linux-kernel - [PATCH 1/2] ftrace: Fix preemption acounting for stacktrace trigger command

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Message-Id: <20250512094246.1167956-1-dolinux.peng@gmail.com>
Date: Mon, 12 May 2025 17:42:45 +0800
From: pengdonglin <dolinux.peng@...il.com>
To: rostedt@...dmis.org
Cc: mhiramat@...nel.org,
	dolinux.peng@...i.com,
	linux-trace-kernel@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	pengdonglin <dolinux.peng@...il.com>
Subject: [PATCH 1/2] ftrace: Fix preemption acounting for stacktrace trigger command

From: pengdonglin <pengdonglin@...omi.com>

While using the stacktrace trigger command to trace syscalls/sys_enter_read,
I noticed that the preemption count was consistently reported as 1, which seemed
incorrect:

root@...ntu22-vm:/sys/kernel/tracing/events/syscalls/sys_enter_read
$ echo stacktrace > trigger
$ echo 1 > enable

    sshd-416     [002] .....   232.864910: sys_read(fd: a, buf: 556b1f3221d0, count: 8000)
    sshd-416     [002] ...1.   232.864913: <stack trace>
 => ftrace_syscall_enter
 => syscall_trace_enter
 => do_syscall_64
 => entry_SYSCALL_64_after_hwframe

The root cause is that the trace framework disables preemption in __DO_TRACE before
invoking the trigger callback.

This patch uses tracing_gen_ctx_dec() to obtain the correct preemption
count within the callback, resulting in accurate reporting:

    sshd-410     [004] .....   210.117660: sys_read(fd: 4, buf: 559b725ba130, count: 40000)
    sshd-410     [004] .....   210.117662: <stack trace>
 => ftrace_syscall_enter
 => syscall_trace_enter
 => do_syscall_64
 => entry_SYSCALL_64_after_hwframe

Signed-off-by: pengdonglin <dolinux.peng@...il.com>
---
 kernel/trace/trace_events_trigger.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/trace_events_trigger.c b/kernel/trace/trace_events_trigger.c
index b66b6d235d91..6e87ae2a1a66 100644
--- a/kernel/trace/trace_events_trigger.c
+++ b/kernel/trace/trace_events_trigger.c
@@ -1560,7 +1560,7 @@ stacktrace_trigger(struct event_trigger_data *data,
 	struct trace_event_file *file = data->private_data;
 
 	if (file)
-		__trace_stack(file->tr, tracing_gen_ctx(), STACK_SKIP);
+		__trace_stack(file->tr, tracing_gen_ctx_dec(), STACK_SKIP);
 	else
 		trace_dump_stack(STACK_SKIP);
 }
-- 
2.25.1