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Date:   Fri, 11 Dec 2020 13:41:48 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Vincent Donnefort <vincent.donnefort@arm.com>
Cc:     Valentin Schneider <valentin.schneider@arm.com>,
        linux-kernel@vger.kernel.org, Qian Cai <cai@redhat.com>,
        tglx@linutronix.de, mingo@kernel.org, bigeasy@linutronix.de,
        qais.yousef@arm.com, 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, tj@kernel.org, ouwen210@hotmail.com
Subject: Re: [PATCH 2/2] workqueue: Fix affinity of kworkers attached during
 late hotplug
Message-ID: <20201211124148.GW2414@hirez.programming.kicks-ass.net>
References: <20201210163830.21514-1-valentin.schneider@arm.com>
 <20201210163830.21514-3-valentin.schneider@arm.com>
 <20201211113920.GA75974@e120877-lin.cambridge.arm.com>
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On Fri, Dec 11, 2020 at 11:39:21AM +0000, Vincent Donnefort wrote:
> Hi Valentin,
> 
> On Thu, Dec 10, 2020 at 04:38:30PM +0000, Valentin Schneider wrote:
> > Per-CPU kworkers forcefully migrated away by hotplug via
> > workqueue_offline_cpu() can end up spawning more kworkers via
> > 
> >   manage_workers() -> maybe_create_worker()
> > 
> > Workers created at this point will be bound using
> > 
> >   pool->attrs->cpumask
> > 
> > which in this case is wrong, as the hotplug state machine already migrated
> > all pinned kworkers away from this CPU. This ends up triggering the BUG_ON
> > condition is sched_cpu_dying() (i.e. there's a kworker enqueued on the
> > dying rq).
> > 
> > Special-case workers being attached to DISASSOCIATED pools and bind them to
> > cpu_active_mask, mimicking them being present when workqueue_offline_cpu()
> > was invoked.
> > 
> > Link: https://lore.kernel.org/r/ff62e3ee994efb3620177bf7b19fab16f4866845.camel@redhat.com
> > Fixes: 06249738a41a ("workqueue: Manually break affinity on hotplug")
> 
> Isn't the problem introduced by 1cf12e0 ("sched/hotplug: Consolidate
> task migration on CPU unplug") ?
> 
> Previously we had:
> 
>  AP_WORKQUEUE_ONLINE -> set POOL_DISASSOCIATED
>    ...
>  TEARDOWN_CPU -> clear CPU in cpu_online_mask
>    |
>    |-AP_SCHED_STARTING -> migrate_tasks()
>    |
>   AP_OFFLINE
> 
> worker_attach_to_pool(), is "protected" by the cpu_online_mask in
> set_cpus_allowed_ptr(). IIUC, now, the tasks being migrated before the
> cpu_online_mask is actually flipped, there's a window, between
> CPUHP_AP_SCHED_WAIT_EMPTY and CPUHP_TEARDOWN_CPU where a kworker can wake-up
> a new one, for the hotunplugged pool that wouldn't be caught by the
> hotunplug migration.

Yes, very much so, however the commit Valentin picked was supposed to
preemptively fix this. So we can consider this a fix for the fix.

But I don't mind an alternative or perhaps even second Fixes tag on
this.