[PATCH bpf-next v6 0/2] Pass external callchain entry to get_perf_callchain

[PATCH bpf-next v6 0/2] Pass external callchain entry to get_perf_callchain

[Tao Chen]

Background
==========
Alexei noted we should use preempt_disable to protect get_perf_callchain
in bpf stackmap.
https://lore.kernel.org/bpf/CAADnVQ+s8B7-fvR1TNO-bniSyKv57cH_ihRszmZV7pQDyV=VDQ@mail.gmail.com

A previous patch was submitted to attempt fixing this issue. And Andrii
suggested teach get_perf_callchain to let us pass that buffer directly to
avoid that unnecessary copy.
https://lore.kernel.org/bpf/20250926153952.1661146-1-chen.dylane@linux.dev

Proposed Solution
=================
Add external perf_callchain_entry parameter for get_perf_callchain to
allow us to use external buffer from BPF side. The biggest advantage is
that it can reduce unnecessary copies.

Todo
====
But I'm not sure if this modification is appropriate. After all, the
implementation of get_callchain_entry in the perf subsystem seems much more
complex than directly using an external buffer.

Comments and suggestions are always welcome.

Change list:
 - v1 -> v2
   From Jiri
   - rebase code, fix conflict
 - v1: https://lore.kernel.org/bpf/20251013174721.2681091-1-chen.dylane@linux.dev
 
 - v2 -> v3:
   From Andrii
   - entries per CPU used in a stack-like fashion
 - v2: https://lore.kernel.org/bpf/20251014100128.2721104-1-chen.dylane@linux.dev

 - v3 -> v4:
   From Peter
   - refactor get_perf_callchain and add three new APIs to use perf
     callchain easily.
   From Andrii
   - reuse the perf callchain management.

   - rename patch1 and patch2.
 - v3: https://lore.kernel.org/bpf/20251019170118.2955346-1-chen.dylane@linux.dev
 
 - v4 -> v5:
   From Yonghong
   - keep add_mark false in stackmap when refactor get_perf_callchain in
     patch1.
   - add atomic operation in get_recursion_context in patch2.
   - rename bpf_put_callchain_entry with bpf_put_perf_callchain in
     patch3.
   - rebase bpf-next master.
 - v4: https://lore.kernel.org/bpf/20251028162502.3418817-1-chen.dylane@linux.dev

 - v5 -> v6:
   From Peter
   - disable preemption from BPF side in patch2.
   From AI
   - use ctx->entry->nr instead of ctx->nr in patch1.
 - v5: https://lore.kernel.org/bpf/20251109163559.4102849-1-chen.dylane@linux.dev

Tao Chen (2):
  perf: Refactor get_perf_callchain
  bpf: Hold the perf callchain entry until used completely

 include/linux/perf_event.h |  9 +++++
 kernel/bpf/stackmap.c      | 67 +++++++++++++++++++++++++++-------
 kernel/events/callchain.c  | 73 ++++++++++++++++++++++++--------------
 3 files changed, 111 insertions(+), 38 deletions(-)

-- 
2.48.1

[PATCH bpf-next v6 1/2] perf: Refactor get_perf_callchain

[Tao Chen]

From BPF stack map, we want to ensure that the callchain buffer
will not be overwritten by other preemptive tasks. Peter
suggested provide more flexible stack-sampling APIs, which
can be used in BPF, and we can still use the perf callchain
entry with the help of these APIs. The next patch will modify
the BPF part.

In the future, these APIs will also make it convenient for us to
add stack-sampling kfuncs in the eBPF subsystem, just as Andrii and
Alexei discussed earlier.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Tao Chen <chen.dylane@linux.dev>
---
 include/linux/perf_event.h |  9 +++++
 kernel/events/callchain.c  | 73 ++++++++++++++++++++++++--------------
 2 files changed, 56 insertions(+), 26 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index fd1d91017b9..edd3058e4d8 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -67,6 +67,7 @@ struct perf_callchain_entry_ctx {
 	u32				nr;
 	short				contexts;
 	bool				contexts_maxed;
+	bool				add_mark;
 };
 
