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

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Date:   Mon, 11 Apr 2022 15:22:07 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Qais Yousef <qais.yousef@....com>
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 Sun, Apr 10, 2022 at 12:38:29AM +0100, 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.

It does not apply to 5.10 at all, how did you test this?

{sigh}

Again, if you want this applied to any stable trees, please test that it
works and send the properly backported patches.

thanks,

greg k-h