linux-kernel - Re: [tip: sched/core] sched/tracing: Don't re-read p->state when emitting sched

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Date:   Fri, 04 Mar 2022 16:13:04 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     tip-bot2 for Valentin Schneider <tip-bot2@...utronix.de>,
        linux-tip-commits@...r.kernel.org
Cc:     Abhijeet Dharmapurikar <adharmap@...cinc.com>,
        "Peter Zijlstra \(Intel\)" <peterz@...radead.org>,
        "Steven Rostedt \(Google\)" <rostedt@...dmis.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [tip: sched/core] sched/tracing: Don't re-read p->state when emitting sched_switch event

On 01/03/22 15:24, tip-bot2 for Valentin Schneider wrote:
> The following commit has been merged into the sched/core branch of tip:
>
> Commit-ID:     fa2c3254d7cfff5f7a916ab928a562d1165f17bb
> Gitweb:        https://git.kernel.org/tip/fa2c3254d7cfff5f7a916ab928a562d1165f17bb
> Author:        Valentin Schneider <valentin.schneider@....com>
> AuthorDate:    Thu, 20 Jan 2022 16:25:19
> Committer:     Peter Zijlstra <peterz@...radead.org>
> CommitterDate: Tue, 01 Mar 2022 16:18:39 +01:00
>
> sched/tracing: Don't re-read p->state when emitting sched_switch event
>
> 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>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Reviewed-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> Link: https://lore.kernel.org/r/20220120162520.570782-2-valentin.schneider@arm.com

So I wasn't sure if that would be considered a "bug", but folks are asking
for this to be backported; do we want to slap

Fixes: c6e7bd7afaeb ("sched/core: Optimize ttwu() spinning on p->on_cpu")

to it?