Re: [PATCH 3/3] sched: Use cpu_dying() to fix balance_push vs hotplug-rollback

From: Peter Zijlstra <peterz@infradead.org>
To: Valentin Schneider <valentin.schneider@arm.com>
Cc: tglx@linutronix.de, mingo@kernel.org, bigeasy@linutronix.de,
	swood@redhat.com, juri.lelli@redhat.com,
	vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
	rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
	bristot@redhat.com, vincent.donnefort@arm.com,
	qais.yousef@arm.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/3] sched: Use cpu_dying() to fix balance_push vs hotplug-rollback
Date: Tue, 13 Apr 2021 08:51:23 +0200	[thread overview]
Message-ID: <YHU/a9HvGLYpOLKZ@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <87r1jfmn8d.mognet@arm.com>

On Mon, Apr 12, 2021 at 06:22:42PM +0100, Valentin Schneider wrote:
> On 12/04/21 14:03, Peter Zijlstra wrote:
> > On Thu, Mar 11, 2021 at 03:13:04PM +0000, Valentin Schneider wrote:
> >> Peter Zijlstra <peterz@infradead.org> writes:
> >> > @@ -7910,6 +7908,14 @@ int sched_cpu_deactivate(unsigned int cp
> >> >    }
> >> >    rq_unlock_irqrestore(rq, &rf);
> >> >
> >> > +	/*
> >> > +	 * From this point forward, this CPU will refuse to run any task that
> >> > +	 * is not: migrate_disable() or KTHREAD_IS_PER_CPU, and will actively
> >> > +	 * push those tasks away until this gets cleared, see
> >> > +	 * sched_cpu_dying().
> >> > +	 */
> >> > +	balance_push_set(cpu, true);
> >> > +
> >>
> >> AIUI with cpu_dying_mask being flipped before even entering
> >> sched_cpu_deactivate(), we don't need this to be before the
> >> synchronize_rcu() anymore; is there more than that to why you're punting it
> >> back this side of it?
> >
> > I think it does does need to be like this, we need to clearly separate
> > the active=true and balance_push_set(). If we were to somehow observe
> > both balance_push_set() and active==false, we'd be in trouble.
> >
> 
> I'm afraid I don't follow; we're replacing a read of rq->balance_push with
> cpu_dying(), and those are still written on the same side of the
> synchronize_rcu(). What am I missing?

Yeah, I'm not sure anymnore either; I tried to work out why I'd done
that but upon closer examination everything fell flat.

Let me try again today :-)

> Oooh, I can't read, only the boot CPU gets its callback uninstalled in
> sched_init()! So secondaries keep push_callback installed up until
> sched_cpu_activate(), but as you said it's not effective unless a rollback
> happens.
> 
> Now, doesn't that mean we should *not* uninstall the callback in
> sched_cpu_dying()? AFAIK it's possible for the initial secondary CPU
> boot to go fine, but the next offline+online cycle fails while going up -
> that would need to rollback with push_callback installed.

Quite; I removed that shortly after sending this; when I tried to write
a comment and found it.