linux-kernel - [PATCH] arm64: Don't trace __switch_to if function graph tracer is enabled

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Date:   Wed, 21 Dec 2016 14:44:46 -0800
From:   Joel Fernandes <joelaf@...gle.com>
To:     linux-kernel@...r.kernel.org
Cc:     Joel Fernandes <joelaf@...gle.com>, Todd Kjos <tkjos@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Jason Cooper <jason@...edaemon.net>,
        Jiri Slaby <jslaby@...e.cz>,
        linux-arm-kernel@...ts.infradead.org (moderated list:ARM64 PORT
        (AARCH64 ARCHITECTURE))
Subject: [PATCH] arm64: Don't trace __switch_to if function graph tracer is enabled

Function graph tracer shows negative time (wrap around) when tracing
__switch_to if the nosleep-time trace option is enabled.

Time compensation for nosleep-time is done by an ftrace probe on
sched_switch. This doesn't work well for the following events (with
letters representing timestamps):
A - sched switch probe called for task T switch out
B - __switch_to calltime is recorded
C - sched_switch probe called for task T switch in
D - __switch_to rettime is recorded

If C - A > D - B, then we end up over compensating for the time spent in
__switch_to giving rise to negative times in the trace output.

On x86, __switch_to is not traced if function graph tracer is enabled.
Do the same for arm64 as well.

Cc: Todd Kjos <tkjos@...gle.com>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Will Deacon <will.deacon@....com>
Cc: Mark Rutland <mark.rutland@....com>
Signed-off-by: Joel Fernandes <joelaf@...gle.com>
---
 arch/arm64/kernel/process.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 01753cd..e84ee27 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -324,7 +324,7 @@ void uao_thread_switch(struct task_struct *next)
 /*
  * Thread switching.
  */
-struct task_struct *__switch_to(struct task_struct *prev,
+__notrace_funcgraph struct task_struct *__switch_to(struct task_struct *prev,
 				struct task_struct *next)
 {
 	struct task_struct *last;
-- 
2.8.0.rc3.226.g39d4020