 typedef unsigned long (*perf_copy_f)(void *dst, const void *src,
@@ -1718,6 +1719,14 @@ 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 void __init_perf_callchain_ctx(struct perf_callchain_entry_ctx *ctx,
+				      struct perf_callchain_entry *entry,
+				      u32 max_stack, bool add_mark);
+
+extern void __get_perf_callchain_kernel(struct perf_callchain_entry_ctx *ctx, struct pt_regs *regs);
+extern void __get_perf_callchain_user(struct perf_callchain_entry_ctx *ctx, 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);
diff --git a/kernel/events/callchain.c b/kernel/events/callchain.c
index 808c0d7a31f..dfb7cdbd470 100644
--- a/kernel/events/callchain.c
+++ b/kernel/events/callchain.c
@@ -216,13 +216,54 @@ static void fixup_uretprobe_trampoline_entries(struct perf_callchain_entry *entr
 #endif
 }
 
+void __init_perf_callchain_ctx(struct perf_callchain_entry_ctx *ctx,
+			       struct perf_callchain_entry *entry,
+			       u32 max_stack, bool add_mark)
+
+{
+	ctx->entry		= entry;
+	ctx->max_stack		= max_stack;
+	ctx->nr			= entry->nr = 0;
+	ctx->contexts		= 0;
+	ctx->contexts_maxed	= false;
+	ctx->add_mark		= add_mark;
+}
+
+void __get_perf_callchain_kernel(struct perf_callchain_entry_ctx *ctx, struct pt_regs *regs)
+{
+	if (user_mode(regs))
+		return;
+
+	if (ctx->add_mark)
+		perf_callchain_store_context(ctx, PERF_CONTEXT_KERNEL);
+	perf_callchain_kernel(ctx, regs);
+}
+
+void __get_perf_callchain_user(struct perf_callchain_entry_ctx *ctx, struct pt_regs *regs)
+{
+	int start_entry_idx;
+
+	if (!user_mode(regs)) {
+		if (current->flags & (PF_KTHREAD | PF_USER_WORKER))
+			return;
+		regs = task_pt_regs(current);
+	}
+
+	if (ctx->add_mark)
+		perf_callchain_store_context(ctx, PERF_CONTEXT_USER);
+
+	start_entry_idx = ctx->entry->nr;
+	perf_callchain_user(ctx, regs);
+	fixup_uretprobe_trampoline_entries(ctx->entry, start_entry_idx);
+}
+
 struct perf_callchain_entry *
 get_perf_callchain(struct pt_regs *regs, bool kernel, bool user,
 		   u32 max_stack, bool crosstask, bool add_mark)
 {
 	struct perf_callchain_entry *entry;
 	struct perf_callchain_entry_ctx ctx;
-	int rctx, start_entry_idx;
+	int rctx;
 
 	/* crosstask is not supported for user stacks */
 	if (crosstask && user && !kernel)
@@ -232,34 +273,14 @@ get_perf_callchain(struct pt_regs *regs, bool kernel, bool user,
 	if (!entry)
 		return NULL;
 
-	ctx.entry		= entry;
-	ctx.max_stack		= max_stack;
-	ctx.nr			= entry->nr = 0;
-	ctx.contexts		= 0;
-	ctx.contexts_maxed	= false;
+	__init_perf_callchain_ctx(&ctx, entry, max_stack, add_mark);
 
-	if (kernel && !user_mode(regs)) {
-		if (add_mark)
-			perf_callchain_store_context(&ctx, PERF_CONTEXT_KERNEL);
-		perf_callchain_kernel(&ctx, regs);
-	}
-
-	if (user && !crosstask) {
-		if (!user_mode(regs)) {
-			if (current->flags & (PF_KTHREAD | PF_USER_WORKER))
-				goto exit_put;
-			regs = task_pt_regs(current);
-		}
+	if (kernel)
+		__get_perf_callchain_kernel(&ctx, regs);
 
-		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);
-	}
+	if (user && !crosstask)
+		__get_perf_callchain_user(&ctx, regs);
 
-exit_put:
 	put_callchain_entry(rctx);
 
 	return entry;
-- 
2.48.1

[PATCH bpf-next v6 2/2] bpf: Hold the perf callchain entry until used completely

[Tao Chen]

As Alexei noted, get_perf_callchain() return values may be reused
if a task is preempted after the BPF program enters migrate disable
mode. The perf_callchain_entres has a small stack of entries, and
we can reuse it as follows:

1. get the perf callchain entry
2. BPF use...
3. put the perf callchain entry

And Peter suggested that get_recursion_context used with preemption
disabled, so we should disable preemption at BPF side.

