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From: Valentin Schneider <valentin.schneider@arm.com>
To: Lai Jiangshan <jiangshanlai@gmail.com>,
	Peter Zijlstra <peterz@infradead.org>
Cc: 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>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	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 14:16:10 +0000	[thread overview]
Message-ID: <jhjturkzzv9.mognet@arm.com> (raw)
In-Reply-To: <CAJhGHyD_xuSpYOp5A9PumWGsBA=DNqM0ge3_NgRkfro7fafGqA@mail.gmail.com>

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.

>> +       }
>>
>>         raw_spin_lock_irq(&pool->lock);
>>
>>
>>

  reply	other threads:[~2021-01-13 14:17 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 [this message]
2021-01-13 17:52       ` Paul E. McKenney
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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