	if (event->cpu < 0)
+		return -EINVAL;
+
+	guard(mutex)(&unwind_deferred_mutex);
+
+	list_for_each_entry(defer, &unwind_deferred_list, list) {
+		if (defer->id == id) {
+			found = true;
+			break;
+		}
+	}
+
+	if (!found) {
+		defer = kzalloc(sizeof(*defer), GFP_KERNEL);
+		if (!defer)
+			return -ENOMEM;
+		list_add(&defer->list, &unwind_deferred_list);
+		defer->id = id;
+	}
+
+	/*
+	 * The deferred desciptor has an array for every CPU.
+	 * Each entry in this array is a link list of all the CPU
+	 * events for the corresponding CPU. This is a quick way to
+	 * find the associated event for a given CPU in
+	 * perf_event_deferred_cpu().
+	 */
+	if (!defer->nr_cpu_events) {
+		cpu_events = kcalloc(num_possible_cpus(),
+				     sizeof(*cpu_events),
+				     GFP_KERNEL);
+		if (!cpu_events) {
+			ret = -ENOMEM;
+			goto free;
+		}
+		for (int cpu = 0; cpu < num_possible_cpus(); cpu++) {
+			rcuwait_init(&cpu_events[cpu].pending_unwind_wait);
+			INIT_LIST_HEAD(&cpu_events[cpu].list);
+		}
+
+		rcu_assign_pointer(defer->cpu_events, cpu_events);
+
+		ret = unwind_deferred_init(&defer->unwind_work,
+					   perf_event_deferred_cpu);
+		if (ret)
+			goto free;
+	}
+	cpu_events = rcu_dereference_protected(defer->cpu_events,
+				lockdep_is_held(&unwind_deferred_mutex));
+
+	if (list_empty(&cpu_events[event->cpu].list))
+		defer->nr_cpu_events++;
+	list_add_tail_rcu(&event->unwind_list, &cpu_events[event->cpu].list);
+
+	event->unwind_deferred = defer;
+	return 0;
+free:
+	if (found)
+		return ret;
+
+	list_del(&defer->list);
+	kfree(cpu_events);
+	kfree(defer);
+	return ret;
+}
+
+static void free_unwind_deferred_rcu(struct rcu_head *head)
+{
+	struct perf_unwind_cpu *cpu_events;
+	struct perf_unwind_deferred *defer =
+		container_of(head, struct perf_unwind_deferred, rcu_head);
+
+	/*
+	 * This is called by call_rcu() and there are no more
+	 * references to cpu_events.
+	 */
+	cpu_events = rcu_dereference_protected(defer->cpu_events, 1);
+	kfree(cpu_events);
+	kfree(defer);
+}
+
+static void perf_remove_unwind_deferred(struct perf_event *event)
+{
+	struct perf_unwind_deferred *defer = event->unwind_deferred;
+	struct perf_unwind_cpu *cpu_events, *cpu_unwind;
+
+	if (!defer)
+		return;
+
+	guard(mutex)(&unwind_deferred_mutex);
+	list_del_rcu(&event->unwind_list);
+
+	cpu_events = rcu_dereference_protected(defer->cpu_events,
+				lockdep_is_held(&unwind_deferred_mutex));
+	cpu_unwind = &cpu_events[event->cpu];
+
+	if (list_empty(&cpu_unwind->list)) {
+		defer->nr_cpu_events--;
+		if (!defer->nr_cpu_events)
+			unwind_deferred_cancel(&defer->unwind_work);
+	}
+
+	event->unwind_deferred = NULL;
+
+	/*
+	 * Make sure perf_event_deferred_cpu() is done with this event.
+	 * That function will set cpu_unwind->processing and then
+	 * call smp_mb() before iterating the list of its events.
+	 * If the event's unwind_deferred is NULL, it will be skipped.
+	 * The smp_mb() in that function matches the mb() in
+	 * rcuwait_wait_event().
+	 */
+	rcuwait_wait_event(&cpu_unwind->pending_unwind_wait,
+				   !cpu_unwind->processing, TASK_UNINTERRUPTIBLE);
+
+	/* Is this still being used by other per CPU events? */
+	if (defer->nr_cpu_events)
+		return;
+
+	list_del(&defer->list);
+	/* The defer->cpu_events is protected by RCU */
+	call_rcu(&defer->rcu_head, free_unwind_deferred_rcu);
+}
+
 static void perf_pending_unwind_sync(struct perf_event *event)
 {
 	might_sleep();
 
+	perf_remove_unwind_deferred(event);
+
 	if (!event->pending_unwind_callback)
 		return;
 
@@ -5609,62 +5784,119 @@ static void perf_pending_unwind_sync(struct perf_event *event)
 
 struct perf_callchain_deferred_event {
 	struct perf_event_header	header;
+	u64				timestamp;
 	u64				nr;
 	u64				ips[];
 };
 
-static void perf_event_callchain_deferred(struct callback_head *work)
+static void perf_event_callchain_deferred(struct perf_event *event,
+					  struct unwind_stacktrace *trace,
+					  u64 timestamp)
 {
-	struct perf_event *event = container_of(work, struct perf_event, pending_unwind_work);
 	struct perf_callchain_deferred_event deferred_event;
 	u64 callchain_context = PERF_CONTEXT_USER;
-	struct unwind_stacktrace trace;
 	struct perf_output_handle handle;
 	struct perf_sample_data data;
 	u64 nr;
 
-	if (!event->pending_unwind_callback)
-		return;
-
-	if (unwind_deferred_trace(&trace) < 0)
-		goto out;
-
-	/*
-	 * All accesses to the event must belong to the same implicit RCU
-	 * read-side critical section as the ->pending_unwind_callback reset.
-	 * See comment in perf_pending_unwind_sync().
-	 */
-	guard(rcu)();
-
 	if (current->flags & (PF_KTHREAD | PF_USER_WORKER))
-		goto out;
+		return;
 
-	nr = trace.nr + 1 ; /* '+1' == callchain_context */
+	nr = trace->nr + 1 ; /* '+1' == callchain_context */
 
 	deferred_event.header.type = PERF_RECORD_CALLCHAIN_DEFERRED;
 	deferred_event.header.misc = PERF_RECORD_MISC_USER;
 	deferred_event.header.size = sizeof(deferred_event) + (nr * sizeof(u64));
 
