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

[Valentin Schneider <valentin.schneider@arm.com>]

On 14/01/21 14:12, Peter Zijlstra wrote:
> On Wed, Jan 13, 2021 at 09:28:13PM +0800, 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.
>
> So something like the below _should_ work, except i'm seeing odd WARNs.
> I'll prod at it some more.
>
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -2371,6 +2371,7 @@ static int worker_thread(void *__worker)
>       /* tell the scheduler that this is a workqueue worker */
>       set_pf_worker(true);
>  woke_up:
> +	kthread_parkme();
>       raw_spin_lock_irq(&pool->lock);
>
>       /* am I supposed to die? */
> @@ -2428,6 +2429,7 @@ static int worker_thread(void *__worker)
>                       move_linked_works(work, &worker->scheduled, NULL);
>                       process_scheduled_works(worker);
>               }
> +		kthread_parkme();
>       } while (keep_working(pool));
>
>       worker_set_flags(worker, WORKER_PREP);
> @@ -4978,9 +4980,9 @@ static void rebind_workers(struct worker
>        * from CPU_ONLINE, the following shouldn't fail.
>        */
>       for_each_pool_worker(worker, pool) {
> -		WARN_ON_ONCE(set_cpus_allowed_ptr(worker->task,
> -						  pool->attrs->cpumask) < 0);
> +		kthread_park(worker->task);

Don't we still need an affinity change here, to undo what was done in
unbind_workers()?

Would something like

  __kthread_bind_mask(worker->task, pool->attrs->cpumask, TASK_PARKED)

even work?

>               kthread_set_per_cpu(worker->task, true);
> +		kthread_unpark(worker->task);
>       }
>
>       raw_spin_lock_irq(&pool->lock);