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

[Peter Zijlstra <peterz@infradead.org>]

On Sat, Jan 16, 2021 at 02:27:09PM +0800, Lai Jiangshan wrote:
> On Thu, Jan 14, 2021 at 11:35 PM Peter Zijlstra <peterz@infradead.org> wrote:
> 
> >
> > -void kthread_set_per_cpu(struct task_struct *k, bool set)
> > +void kthread_set_per_cpu(struct task_struct *k, int cpu)
> >  {
> >         struct kthread *kthread = to_kthread(k);
> >         if (!kthread)
> >                 return;
> >
> > -       if (set) {
> > -               WARN_ON_ONCE(!(k->flags & PF_NO_SETAFFINITY));
> > -               WARN_ON_ONCE(k->nr_cpus_allowed != 1);
> > -               set_bit(KTHREAD_IS_PER_CPU, &kthread->flags);
> > -       } else {
> > +       WARN_ON_ONCE(!(k->flags & PF_NO_SETAFFINITY));
> > +
> > +       if (cpu < 0) {
> >                 clear_bit(KTHREAD_IS_PER_CPU, &kthread->flags);
> > +               return;
> >         }
> > +
> > +       kthread->cpu = cpu;
> > +       set_bit(KTHREAD_IS_PER_CPU, &kthread->flags);
> >  }
> >
> 
> I don't see the code to set the mask of the cpu to the task
> since set_cpus_allowed_ptr() is removed from rebind_worker().
> 
> Is it somewhere I missed?

kthread_unpark().

> > @@ -4978,9 +4982,9 @@ static void rebind_workers(struct worker_pool *pool)
> >          * 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_set_per_cpu(worker->task, true);
> > +               WARN_ON_ONCE(kthread_park(worker->task) < 0);
> > +               kthread_set_per_cpu(worker->task, pool->cpu);
> > +               kthread_unpark(worker->task);
> 
> I feel nervous to use kthread_park() here and kthread_parkme() in
> worker thread.  And adding kthread_should_park() to the fast path
> also daunt me.

Is that really such a hot path that an additional load is problematic?

> How about using a new KTHREAD_XXXX instead of KTHREAD_IS_PER_CPU,
> so that we can set and clear KTHREAD_XXXX freely, especially before
> set_cpus_allowed_ptr().

KTHREAD_IS_PER_CPU is exactly what we need, why make another flag?

The above sequence is nice in that it restores both the
KTHREAD_IS_PER_CPU flag and affinity while the task is frozen, so there
are no races where one is observed and not the other.

It is also the exact sequence normal per-cpu threads (smpboot) use to
preserve affinity.