+	deferred_event.timestamp = timestamp;
 	deferred_event.nr = nr;
 
 	perf_event_header__init_id(&deferred_event.header, &data, event);
 
 	if (perf_output_begin(&handle, &data, event, deferred_event.header.size))
-		goto out;
+		return;
 
 	perf_output_put(&handle, deferred_event);
 	perf_output_put(&handle, callchain_context);
-	perf_output_copy(&handle, trace.entries, trace.nr * sizeof(u64));
+	perf_output_copy(&handle, trace->entries, trace->nr * sizeof(u64));
 	perf_event__output_id_sample(event, &handle, &data);
 
 	perf_output_end(&handle);
+}
+
+/* Deferred unwinding callback for task specific events */
+static void perf_event_deferred_task(struct callback_head *work)
+{
+	struct perf_event *event = container_of(work, struct perf_event, pending_unwind_work);
+	struct unwind_stacktrace trace;
+
+	if (!event->pending_unwind_callback)
+		return;
+
+	if (unwind_user_faultable(&trace) >= 0) {
+
+		/*
+		 * All accesses to the event must belong to the same implicit RCU
+		 * read-side critical section as the ->pending_unwind_callback reset.
+		 * See comment in perf_pending_unwind_sync().
+		 */
+		guard(rcu)();
+		perf_event_callchain_deferred(event, &trace, 0);
+	}
 
-out:
 	event->pending_unwind_callback = 0;
 	local_dec(&event->ctx->nr_no_switch_fast);
 	rcuwait_wake_up(&event->pending_unwind_wait);
 }
 
+/*
+ * Deferred unwinding callback for per CPU events.
+ * Note, the request for the deferred unwinding may have happened
+ * on a different CPU.
+ */
+static void perf_event_deferred_cpu(struct unwind_work *work,
+				    struct unwind_stacktrace *trace, u64 timestamp)
+{
+	struct perf_unwind_deferred *defer =
+		container_of(work, struct perf_unwind_deferred, unwind_work);
+	struct perf_unwind_cpu *cpu_events, *cpu_unwind;
+	struct perf_event *event;
+	int cpu;
+
+	guard(rcu)();
+	guard(preempt)();
+
+	cpu = smp_processor_id();
+	cpu_events = rcu_dereference(defer->cpu_events);
+	cpu_unwind = &cpu_events[cpu];
+
+	WRITE_ONCE(cpu_unwind->processing, 1);
+	/*
+	 * Make sure the above is seen before the event->unwind_deferred
+	 * is checked. This matches the mb() in rcuwait_rcu_wait_event() in
+	 * perf_remove_unwind_deferred().
+	 */
+	smp_mb();
+
+	list_for_each_entry_rcu(event, &cpu_unwind->list, unwind_list) {
+		/* If unwind_deferred is NULL the event is going away */
+		if (unlikely(!event->unwind_deferred))
+			continue;
+		perf_event_callchain_deferred(event, trace, timestamp);
+		/* Only the first CPU event gets the trace */
+		break;
+	}
+
+	/*
+	 * The perf_event_callchain_deferred() must finish before setting
+	 * cpu_unwind->processing to zero. This is also to synchronize
+	 * with the rcuwait in perf_remove_unwind_deferred().
+	 */
+	smp_mb();
+	WRITE_ONCE(cpu_unwind->processing, 0);
+	rcuwait_wake_up(&cpu_unwind->pending_unwind_wait);
+}
+
 static void perf_free_addr_filters(struct perf_event *event);
 
 /* vs perf_event_alloc() error */
@@ -8246,6 +8478,16 @@ static u64 perf_get_page_size(unsigned long addr)
 
 static struct perf_callchain_entry __empty_callchain = { .nr = 0, };
 
+
+static int deferred_unwind_request(struct perf_unwind_deferred *defer)
+{
+	u64 timestamp;
+	int ret;
+
+	ret = unwind_deferred_request(&defer->unwind_work, &timestamp);
+	return ret < 0 ? ret : 0;
+}
+
 /*
  * Returns:
 *     > 0 : if already queued.
@@ -8255,17 +8497,22 @@ static struct perf_callchain_entry __empty_callchain = { .nr = 0, };
 static int deferred_request(struct perf_event *event)
 {
 	struct callback_head *work = &event->pending_unwind_work;
+	struct perf_unwind_deferred *defer;
 	int pending;
 	int ret;
 
-	/* Only defer for task events */
-	if (!event->ctx->task)
-		return -EINVAL;
-
 	if ((current->flags & (PF_KTHREAD | PF_USER_WORKER)) ||
 	    !user_mode(task_pt_regs(current)))
 		return -EINVAL;
 
+	defer = READ_ONCE(event->unwind_deferred);
+	if (defer)
+		return deferred_unwind_request(defer);
+
+	/* Per CPU events should have had unwind_deferred set! */
+	if (WARN_ON_ONCE(!event->ctx->task))
+		return -EINVAL;
+
 	guard(irqsave)();
 
 	/* callback already pending? */
@@ -13155,13 +13402,20 @@ perf_event_alloc(struct perf_event_attr *attr, int cpu,
 		}
 	}
 
+	/* Setup unwind deferring for per CPU events */
+	if (event->attr.defer_callchain && !task) {
+		err = perf_add_unwind_deferred(event);
+		if (err)
+			return ERR_PTR(err);
+	}
+
 	err = security_perf_event_alloc(event);
 	if (err)
 		return ERR_PTR(err);
 
 	if (event->attr.defer_callchain)
 		init_task_work(&event->pending_unwind_work,
-			       perf_event_callchain_deferred);
+			       perf_event_deferred_task);
 
 	/* symmetric to unaccount_event() in _free_event() */
 	account_event(event);
-- 
2.47.2

Re: [PATCH v12 07/11] perf: Support deferred user callchains for per CPU events

[Jens Remus]

Hello Steve!

On 01.07.2025 20:04, Steven Rostedt wrote:

> diff --git a/kernel/events/core.c b/kernel/events/core.c

> @@ -5609,62 +5784,119 @@ static void perf_pending_unwind_sync(struct perf_event *event)
>  
>  struct perf_callchain_deferred_event {
>  	struct perf_event_header	header;
> +	u64				timestamp;
>  	u64				nr;
>  	u64				ips[];
>  };

Nit: Please update the following related comments when making changes to
struct perf_callchain_deferred_event.

diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
index 184740d1e79d..08ce721e79bc 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -1248,6 +1248,7 @@ enum perf_event_type {
 	 *
 	 * struct {
 	 *	struct perf_event_header	header;
+	 *	u64				timestamp;
 	 *	u64				nr;
 	 *	u64				ips[nr];
 	 *	struct sample_id		sample_id;
diff --git a/tools/include/uapi/linux/perf_event.h b/tools/include/uapi/linux/perf_event.h
index 184740d1e79d..08ce721e79bc 100644
--- a/tools/include/uapi/linux/perf_event.h
+++ b/tools/include/uapi/linux/perf_event.h
@@ -1248,6 +1248,7 @@ enum perf_event_type {
 	 *
 	 * struct {
 	 *	struct perf_event_header	header;
+	 *	u64				timestamp;
 	 *	u64				nr;
 	 *	u64				ips[nr];
 	 *	struct sample_id		sample_id;

Thanks and regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@de.ibm.com

IBM

IBM Deutschland Research & Development GmbH; Vorsitzender des Aufsichtsrats: Wolfgang Wendt; Geschäftsführung: David Faller; Sitz der Gesellschaft: Böblingen; Registergericht: Amtsgericht Stuttgart, HRB 243294
IBM Data Privacy Statement: https://www.ibm.com/privacy/

[PATCH v12 08/11] perf tools: Minimal CALLCHAIN_DEFERRED support

[Steven Rostedt]

From: Namhyung Kim <namhyung@kernel.org>

Add a new event type for deferred callchains and a new callback for the
struct perf_tool.  For now it doesn't actually handle the deferred
callchains but it just marks the sample if it has the PERF_CONTEXT_
USER_DEFFERED in the callchain array.

