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SubjectRe: [PATCH 3/4] workqueue: Tag bound workers with KTHREAD_IS_PER_CPU
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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);

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