Signed-off-by: Tao Chen <chen.dylane@linux.dev>
---
 kernel/bpf/stackmap.c | 67 +++++++++++++++++++++++++++++++++++--------
 1 file changed, 55 insertions(+), 12 deletions(-)

diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index 2365541c81d..64ace4ed50e 100644
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -210,13 +210,14 @@ static void stack_map_get_build_id_offset(struct bpf_stack_build_id *id_offs,
 }
 
 static struct perf_callchain_entry *
-get_callchain_entry_for_task(struct task_struct *task, u32 max_depth)
+get_callchain_entry_for_task(int *rctx, struct task_struct *task, u32 max_depth)
 {
 #ifdef CONFIG_STACKTRACE
 	struct perf_callchain_entry *entry;
-	int rctx;
 
-	entry = get_callchain_entry(&rctx);
+	preempt_disable();
+	entry = get_callchain_entry(rctx);
+	preempt_enable();
 
 	if (!entry)
 		return NULL;
@@ -238,8 +239,6 @@ get_callchain_entry_for_task(struct task_struct *task, u32 max_depth)
 			to[i] = (u64)(from[i]);
 	}
 
-	put_callchain_entry(rctx);
-
 	return entry;
 #else /* CONFIG_STACKTRACE */
 	return NULL;
@@ -320,6 +319,34 @@ static long __bpf_get_stackid(struct bpf_map *map,
 	return id;
 }
 
+static struct perf_callchain_entry *
+bpf_get_perf_callchain(int *rctx, struct pt_regs *regs, bool kernel, bool user,
+		       int max_stack, bool crosstask)
+{
+	struct perf_callchain_entry_ctx ctx;
+	struct perf_callchain_entry *entry;
+
+	preempt_disable();
+	entry = get_callchain_entry(rctx);
+	preempt_enable();
+
+	if (unlikely(!entry))
+		return NULL;
+
+	__init_perf_callchain_ctx(&ctx, entry, max_stack, false);
+	if (kernel)
+		__get_perf_callchain_kernel(&ctx, regs);
+	if (user && !crosstask)
+		__get_perf_callchain_user(&ctx, regs);
+
+	return entry;
+}
+
+static void bpf_put_perf_callchain(int rctx)
+{
+	put_callchain_entry(rctx);
+}
+
 BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,
 	   u64, flags)
 {
@@ -328,20 +355,24 @@ BPF_CALL_3(bpf_get_stackid, struct pt_regs *, regs, struct bpf_map *, map,
 	struct perf_callchain_entry *trace;
 	bool kernel = !user;
 	u32 max_depth;
+	int rctx, ret;
 
 	if (unlikely(flags & ~(BPF_F_SKIP_FIELD_MASK | BPF_F_USER_STACK |
 			       BPF_F_FAST_STACK_CMP | BPF_F_REUSE_STACKID)))
 		return -EINVAL;
 
 	max_depth = stack_map_calculate_max_depth(map->value_size, elem_size, flags);
-	trace = get_perf_callchain(regs, kernel, user, max_depth,
-				   false, false);
+	trace = bpf_get_perf_callchain(&rctx, regs, kernel, user, max_depth,
+				       false);
 
 	if (unlikely(!trace))
 		/* couldn't fetch the stack trace */
 		return -EFAULT;
 
-	return __bpf_get_stackid(map, trace, flags);
+	ret = __bpf_get_stackid(map, trace, flags);
+	bpf_put_perf_callchain(rctx);
+
+	return ret;
 }
 
 const struct bpf_func_proto bpf_get_stackid_proto = {
@@ -435,6 +466,7 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
 	bool kernel = !user;
 	int err = -EINVAL;
 	u64 *ips;
+	int rctx;
 
 	if (unlikely(flags & ~(BPF_F_SKIP_FIELD_MASK | BPF_F_USER_STACK |
 			       BPF_F_USER_BUILD_ID)))
@@ -467,18 +499,26 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
 		trace = trace_in;
 		trace->nr = min_t(u32, trace->nr, max_depth);
 	} else if (kernel && task) {
-		trace = get_callchain_entry_for_task(task, max_depth);
+		trace = get_callchain_entry_for_task(&rctx, task, max_depth);
 	} else {
-		trace = get_perf_callchain(regs, kernel, user, max_depth,
-					   crosstask, false);
+		trace = bpf_get_perf_callchain(&rctx, regs, kernel, user, max_depth,
+					       crosstask);
 	}
 