At least, perf report can dump the raw data with this change.  Actually
this requires the next commit to enable attr.defer_callchain, but if you
already have a data file, it'll show the following result.

  $ perf report -D
  ...
  0x5fe0@perf.data [0x40]: event: 22
  .
  . ... raw event: size 64 bytes
  .  0000:  16 00 00 00 02 00 40 00 02 00 00 00 00 00 00 00  ......@.........
  .  0010:  00 fe ff ff ff ff ff ff 4b d3 3f 25 45 7f 00 00  ........K.?%E...
  .  0020:  21 03 00 00 21 03 00 00 43 02 12 ab 05 00 00 00  !...!...C.......
  .  0030:  00 00 00 00 00 00 00 00 09 00 00 00 00 00 00 00  ................

  0 24344920643 0x5fe0 [0x40]: PERF_RECORD_CALLCHAIN_DEFERRED(IP, 0x2): 801/801: 0
  ... FP chain: nr:2
  .....  0: fffffffffffffe00
  .....  1: 00007f45253fd34b
  : unhandled!

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 tools/lib/perf/include/perf/event.h       |  7 +++++++
 tools/perf/util/event.c                   |  1 +
 tools/perf/util/evsel.c                   | 15 +++++++++++++++
 tools/perf/util/machine.c                 |  1 +
 tools/perf/util/perf_event_attr_fprintf.c |  1 +
 tools/perf/util/sample.h                  |  3 ++-
 tools/perf/util/session.c                 | 17 +++++++++++++++++
 tools/perf/util/tool.c                    |  1 +
 tools/perf/util/tool.h                    |  3 ++-
 9 files changed, 47 insertions(+), 2 deletions(-)

diff --git a/tools/lib/perf/include/perf/event.h b/tools/lib/perf/include/perf/event.h
index 09b7c643ddac..4590d62ca913 100644
--- a/tools/lib/perf/include/perf/event.h
+++ b/tools/lib/perf/include/perf/event.h
@@ -151,6 +151,12 @@ struct perf_record_switch {
 	__u32			 next_prev_tid;
 };
 
+struct perf_record_callchain_deferred {
+	struct perf_event_header header;
+	__u64			 nr;
+	__u64			 ips[];
+};
+
 struct perf_record_header_attr {
 	struct perf_event_header header;
 	struct perf_event_attr	 attr;
@@ -505,6 +511,7 @@ union perf_event {
 	struct perf_record_read			read;
 	struct perf_record_throttle		throttle;
 	struct perf_record_sample		sample;
+	struct perf_record_callchain_deferred	callchain_deferred;
 	struct perf_record_bpf_event		bpf;
 	struct perf_record_ksymbol		ksymbol;
 	struct perf_record_text_poke_event	text_poke;
diff --git a/tools/perf/util/event.c b/tools/perf/util/event.c
index 7544a3104e21..720682fea9be 100644
--- a/tools/perf/util/event.c
+++ b/tools/perf/util/event.c
@@ -58,6 +58,7 @@ static const char *perf_event__names[] = {
 	[PERF_RECORD_CGROUP]			= "CGROUP",
 	[PERF_RECORD_TEXT_POKE]			= "TEXT_POKE",
 	[PERF_RECORD_AUX_OUTPUT_HW_ID]		= "AUX_OUTPUT_HW_ID",
+	[PERF_RECORD_CALLCHAIN_DEFERRED]	= "CALLCHAIN_DEFERRED",
 	[PERF_RECORD_HEADER_ATTR]		= "ATTR",
 	[PERF_RECORD_HEADER_EVENT_TYPE]		= "EVENT_TYPE",
 	[PERF_RECORD_HEADER_TRACING_DATA]	= "TRACING_DATA",
diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
index d55482f094bf..6176c31b57ea 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -2978,6 +2978,18 @@ int evsel__parse_sample(struct evsel *evsel, union perf_event *event,
 	data->data_src = PERF_MEM_DATA_SRC_NONE;
 	data->vcpu = -1;
 
+	if (event->header.type == PERF_RECORD_CALLCHAIN_DEFERRED) {
+		const u64 max_callchain_nr = UINT64_MAX / sizeof(u64);
+
+		data->callchain = (struct ip_callchain *)&event->callchain_deferred.nr;
+		if (data->callchain->nr > max_callchain_nr)
+			return -EFAULT;
+
+		if (evsel->core.attr.sample_id_all)
+			perf_evsel__parse_id_sample(evsel, event, data);
+		return 0;
+	}
+
 	if (event->header.type != PERF_RECORD_SAMPLE) {
 		if (!evsel->core.attr.sample_id_all)
 			return 0;
@@ -3108,6 +3120,9 @@ int evsel__parse_sample(struct evsel *evsel, union perf_event *event,
 		if (data->callchain->nr > max_callchain_nr)
 			return -EFAULT;
 		sz = data->callchain->nr * sizeof(u64);
+		if (evsel->core.attr.defer_callchain && data->callchain->nr >= 1 &&
+		    data->callchain->ips[data->callchain->nr - 1] == PERF_CONTEXT_USER_DEFERRED)
+			data->deferred_callchain = true;
 		OVERFLOW_CHECK(array, sz, max_size);
 		array = (void *)array + sz;
 	}
diff --git a/tools/perf/util/machine.c b/tools/perf/util/machine.c
index 7ec12c207970..de6100366eee 100644
--- a/tools/perf/util/machine.c
+++ b/tools/perf/util/machine.c
@@ -2124,6 +2124,7 @@ static int add_callchain_ip(struct thread *thread,
 				*cpumode = PERF_RECORD_MISC_KERNEL;
 				break;
 			case PERF_CONTEXT_USER:
+			case PERF_CONTEXT_USER_DEFERRED:
 				*cpumode = PERF_RECORD_MISC_USER;
 				break;
 			default:
diff --git a/tools/perf/util/perf_event_attr_fprintf.c b/tools/perf/util/perf_event_attr_fprintf.c
index 66b666d9ce64..abfd9b9a718c 100644
--- a/tools/perf/util/perf_event_attr_fprintf.c
+++ b/tools/perf/util/perf_event_attr_fprintf.c
@@ -343,6 +343,7 @@ int perf_event_attr__fprintf(FILE *fp, struct perf_event_attr *attr,
 	PRINT_ATTRf(inherit_thread, p_unsigned);
 	PRINT_ATTRf(remove_on_exec, p_unsigned);
 	PRINT_ATTRf(sigtrap, p_unsigned);
+	PRINT_ATTRf(defer_callchain, p_unsigned);
 
