linux-kernel - Re: [PATCH 3/4] workqueue: Tag bound workers with KTHREAD_IS_PER

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Date:   Wed, 13 Jan 2021 14:16:10 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Lai Jiangshan <jiangshanlai@...il.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, Qian Cai <cai@...hat.com>,
        Vincent Donnefort <vincent.donnefort@....com>,
        Dexuan Cui <decui@...rosoft.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Steven Rostedt <rostedt@...dmis.org>, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 3/4] workqueue: Tag bound workers with KTHREAD_IS_PER_CPU

On 13/01/21 21:28, Lai Jiangshan wrote:
> On Tue, Jan 12, 2021 at 10:51 PM Peter Zijlstra <peterz@...radead.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);
>>
>>
>>