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

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Message-Id: <20250512094246.1167956-2-dolinux.peng@gmail.com>
Date: Mon, 12 May 2025 17:42:46 +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 2/2] ftrace: Fix preemption acounting for stacktrace filter command

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

While using the stacktrace filter command to trace ksys_read, I noticed
that the preemption count was consistently reported as 1, which seemed
incorrect:

$ echo ksys_read:stacktrace > set_ftrace_filter

   <...>-453     [004] ...1.    38.308956: <stack trace>
=> ksys_read
=> do_syscall_64
=> entry_SYSCALL_64_after_hwframe

The root cause is that the trace framework disables preemption when
invoking the filter command callback in function_trace_probe_call:

   preempt_disable_notrace();
   probe_ops->func(ip, parent_ip, probe_opsbe->tr, probe_ops, probe->data);
   preempt_enable_notrace();

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

$ echo ksys_read:stacktrace > set_ftrace_filter

   <...>-410     [006] .....    31.420396: <stack trace>
=> ksys_read
=> do_syscall_64
=> entry_SYSCALL_64_after_hwframe

Signed-off-by: pengdonglin <dolinux.peng@...il.com>
---
 kernel/trace/trace_functions.c | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/kernel/trace/trace_functions.c b/kernel/trace/trace_functions.c
index 98ccf3f00c51..4e37a0f6aaa3 100644
--- a/kernel/trace/trace_functions.c
+++ b/kernel/trace/trace_functions.c
@@ -633,11 +633,7 @@ ftrace_traceoff(unsigned long ip, unsigned long parent_ip,
 
 static __always_inline void trace_stack(struct trace_array *tr)
 {
-	unsigned int trace_ctx;
-
-	trace_ctx = tracing_gen_ctx();
-
-	__trace_stack(tr, trace_ctx, FTRACE_STACK_SKIP);
+	__trace_stack(tr, tracing_gen_ctx_dec(), FTRACE_STACK_SKIP);
 }
 
 static void
-- 
2.25.1