 	PRINT_ATTRn("{ wakeup_events, wakeup_watermark }", wakeup_events, p_unsigned, false);
 	PRINT_ATTRf(bp_type, p_unsigned);
diff --git a/tools/perf/util/sample.h b/tools/perf/util/sample.h
index 0e96240052e9..9d6e2f14551c 100644
--- a/tools/perf/util/sample.h
+++ b/tools/perf/util/sample.h
@@ -108,7 +108,8 @@ struct perf_sample {
 		u16 p_stage_cyc;
 		u16 retire_lat;
 	};
-	bool no_hw_idx;		/* No hw_idx collected in branch_stack */
+	bool no_hw_idx;			/* No hw_idx collected in branch_stack */
+	bool deferred_callchain;	/* Has deferred user callchains */
 	char insn[MAX_INSN];
 	void *raw_data;
 	struct ip_callchain *callchain;
diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
index a320672c264e..e6be01459352 100644
--- a/tools/perf/util/session.c
+++ b/tools/perf/util/session.c
@@ -715,6 +715,7 @@ static perf_event__swap_op perf_event__swap_ops[] = {
 	[PERF_RECORD_CGROUP]		  = perf_event__cgroup_swap,
 	[PERF_RECORD_TEXT_POKE]		  = perf_event__text_poke_swap,
 	[PERF_RECORD_AUX_OUTPUT_HW_ID]	  = perf_event__all64_swap,
+	[PERF_RECORD_CALLCHAIN_DEFERRED]  = perf_event__all64_swap,
 	[PERF_RECORD_HEADER_ATTR]	  = perf_event__hdr_attr_swap,
 	[PERF_RECORD_HEADER_EVENT_TYPE]	  = perf_event__event_type_swap,
 	[PERF_RECORD_HEADER_TRACING_DATA] = perf_event__tracing_data_swap,
@@ -1118,6 +1119,19 @@ static void dump_sample(struct evsel *evsel, union perf_event *event,
 		sample_read__printf(sample, evsel->core.attr.read_format);
 }
 
+static void dump_deferred_callchain(struct evsel *evsel, union perf_event *event,
+				    struct perf_sample *sample)
+{
+	if (!dump_trace)
+		return;
+
+	printf("(IP, 0x%x): %d/%d: %#" PRIx64 "\n",
+	       event->header.misc, sample->pid, sample->tid, sample->ip);
+
+	if (evsel__has_callchain(evsel))
+		callchain__printf(evsel, sample);
+}
+
 static void dump_read(struct evsel *evsel, union perf_event *event)
 {
 	struct perf_record_read *read_event = &event->read;
@@ -1348,6 +1362,9 @@ static int machines__deliver_event(struct machines *machines,
 		return tool->text_poke(tool, event, sample, machine);
 	case PERF_RECORD_AUX_OUTPUT_HW_ID:
 		return tool->aux_output_hw_id(tool, event, sample, machine);
+	case PERF_RECORD_CALLCHAIN_DEFERRED:
+		dump_deferred_callchain(evsel, event, sample);
+		return tool->callchain_deferred(tool, event, sample, evsel, machine);
 	default:
 		++evlist->stats.nr_unknown_events;
 		return -1;
diff --git a/tools/perf/util/tool.c b/tools/perf/util/tool.c
index 37bd8ac63b01..e1d60abb4e41 100644
--- a/tools/perf/util/tool.c
+++ b/tools/perf/util/tool.c
@@ -266,6 +266,7 @@ void perf_tool__init(struct perf_tool *tool, bool ordered_events)
 	tool->read = process_event_sample_stub;
 	tool->throttle = process_event_stub;
 	tool->unthrottle = process_event_stub;
+	tool->callchain_deferred = process_event_sample_stub;
 	tool->attr = process_event_synth_attr_stub;
 	tool->event_update = process_event_synth_event_update_stub;
 	tool->tracing_data = process_event_synth_tracing_data_stub;
diff --git a/tools/perf/util/tool.h b/tools/perf/util/tool.h
index db1c7642b0d1..9987bbde6d5e 100644
--- a/tools/perf/util/tool.h
+++ b/tools/perf/util/tool.h
@@ -42,7 +42,8 @@ enum show_feature_header {
 
 struct perf_tool {
 	event_sample	sample,
-			read;
+			read,
+			callchain_deferred;
 	event_op	mmap,
 			mmap2,
 			comm,
-- 
2.47.2

Re: [PATCH v12 08/11] perf tools: Minimal CALLCHAIN_DEFERRED support

[Jens Remus]

Hello Steve!

On 01.07.2025 20:04, Steven Rostedt wrote:
> From: Namhyung Kim <namhyung@kernel.org>
> 
> Add a new event type for deferred callchains and a new callback for the
> struct perf_tool.  For now it doesn't actually handle the deferred
> callchains but it just marks the sample if it has the PERF_CONTEXT_
> USER_DEFFERED in the callchain array.

> diff --git a/tools/lib/perf/include/perf/event.h b/tools/lib/perf/include/perf/event.h

> @@ -151,6 +151,12 @@ struct perf_record_switch {
>  	__u32			 next_prev_tid;
>  };
>  
> +struct perf_record_callchain_deferred {
> +	struct perf_event_header header;

At minimum the timestamp field added to perf with "[PATCH v12 07/11]
perf: Support deferred user callchains for per CPU events" needs to be
added here as well:

	__u64			 timestamp;

Otherwise this and any subsequent enhancements of the perf tools do no
longer work at all.  But probably the timestamp field also needs to be
used for some purpose?

> +	__u64			 nr;
> +	__u64			 ips[];
> +};
> +

Thanks and regards,
Jens
-- 
Jens Remus
Linux on Z Development (D3303)
+49-7031-16-1128 Office
jremus@de.ibm.com

IBM

IBM Deutschland Research & Development GmbH; Vorsitzender des Aufsichtsrats: Wolfgang Wendt; Geschäftsführung: David Faller; Sitz der Gesellschaft: Böblingen; Registergericht: Amtsgericht Stuttgart, HRB 243294
IBM Data Privacy Statement: https://www.ibm.com/privacy/

Re: [PATCH v12 08/11] perf tools: Minimal CALLCHAIN_DEFERRED support

[Steven Rostedt]

On Wed, 2 Jul 2025 14:23:24 +0200
Jens Remus <jremus@linux.ibm.com> wrote:

> > +struct perf_record_callchain_deferred {
> > +	struct perf_event_header header;  
> 
> At minimum the timestamp field added to perf with "[PATCH v12 07/11]
> perf: Support deferred user callchains for per CPU events" needs to be
> added here as well:

Thanks for the review.