-	if (unlikely(!trace) || trace->nr < skip) {
+	if (unlikely(!trace)) {
 		if (may_fault)
 			rcu_read_unlock();
 		goto err_fault;
 	}
 
+	if (trace->nr < skip) {
+		if (may_fault)
+			rcu_read_unlock();
+		if (!trace_in)
+			bpf_put_perf_callchain(rctx);
+		goto err_fault;
+	}
+
 	trace_nr = trace->nr - skip;
 	copy_len = trace_nr * elem_size;
 
@@ -497,6 +537,9 @@ static long __bpf_get_stack(struct pt_regs *regs, struct task_struct *task,
 	if (may_fault)
 		rcu_read_unlock();
 
+	if (!trace_in)
+		bpf_put_perf_callchain(rctx);
+
 	if (user_build_id)
 		stack_map_get_build_id_offset(buf, trace_nr, user, may_fault);
 
-- 
2.48.1

Re: [PATCH bpf-next v6 2/2] bpf: Hold the perf callchain entry until used completely

[Yonghong Song]

On 11/12/25 8:31 AM, Tao Chen wrote:
> As Alexei noted, get_perf_callchain() return values may be reused
> if a task is preempted after the BPF program enters migrate disable
> mode. The perf_callchain_entres has a small stack of entries, and
> we can reuse it as follows:
>
> 1. get the perf callchain entry
> 2. BPF use...
> 3. put the perf callchain entry
>
> And Peter suggested that get_recursion_context used with preemption
> disabled, so we should disable preemption at BPF side.
>
> Signed-off-by: Tao Chen <chen.dylane@linux.dev>

Acked-by: Yonghong Song <yonghong.song@linux.dev>

Re: [PATCH bpf-next v6 2/2] bpf: Hold the perf callchain entry until used completely

[Tao Chen]

在 2025/11/14 00:01, Yonghong Song 写道:
> 
> 
> On 11/12/25 8:31 AM, Tao Chen wrote:
>> As Alexei noted, get_perf_callchain() return values may be reused
>> if a task is preempted after the BPF program enters migrate disable
>> mode. The perf_callchain_entres has a small stack of entries, and
>> we can reuse it as follows:
>>
>> 1. get the perf callchain entry
>> 2. BPF use...
>> 3. put the perf callchain entry
>>
>> And Peter suggested that get_recursion_context used with preemption
>> disabled, so we should disable preemption at BPF side.
>>
>> Signed-off-by: Tao Chen <chen.dylane@linux.dev>
> 
> Acked-by: Yonghong Song <yonghong.song@linux.dev>
> 

Hi eBPF maintainers,

This patch appears to have missed the eBPF maintainers on the list, 
please review it again, thanks.

-- 
Best Regards
Tao Chen

Re: [PATCH bpf-next v6 2/2] bpf: Hold the perf callchain entry until used completely

[Tao Chen]

在 2025/11/26 17:29, Tao Chen 写道:
> 在 2025/11/14 00:01, Yonghong Song 写道:
>>
>>
>> On 11/12/25 8:31 AM, Tao Chen wrote:
>>> As Alexei noted, get_perf_callchain() return values may be reused
>>> if a task is preempted after the BPF program enters migrate disable
>>> mode. The perf_callchain_entres has a small stack of entries, and
>>> we can reuse it as follows:
>>>
>>> 1. get the perf callchain entry
>>> 2. BPF use...
>>> 3. put the perf callchain entry
>>>
>>> And Peter suggested that get_recursion_context used with preemption
>>> disabled, so we should disable preemption at BPF side.
>>>
>>> Signed-off-by: Tao Chen <chen.dylane@linux.dev>
>>
>> Acked-by: Yonghong Song <yonghong.song@linux.dev>
>>
> 
> Hi eBPF maintainers,
> 
> This patch appears to have missed the eBPF maintainers on the list, 
> please review it again, thanks.
> 

ping...

-- 
Best Regards
Tao Chen