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Subject: Re: [PATCH -v2 1/5] sched: Fix ttwu() race
From:   Chris Wilson <chris@chris-wilson.co.uk>
Cc:     mingo@kernel.org, tglx@linutronix.de, linux-kernel@vger.kernel.org,
        juri.lelli@redhat.com, vincent.guittot@linaro.org,
        dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com,
        mgorman@suse.de, paulmck@kernel.org, frederic@kernel.org,
        torvalds@linux-foundation.org, hch@lst.de
To:     Peter Zijlstra <peterz@infradead.org>
Date:   Thu, 23 Jul 2020 20:41:03 +0100
Message-ID: <159553326368.21069.3167204000119437062@build.alporthouse.com>
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Quoting Peter Zijlstra (2020-07-23 19:28:41)
> On Wed, Jul 22, 2020 at 10:57:56AM +0100, Chris Wilson wrote:
> 
> > Perhaps more damning is that I can replace WF_ON_CPU with p->on_cpu to
> > suppress the warning:
> 
> *argh*, I'm starting to go mad...
> 
> Chris, could you please try the below patch?

 ttwu-IPI-self: 1==1, p->on_cpu=0;0, task_cpu(p)=1;1
 ttwu-IPI-self: 1==1, p->on_cpu=0;0, task_cpu(p)=1;1
 ttwu-IPI-self: 0==0, p->on_cpu=0;0, task_cpu(p)=0;0
 ttwu-IPI-self: 3==3, p->on_cpu=0;0, task_cpu(p)=3;3
 ttwu-IPI-self: 2==2, p->on_cpu=0;0, task_cpu(p)=2;2
 ttwu-IPI-self: 1==1, p->on_cpu=0;0, task_cpu(p)=1;1
 ttwu-IPI-self: 2==2, p->on_cpu=0;0, task_cpu(p)=2;2
 ttwu-IPI-self: 2==2, p->on_cpu=0;0, task_cpu(p)=2;2
 ttwu-IPI-self: 2==2, p->on_cpu=0;0, task_cpu(p)=2;2

> Can you also confirm that if you do:
> 
> $ echo NO_TTWU_QUEUE_ON_CPU > /debug/sched_features

With,

 sched_feat_disable(10):TTWU_QUEUE_ON_CPU

the pr_alert is still being hit

 ttwu-IPI-self: 3==3, p->on_cpu=0;0, task_cpu(p)=3;3

At which point, it darns on me. Mea culpa, stray bits being passed into
default_wake_function.

I am very sorry for the wild goose chase.
-Chris