> 
> 	__u64			 timestamp;
> 
> Otherwise this and any subsequent enhancements of the perf tools do no
> longer work at all.  But probably the timestamp field also needs to be
> used for some purpose?

OK, so this may be part of the discussion about using this as a
timestamp. The timestamp is the timestamp given by the deferred trace
infrastructure. It holds the time that this stack trace is valid for.
But that's assuming that the timestamp is the same as what perf is
using.

In case of dropped events, we could have the case of:

  system_call() {
    <nmi> {
      take kernel stack trace
      ask for deferred trace.

   [EVENTS START DROPPING HERE]
    }
    Call deferred callback to record trace [ BUT IS DROPPED ]
  }

  system_call() {
    <nmi> {
      take kernel stack trace
      ask for deferred trace [ STILL DROPPING ]
    }
    [ READER CATCHES UP AND STARTS READING EVENTS AGAIN]

    Call deferred callback to record trace
  }

The user space tool will see that kernel stack traces of the first
system call, then it will see events dropped, and then it will see the
deferred user space stack trace of the second call.

If the timestamps are in sync with perf and what is passed in, then the
tool will see that the kernel stack traces from the first system call
are older than the timestamp of the user stack trace and know that they
are not related.

Without either saving the timestamp along with the kernel stack traces,
or having the timestamps in sync, user space will not know whether or
not if the user space stack trace of the second system call belongs to
the kernel stack traces that were recorded in the first system call.

I guess the question is, do we just not associate stack traces beyond
where events are dropped? If so, we don't even need to save the
timestamp.

-- Steve


> 
> > +	__u64			 nr;
> > +	__u64			 ips[];
> > +};

[PATCH v12 09/11] perf record: Enable defer_callchain for user callchains

[Steven Rostedt]

From: Namhyung Kim <namhyung@kernel.org>

And add the missing feature detection logic to clear the flag on old
kernels.

  $ perf record -g -vv true
  ...
  ------------------------------------------------------------
  perf_event_attr:
    type                             0 (PERF_TYPE_HARDWARE)
    size                             136
    config                           0 (PERF_COUNT_HW_CPU_CYCLES)
    { sample_period, sample_freq }   4000
    sample_type                      IP|TID|TIME|CALLCHAIN|PERIOD
    read_format                      ID|LOST
    disabled                         1
    inherit                          1
    mmap                             1
    comm                             1
    freq                             1
    enable_on_exec                   1
    task                             1
    sample_id_all                    1
    mmap2                            1
    comm_exec                        1
    ksymbol                          1
    bpf_event                        1
    defer_callchain                  1
  ------------------------------------------------------------
  sys_perf_event_open: pid 162755  cpu 0  group_fd -1  flags 0x8
  sys_perf_event_open failed, error -22
  switching off deferred callchain support

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 tools/perf/util/evsel.c | 24 ++++++++++++++++++++++++
 tools/perf/util/evsel.h |  1 +
 2 files changed, 25 insertions(+)

diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
index 6176c31b57ea..c942983b870e 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -1076,6 +1076,14 @@ static void __evsel__config_callchain(struct evsel *evsel, struct record_opts *o
 		}
 	}
 
+	if (param->record_mode == CALLCHAIN_FP && !attr->exclude_callchain_user) {
+		/*
+		 * Enable deferred callchains optimistically.  It'll be switched
+		 * off later if the kernel doesn't support it.
+		 */
+		attr->defer_callchain = 1;
+	}
+
 	if (function) {
 		pr_info("Disabling user space callchains for function trace event.\n");
 		attr->exclude_callchain_user = 1;
@@ -2124,6 +2132,8 @@ static int __evsel__prepare_open(struct evsel *evsel, struct perf_cpu_map *cpus,
 
 static void evsel__disable_missing_features(struct evsel *evsel)
 {
+	if (perf_missing_features.defer_callchain)
+		evsel->core.attr.defer_callchain = 0;
 	if (perf_missing_features.inherit_sample_read && evsel->core.attr.inherit &&
 	    (evsel->core.attr.sample_type & PERF_SAMPLE_READ))
 		evsel->core.attr.inherit = 0;
@@ -2398,6 +2408,15 @@ static bool evsel__detect_missing_features(struct evsel *evsel, struct perf_cpu
 
 	/* Please add new feature detection here. */
 
+	attr.defer_callchain = true;
+	attr.sample_type = PERF_SAMPLE_CALLCHAIN;
+	if (has_attr_feature(&attr, /*flags=*/0))
+		goto found;
+	perf_missing_features.defer_callchain = true;
+	pr_debug2("switching off deferred callchain support\n");
+	attr.defer_callchain = false;
+	attr.sample_type = 0;
+
 	attr.inherit = true;
 	attr.sample_type = PERF_SAMPLE_READ;
 	if (has_attr_feature(&attr, /*flags=*/0))
@@ -2509,6 +2528,11 @@ static bool evsel__detect_missing_features(struct evsel *evsel, struct perf_cpu
 	errno = old_errno;
 
 check:
+	if (evsel->core.attr.defer_callchain &&
+	    evsel->core.attr.sample_type & PERF_SAMPLE_CALLCHAIN &&
+	    perf_missing_features.defer_callchain)
+		return true;
+
 	if (evsel->core.attr.inherit &&
 	    (evsel->core.attr.sample_type & PERF_SAMPLE_READ) &&
 	    perf_missing_features.inherit_sample_read)
diff --git a/tools/perf/util/evsel.h b/tools/perf/util/evsel.h
index 6dbc9690e0c9..a01c1de8f95f 100644
--- a/tools/perf/util/evsel.h
+++ b/tools/perf/util/evsel.h
@@ -221,6 +221,7 @@ struct perf_missing_features {
 	bool branch_counters;
 	bool aux_action;
 	bool inherit_sample_read;
+	bool defer_callchain;
 };
 
 extern struct perf_missing_features perf_missing_features;
-- 
2.47.2

[PATCH v12 10/11] perf script: Display PERF_RECORD_CALLCHAIN_DEFERRED

[Steven Rostedt]

From: Namhyung Kim <namhyung@kernel.org>

Handle the deferred callchains in the script output.

