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

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Date:   Sun, 10 Apr 2022 23:06:08 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        Abhijeet Dharmapurikar <adharmap@...cinc.com>,
        Valentin Schneider <valentin.schneider@....com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        "Steven Rostedt (Google)" <rostedt@...dmis.org>, x86@...nel.org,
        stable@...r.kernel.org
Subject: Re: [tip: sched/core] sched/tracing: Don't re-read p->state when
 emitting sched_switch event

On 04/10/22 00:38, Qais Yousef wrote:
> On 03/08/22 18:51, Qais Yousef wrote:
> > On 03/08/22 19:10, Greg KH wrote:
> > > On Tue, Mar 08, 2022 at 06:02:40PM +0000, Qais Yousef wrote:
> > > > +CC stable
> > > > 
> > > > On 03/01/22 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
> > > > 
> > > > Any objection to picking this for stable? I'm interested in this one for some
> > > > Android users but prefer if it can be taken by stable rather than backport it
> > > > individually.
> > > > 
> > > > I think it makes sense to pick the next one in the series too.
> > > 
> > > What commit does this fix in Linus's tree?
> > 
> > It should be this one: c6e7bd7afaeb ("sched/core: Optimize ttwu() spinning on p->on_cpu")
> 
> Should this be okay to be picked up by stable now? I can see AUTOSEL has picked
> it up for v5.15+, but it impacts v5.10 too.

commit: fa2c3254d7cfff5f7a916ab928a562d1165f17bb
subject: sched/tracing: Don't re-read p->state when emitting sched_switch event

This patch has an impact on Android 5.10 users who experience tooling breakage.
Is it possible to include in 5.10 LTS please?

It was already picked up for 5.15+ by AUTOSEL and only 5.10 is missing.

Thanks

--
Qais Yousef