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

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Date:   Wed, 4 Jan 2017 15:12:36 +0000
From:   Will Deacon <will.deacon@....com>
To:     Joel Fernandes <joelaf@...gle.com>
Cc:     linux-kernel@...r.kernel.org, Todd Kjos <tkjos@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        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>,
        "moderated list:ARM64 PORT (AARCH64 ARCHITECTURE)" 
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] arm64: Don't trace __switch_to if function graph tracer
 is enabled

On Wed, Dec 21, 2016 at 02:44:46PM -0800, Joel Fernandes wrote:
> 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(-)

Thanks, queued for 4.11.

Will