  $ perf script
  perf     801 [000]    18.031793:          1 cycles:P:
          ffffffff91a14c36 __intel_pmu_enable_all.isra.0+0x56 ([kernel.kallsyms])
          ffffffff91d373e9 perf_ctx_enable+0x39 ([kernel.kallsyms])
          ffffffff91d36af7 event_function+0xd7 ([kernel.kallsyms])
          ffffffff91d34222 remote_function+0x42 ([kernel.kallsyms])
          ffffffff91c1ebe1 generic_exec_single+0x61 ([kernel.kallsyms])
          ffffffff91c1edac smp_call_function_single+0xec ([kernel.kallsyms])
          ffffffff91d37a9d event_function_call+0x10d ([kernel.kallsyms])
          ffffffff91d33557 perf_event_for_each_child+0x37 ([kernel.kallsyms])
          ffffffff91d47324 _perf_ioctl+0x204 ([kernel.kallsyms])
          ffffffff91d47c43 perf_ioctl+0x33 ([kernel.kallsyms])
          ffffffff91e2f216 __x64_sys_ioctl+0x96 ([kernel.kallsyms])
          ffffffff9265f1ae do_syscall_64+0x9e ([kernel.kallsyms])
          ffffffff92800130 entry_SYSCALL_64+0xb0 ([kernel.kallsyms])

  perf     801 [000]    18.031814: DEFERRED CALLCHAIN
              7fb5fc22034b __GI___ioctl+0x3b (/usr/lib/x86_64-linux-gnu/libc.so.6)

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 tools/perf/builtin-script.c | 89 +++++++++++++++++++++++++++++++++++++
 1 file changed, 89 insertions(+)

diff --git a/tools/perf/builtin-script.c b/tools/perf/builtin-script.c
index 6c3bf74dd78c..a6f8209256fe 100644
--- a/tools/perf/builtin-script.c
+++ b/tools/perf/builtin-script.c
@@ -2483,6 +2483,93 @@ static int process_sample_event(const struct perf_tool *tool,
 	return ret;
 }
 
+static int process_deferred_sample_event(const struct perf_tool *tool,
+					 union perf_event *event,
+					 struct perf_sample *sample,
+					 struct evsel *evsel,
+					 struct machine *machine)
+{
+	struct perf_script *scr = container_of(tool, struct perf_script, tool);
+	struct perf_event_attr *attr = &evsel->core.attr;
+	struct evsel_script *es = evsel->priv;
+	unsigned int type = output_type(attr->type);
+	struct addr_location al;
+	FILE *fp = es->fp;
+	int ret = 0;
+
+	if (output[type].fields == 0)
+		return 0;
+
+	/* Set thread to NULL to indicate addr_al and al are not initialized */
+	addr_location__init(&al);
+
+	if (perf_time__ranges_skip_sample(scr->ptime_range, scr->range_num,
+					  sample->time)) {
+		goto out_put;
+	}
+
+	if (debug_mode) {
+		if (sample->time < last_timestamp) {
+			pr_err("Samples misordered, previous: %" PRIu64
+				" this: %" PRIu64 "\n", last_timestamp,
+				sample->time);
+			nr_unordered++;
+		}
+		last_timestamp = sample->time;
+		goto out_put;
+	}
+
+	if (filter_cpu(sample))
+		goto out_put;
+
+	if (machine__resolve(machine, &al, sample) < 0) {
+		pr_err("problem processing %d event, skipping it.\n",
+		       event->header.type);
+		ret = -1;
+		goto out_put;
+	}
+
+	if (al.filtered)
+		goto out_put;
+
+	if (!show_event(sample, evsel, al.thread, &al, NULL))
+		goto out_put;
+
+	if (evswitch__discard(&scr->evswitch, evsel))
+		goto out_put;
+
+	perf_sample__fprintf_start(scr, sample, al.thread, evsel,
+				   PERF_RECORD_CALLCHAIN_DEFERRED, fp);
+	fprintf(fp, "DEFERRED CALLCHAIN");
+
+	if (PRINT_FIELD(IP)) {
+		struct callchain_cursor *cursor = NULL;
+
+		if (symbol_conf.use_callchain && sample->callchain) {
+			cursor = get_tls_callchain_cursor();
+			if (thread__resolve_callchain(al.thread, cursor, evsel,
+						      sample, NULL, NULL,
+						      scripting_max_stack)) {
+				pr_info("cannot resolve deferred callchains\n");
+				cursor = NULL;
+			}
+		}
+
+		fputc(cursor ? '\n' : ' ', fp);
+		sample__fprintf_sym(sample, &al, 0, output[type].print_ip_opts,
+				    cursor, symbol_conf.bt_stop_list, fp);
+	}
+
+	fprintf(fp, "\n");
+
+	if (verbose > 0)
+		fflush(fp);
+
+out_put:
+	addr_location__exit(&al);
+	return ret;
+}
+
 // Used when scr->per_event_dump is not set
 static struct evsel_script es_stdout;
 
@@ -4069,6 +4156,7 @@ int cmd_script(int argc, const char **argv)
 
 	perf_tool__init(&script.tool, !unsorted_dump);
 	script.tool.sample		 = process_sample_event;
+	script.tool.callchain_deferred	 = process_deferred_sample_event;
 	script.tool.mmap		 = perf_event__process_mmap;
 	script.tool.mmap2		 = perf_event__process_mmap2;
 	script.tool.comm		 = perf_event__process_comm;
@@ -4095,6 +4183,7 @@ int cmd_script(int argc, const char **argv)
 	script.tool.throttle		 = process_throttle_event;
 	script.tool.unthrottle		 = process_throttle_event;
 	script.tool.ordering_requires_timestamps = true;
+	script.tool.merge_deferred_callchains = false;
 	session = perf_session__new(&data, &script.tool);
 	if (IS_ERR(session))
 		return PTR_ERR(session);
-- 
2.47.2

[PATCH v12 11/11] perf tools: Merge deferred user callchains

[Steven Rostedt]

From: Namhyung Kim <namhyung@kernel.org>

Save samples with deferred callchains in a separate list and deliver
them after merging the user callchains.  If users don't want to merge
they can set tool->merge_deferred_callchains to false to prevent the
behavior.

With previous result, now perf script will show the merged callchains.

  $ perf script
  perf     801 [000]    18.031793:          1 cycles:P:
          ffffffff91a14c36 __intel_pmu_enable_all.isra.0+0x56 ([kernel.kallsyms])
          ffffffff91d373e9 perf_ctx_enable+0x39 ([kernel.kallsyms])
          ffffffff91d36af7 event_function+0xd7 ([kernel.kallsyms])
          ffffffff91d34222 remote_function+0x42 ([kernel.kallsyms])
          ffffffff91c1ebe1 generic_exec_single+0x61 ([kernel.kallsyms])
          ffffffff91c1edac smp_call_function_single+0xec ([kernel.kallsyms])
          ffffffff91d37a9d event_function_call+0x10d ([kernel.kallsyms])
          ffffffff91d33557 perf_event_for_each_child+0x37 ([kernel.kallsyms])
          ffffffff91d47324 _perf_ioctl+0x204 ([kernel.kallsyms])
          ffffffff91d47c43 perf_ioctl+0x33 ([kernel.kallsyms])
          ffffffff91e2f216 __x64_sys_ioctl+0x96 ([kernel.kallsyms])
          ffffffff9265f1ae do_syscall_64+0x9e ([kernel.kallsyms])
          ffffffff92800130 entry_SYSCALL_64+0xb0 ([kernel.kallsyms])
              7fb5fc22034b __GI___ioctl+0x3b (/usr/lib/x86_64-linux-gnu/libc.so.6)
  ...

