Re: [PATCH 3/4] workqueue: Tag bound workers with KTHREAD_IS_PER_CPU

From: "Paul E. McKenney" <paulmck@kernel.org>
To: Valentin Schneider <valentin.schneider@arm.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	LKML <linux-kernel@vger.kernel.org>, Qian Cai <cai@redhat.com>,
	Vincent Donnefort <vincent.donnefort@arm.com>,
	Dexuan Cui <decui@microsoft.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Steven Rostedt <rostedt@goodmis.org>, Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH 3/4] workqueue: Tag bound workers with KTHREAD_IS_PER_CPU
Date: Wed, 13 Jan 2021 09:52:49 -0800	[thread overview]
Message-ID: <20210113175249.GA27312@paulmck-ThinkPad-P72> (raw)
In-Reply-To: <jhjturkzzv9.mognet@arm.com>

On Wed, Jan 13, 2021 at 02:16:10PM +0000, Valentin Schneider wrote:
> On 13/01/21 21:28, Lai Jiangshan wrote:
> > On Tue, Jan 12, 2021 at 10:51 PM Peter Zijlstra <peterz@infradead.org> wrote:
> >> @@ -4972,9 +4977,11 @@ static void rebind_workers(struct worker
> >>          * of all workers first and then clear UNBOUND.  As we're called
> >>          * from CPU_ONLINE, the following shouldn't fail.
> >>          */
> >> -       for_each_pool_worker(worker, pool)
> >> +       for_each_pool_worker(worker, pool) {
> >>                 WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task,
> >>                                                   pool->attrs->cpumask) < 0);
> >> +               kthread_set_per_cpu(worker->task, true);
> >
> > Will the schedule break affinity in the middle of these two lines due to
> > patch4 allowing it and result in Paul's reported splat.
> >
> 
> You might be right; at this point we would still have BALANCE_PUSH set,
> so something like the below could happen
> 
>   rebind_workers()
>     set_cpus_allowed_ptr()
>       affine_move_task()
>         task_running() => stop_one_cpu()
> 
>   ... // Stopper migrates the kworker here in the meantime
> 
>   switch_to(<pcpu kworker>) // Both cpuhp thread and kworker should be enqueued
>                             // here, so one or the other could be picked
>   balance_switch()
>     balance_push()
>     ^-- no KTHREAD_IS_PER_CPU !
> 
> This should however trigger the WARN_ON_ONCE() in kthread_set_per_cpu()
> *before* the one in process_one_work(), which I haven't seen in Paul's
> mails.

The 56 instances of one-hour SRCU-P scenarios hit the WARN_ON_ONCE()
in process_one_work() once, but there is no sign of a WARN_ON_ONCE()
from kthread_set_per_cpu().  But to your point, this does appear to be
a rather low-probability race condition, once per some tens of hours
of SRCU-P.

Is there a more focused check for the race condition above?

							Thanx, Paul