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
next prev parent reply other threads:[~2021-01-13 17:53 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-01-12 14:43 [PATCH 0/4] sched: Fix hot-unplug regressions Peter Zijlstra
2021-01-12 14:43 ` [PATCH 2/4] kthread: Extract KTHREAD_IS_PER_CPU Peter Zijlstra
2021-01-12 14:43 ` [PATCH 3/4] workqueue: Tag bound workers with KTHREAD_IS_PER_CPU Peter Zijlstra
2021-01-12 16:36 ` Lai Jiangshan
2021-01-13 11:43 ` Peter Zijlstra
2021-01-12 17:57 ` Valentin Schneider
2021-01-13 13:28 ` Lai Jiangshan
2021-01-13 14:16 ` Valentin Schneider
2021-01-13 17:52 ` Paul E. McKenney [this message]
2021-01-13 18:43 ` Valentin Schneider
2021-01-13 18:59 ` Paul E. McKenney
2021-01-14 13:12 ` Peter Zijlstra
2021-01-14 13:21 ` Valentin Schneider
2021-01-14 15:34 ` Peter Zijlstra
2021-01-16 6:27 ` Lai Jiangshan
2021-01-16 12:45 ` Peter Zijlstra
2021-01-16 14:45 ` Lai Jiangshan
2021-01-16 15:16 ` Peter Zijlstra
2021-01-16 16:14 ` Lai Jiangshan
2021-01-16 18:46 ` Peter Zijlstra
2021-01-17 9:54 ` Peter Zijlstra
2021-01-16 15:13 ` Peter Zijlstra
2021-01-12 14:43 ` [PATCH 4/4] sched: Fix CPU hotplug / tighten is_per_cpu_kthread() Peter Zijlstra
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