The old output can be get using --no-merge-callchain option.
Also perf report can get the user callchain entry at the end.

  $ perf report --no-children --percent-limit=0 --stdio -q -S __intel_pmu_enable_all.isra.0
  # symbol: __intel_pmu_enable_all.isra.0
       0.00%  perf     [kernel.kallsyms]
              |
              ---__intel_pmu_enable_all.isra.0
                 perf_ctx_enable
                 event_function
                 remote_function
                 generic_exec_single
                 smp_call_function_single
                 event_function_call
                 perf_event_for_each_child
                 _perf_ioctl
                 perf_ioctl
                 __x64_sys_ioctl
                 do_syscall_64
                 entry_SYSCALL_64
                 __GI___ioctl

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
---
 tools/perf/Documentation/perf-script.txt |  5 ++
 tools/perf/builtin-script.c              |  5 +-
 tools/perf/util/callchain.c              | 24 +++++++++
 tools/perf/util/callchain.h              |  3 ++
 tools/perf/util/evlist.c                 |  1 +
 tools/perf/util/evlist.h                 |  1 +
 tools/perf/util/session.c                | 63 +++++++++++++++++++++++-
 tools/perf/util/tool.c                   |  1 +
 tools/perf/util/tool.h                   |  1 +
 9 files changed, 102 insertions(+), 2 deletions(-)

diff --git a/tools/perf/Documentation/perf-script.txt b/tools/perf/Documentation/perf-script.txt
index 28bec7e78bc8..03d112960632 100644
--- a/tools/perf/Documentation/perf-script.txt
+++ b/tools/perf/Documentation/perf-script.txt
@@ -527,6 +527,11 @@ include::itrace.txt[]
 	The known limitations include exception handing such as
 	setjmp/longjmp will have calls/returns not match.
 
+--merge-callchains::
+	Enable merging deferred user callchains if available.  This is the
+	default behavior.  If you want to see separate CALLCHAIN_DEFERRED
+	records for some reason, use --no-merge-callchains explicitly.
+
 :GMEXAMPLECMD: script
 :GMEXAMPLESUBCMD:
 include::guest-files.txt[]
diff --git a/tools/perf/builtin-script.c b/tools/perf/builtin-script.c
index a6f8209256fe..b50442cca540 100644
--- a/tools/perf/builtin-script.c
+++ b/tools/perf/builtin-script.c
@@ -3775,6 +3775,7 @@ int cmd_script(int argc, const char **argv)
 	bool header_only = false;
 	bool script_started = false;
 	bool unsorted_dump = false;
+	bool merge_deferred_callchains = true;
 	char *rec_script_path = NULL;
 	char *rep_script_path = NULL;
 	struct perf_session *session;
@@ -3928,6 +3929,8 @@ int cmd_script(int argc, const char **argv)
 		    "Guest code can be found in hypervisor process"),
 	OPT_BOOLEAN('\0', "stitch-lbr", &script.stitch_lbr,
 		    "Enable LBR callgraph stitching approach"),
+	OPT_BOOLEAN('\0', "merge-callchains", &merge_deferred_callchains,
+		    "Enable merge deferred user callchains"),
 	OPTS_EVSWITCH(&script.evswitch),
 	OPT_END()
 	};
@@ -4183,7 +4186,7 @@ int cmd_script(int argc, const char **argv)
 	script.tool.throttle		 = process_throttle_event;
 	script.tool.unthrottle		 = process_throttle_event;
 	script.tool.ordering_requires_timestamps = true;
-	script.tool.merge_deferred_callchains = false;
+	script.tool.merge_deferred_callchains = merge_deferred_callchains;
 	session = perf_session__new(&data, &script.tool);
 	if (IS_ERR(session))
 		return PTR_ERR(session);
diff --git a/tools/perf/util/callchain.c b/tools/perf/util/callchain.c
index d7b7eef740b9..6d423d92861b 100644
--- a/tools/perf/util/callchain.c
+++ b/tools/perf/util/callchain.c
@@ -1828,3 +1828,27 @@ int sample__for_each_callchain_node(struct thread *thread, struct evsel *evsel,
 	}
 	return 0;
 }
+
+int sample__merge_deferred_callchain(struct perf_sample *sample_orig,
+				     struct perf_sample *sample_callchain)
+{
+	u64 nr_orig = sample_orig->callchain->nr - 1;
+	u64 nr_deferred = sample_callchain->callchain->nr;
+	struct ip_callchain *callchain;
+
+	callchain = calloc(1 + nr_orig + nr_deferred, sizeof(u64));
+	if (callchain == NULL) {
+		sample_orig->deferred_callchain = false;
+		return -ENOMEM;
+	}
+
+	callchain->nr = nr_orig + nr_deferred;
+	/* copy except for the last PERF_CONTEXT_USER_DEFERRED */
+	memcpy(callchain->ips, sample_orig->callchain->ips, nr_orig * sizeof(u64));
+	/* copy deferred use callchains */
+	memcpy(&callchain->ips[nr_orig], sample_callchain->callchain->ips,
+	       nr_deferred * sizeof(u64));
+
+	sample_orig->callchain = callchain;
+	return 0;
+}
diff --git a/tools/perf/util/callchain.h b/tools/perf/util/callchain.h
index 86ed9e4d04f9..89785125ed25 100644
--- a/tools/perf/util/callchain.h
+++ b/tools/perf/util/callchain.h
@@ -317,4 +317,7 @@ int sample__for_each_callchain_node(struct thread *thread, struct evsel *evsel,
 				    struct perf_sample *sample, int max_stack,
 				    bool symbols, callchain_iter_fn cb, void *data);
 
+int sample__merge_deferred_callchain(struct perf_sample *sample_orig,
+				     struct perf_sample *sample_callchain);
+
 #endif	/* __PERF_CALLCHAIN_H */
diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
index dcd1130502df..1d5f50f83b31 100644
--- a/tools/perf/util/evlist.c
+++ b/tools/perf/util/evlist.c
@@ -83,6 +83,7 @@ void evlist__init(struct evlist *evlist, struct perf_cpu_map *cpus,
 	evlist->ctl_fd.ack = -1;
 	evlist->ctl_fd.pos = -1;
 	evlist->nr_br_cntr = -1;
+	INIT_LIST_HEAD(&evlist->deferred_samples);
 }
 
 struct evlist *evlist__new(void)
diff --git a/tools/perf/util/evlist.h b/tools/perf/util/evlist.h
index 85859708393e..d2895c8be167 100644
--- a/tools/perf/util/evlist.h
+++ b/tools/perf/util/evlist.h
@@ -86,6 +86,7 @@ struct evlist {
 		int	pos;	/* index at evlist core object to check signals */
 	} ctl_fd;
 	struct event_enable_timer *eet;
+	struct list_head deferred_samples;
 };
 
 struct evsel_str_handler {
diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
index e6be01459352..21b0958e8229 100644
--- a/tools/perf/util/session.c
+++ b/tools/perf/util/session.c
@@ -1277,6 +1277,56 @@ static int evlist__deliver_sample(struct evlist *evlist, const struct perf_tool
 					    per_thread);
 }
 
