linux-kernel - Re: [RFC PATCH 1/2] sched/tracing: Don't re-read p->state when emitting sched

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Message-ID: <20211208151203.2196088c@gandalf.local.home>
Date:   Wed, 8 Dec 2021 15:12:03 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel@...r.kernel.org,
        Abhijeet Dharmapurikar <adharmap@...cinc.com>,
        Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Juri Lelli <juri.lelli@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 1/2] sched/tracing: Don't re-read p->state when
 emitting sched_switch event

On Mon, 29 Nov 2021 12:36:00 +0000
Valentin Schneider <valentin.schneider@....com> wrote:

> As of commit
> 
>   c6e7bd7afaeb ("sched/core: Optimize ttwu() spinning on p->on_cpu")
> 
> the following sequence becomes possible:
> 
> 		      p->__state = TASK_INTERRUPTIBLE;
> 		      __schedule()
> 			deactivate_task(p);
>   ttwu()
>     READ !p->on_rq
>     p->__state=TASK_WAKING
> 			trace_sched_switch()
> 			  __trace_sched_switch_state()
> 			    task_state_index()
> 			      return 0;
> 
> TASK_WAKING isn't in TASK_REPORT, so the task appears as TASK_RUNNING in
> the trace event.
> 
> Prevent this by pushing the value read from __schedule() down the trace
> event.
> 
> Reported-by: Abhijeet Dharmapurikar <adharmap@...cinc.com>
> Signed-off-by: Valentin Schneider <valentin.schneider@....com>
> ---
>  include/linux/sched.h             | 11 ++++++++---
>  include/trace/events/sched.h      | 11 +++++++----
>  kernel/sched/core.c               |  4 ++--
>  kernel/trace/fgraph.c             |  4 +++-
>  kernel/trace/ftrace.c             |  4 +++-
>  kernel/trace/trace_events.c       |  8 ++++++--
>  kernel/trace/trace_sched_switch.c |  1 +

I believe you may have missed some functions that register the sched_switch
event. Do a git grep on register_trace_sched_switch.

-- Steve


>  7 files changed, 30 insertions(+), 13 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index d2e261adb8ea..d00837d12b9d 100644