linux-kernel - Re: [PATCH 3/3] sched: Use cpu_dying() to fix balance

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Date:   Tue, 20 Apr 2021 16:39:04 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Donnefort <vincent.donnefort@....com>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        tglx@...utronix.de, mingo@...nel.org, bigeasy@...utronix.de,
        swood@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, qais.yousef@....com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] sched: Use cpu_dying() to fix balance_push vs
 hotplug-rollback

On Tue, Apr 20, 2021 at 04:20:56PM +0200, Peter Zijlstra wrote:
> On Tue, Apr 20, 2021 at 10:46:33AM +0100, Vincent Donnefort wrote:
> 
> > Found the issue:
> > 
> > $ cat hotplug/states:
> > 219: sched:active
> > 220: online
> > 
> > CPU0: 
> > 
> > $ echo 219 > hotplug/fail
> > $ echo 0 > online
> > 
> > => cpu_active = 1 cpu_dying = 1
> > 
> > which means that later on, for another CPU hotunplug, in
> > __balance_push_cpu_stop(), the fallback rq for a kthread can select that
> > CPU0, but __migrate_task() would fail and we end-up in an infinite loop,
> > trying to migrate that task to CPU0.
> > 
> > The problem is that for a failure in sched:active, as "online" has no callback,
> > there will be no call to cpuhp_invoke_callback(). Hence, the cpu_dying bit would
> > not be reset.
> 
> Urgh! Good find.
> 
> > Maybe cpuhp_reset_state() and cpuhp_set_state() would then be a better place to
> > switch the dying bit?
> 
> Yes, except now cpuhp_invoke_ap_callback() makes my head hurt, that runs
> the callbacks out of order. I _think_ we can ignore it, but ....
> 
> Something like the below, let me see if I can reproduce and test.

I seem to have triggered the BUG() in select_fallback_rq() with your recipie.
Have cpu0 fail on sched:active, then offline all other CPUs.

Now lemme add that patch.