+struct deferred_event {
+	struct list_head list;
+	union perf_event *event;
+};
+
+static int evlist__deliver_deferred_samples(struct evlist *evlist,
+					    const struct perf_tool *tool,
+					    union  perf_event *event,
+					    struct perf_sample *sample,
+					    struct machine *machine)
+{
+	struct deferred_event *de, *tmp;
+	struct evsel *evsel;
+	int ret = 0;
+
+	if (!tool->merge_deferred_callchains) {
+		evsel = evlist__id2evsel(evlist, sample->id);
+		return tool->callchain_deferred(tool, event, sample,
+						evsel, machine);
+	}
+
+	list_for_each_entry_safe(de, tmp, &evlist->deferred_samples, list) {
+		struct perf_sample orig_sample;
+
+		ret = evlist__parse_sample(evlist, de->event, &orig_sample);
+		if (ret < 0) {
+			pr_err("failed to parse original sample\n");
+			break;
+		}
+
+		if (sample->tid != orig_sample.tid)
+			continue;
+
+		evsel = evlist__id2evsel(evlist, orig_sample.id);
+		sample__merge_deferred_callchain(&orig_sample, sample);
+		ret = evlist__deliver_sample(evlist, tool, de->event,
+					     &orig_sample, evsel, machine);
+
+		if (orig_sample.deferred_callchain)
+			free(orig_sample.callchain);
+
+		list_del(&de->list);
+		free(de);
+
+		if (ret)
+			break;
+	}
+	return ret;
+}
+
 static int machines__deliver_event(struct machines *machines,
 				   struct evlist *evlist,
 				   union perf_event *event,
@@ -1305,6 +1355,16 @@ static int machines__deliver_event(struct machines *machines,
 			return 0;
 		}
 		dump_sample(evsel, event, sample, perf_env__arch(machine->env));
+		if (sample->deferred_callchain && tool->merge_deferred_callchains) {
+			struct deferred_event *de = malloc(sizeof(*de));
+
+			if (de == NULL)
+				return -ENOMEM;
+
+			de->event = event;
+			list_add_tail(&de->list, &evlist->deferred_samples);
+			return 0;
+		}
 		return evlist__deliver_sample(evlist, tool, event, sample, evsel, machine);
 	case PERF_RECORD_MMAP:
 		return tool->mmap(tool, event, sample, machine);
@@ -1364,7 +1424,8 @@ static int machines__deliver_event(struct machines *machines,
 		return tool->aux_output_hw_id(tool, event, sample, machine);
 	case PERF_RECORD_CALLCHAIN_DEFERRED:
 		dump_deferred_callchain(evsel, event, sample);
-		return tool->callchain_deferred(tool, event, sample, evsel, machine);
+		return evlist__deliver_deferred_samples(evlist, tool, event,
+							sample, machine);
 	default:
 		++evlist->stats.nr_unknown_events;
 		return -1;
diff --git a/tools/perf/util/tool.c b/tools/perf/util/tool.c
index e1d60abb4e41..61fdba70b0a4 100644
--- a/tools/perf/util/tool.c
+++ b/tools/perf/util/tool.c
@@ -245,6 +245,7 @@ void perf_tool__init(struct perf_tool *tool, bool ordered_events)
 	tool->cgroup_events = false;
 	tool->no_warn = false;
 	tool->show_feat_hdr = SHOW_FEAT_NO_HEADER;
+	tool->merge_deferred_callchains = true;
 
 	tool->sample = process_event_sample_stub;
 	tool->mmap = process_event_stub;
diff --git a/tools/perf/util/tool.h b/tools/perf/util/tool.h
index 9987bbde6d5e..d06580478ab1 100644
--- a/tools/perf/util/tool.h
+++ b/tools/perf/util/tool.h
@@ -87,6 +87,7 @@ struct perf_tool {
 	bool		cgroup_events;
 	bool		no_warn;
 	bool		dont_split_sample_group;
+	bool		merge_deferred_callchains;
 	enum show_feature_header show_feat_hdr;
 };
 
-- 
2.47.2

Re: [PATCH v12 00/11] perf: Support the deferred unwinding infrastructure

[Steven Rostedt]

On Tue, 01 Jul 2025 14:04:10 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> Josh Poimboeuf (5):
>       perf: Remove get_perf_callchain() init_nr argument
>       perf: Have get_perf_callchain() return NULL if crosstask and user are set
>       perf: Simplify get_perf_callchain() user logic
>       perf: Skip user unwind if the task is a kernel thread
>       perf: Support deferred user callchains
> 
> Namhyung Kim (4):
>       perf tools: Minimal CALLCHAIN_DEFERRED support
>       perf record: Enable defer_callchain for user callchains
>       perf script: Display PERF_RECORD_CALLCHAIN_DEFERRED
>       perf tools: Merge deferred user callchains
> 
> Steven Rostedt (2):
>       perf: Use current->flags & PF_KTHREAD|PF_USER_WORKER instead of current->mm == NULL
>       perf: Support deferred user callchains for per CPU events

Hmm, patches 5-11 seemed to be dropped, and I sent out 12 patches of
the latest sframe work that I can't find anywhere. I think my ISP is
thinking I'm spamming so it dropped the emails. That's all I can figure.

I originally used kernel.org as my smtp server, but gmail starting
putting my emails into spam because kernel.org can't validate
goodmis.org with DKIM (my DNS has kernel.org set for SPF, but that's
not enough these days :-p).

So I went back to my ISP (hover.com) as my SMTP server, but now I think
it doesn't like my scripts that sends out a series of patches to a lot
of people. It likely thinks I'm a spam bot (although maybe I am!).

I guess I'll change my scripts to use rostedt@kernel.org to send and go
back to using kernel.org as the SMTP server for my scripts. :-/

Expect a resend. Sorry for all the noise.

-- Steve

Re: [PATCH v12 00/11] perf: Support the deferred unwinding infrastructure

[Steven Rostedt]

On Tue, 1 Jul 2025 15:17:15 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> Hmm, patches 5-11 seemed to be dropped, and I sent out 12 patches of
> the latest sframe work that I can't find anywhere. I think my ISP is
> thinking I'm spamming so it dropped the emails. That's all I can figure.

Hah! After I write this, my patches start showing up! :-p

OK, no resend.

-- Steve