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

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Date:   Tue, 8 Mar 2022 18:51:38 +0000
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 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")

Not sure about the 2nd patch, but I can try to figure it out.

> Once it hits Linus's tree, let stable@...r.kernel.org know what the git
> commit id is in Linus's tree.

Sure. My bad if I rushed the request. I just wanted to know whether it's okay
for this to go into stable. If no one shouts, I'll ping once it lands in
Linus'.


Thanks!

--
Qais Yousef