Re: [RFC][PATCH] sched/core: Tweak wait_task_inactive() to force dequeue sched_delayed tasks

Peter Zijlstra
2025-04-22T10:19:37Z

On Tue, Apr 22, 2025 at 11:55:31AM +0200, Frederic Weisbecker wrote:
> Le Tue, Apr 22, 2025 at 10:56:28AM +0200, Peter Zijlstra a écrit :
> > On Mon, Apr 21, 2025 at 09:43:45PM -0700, John Stultz wrote:
> > > It was reported that in 6.12, smpboot_create_threads() was
> > > taking much longer then in 6.6.
> > >
> > > I narrowed down the call path to:
> > > smpboot_create_threads()
> > > -> kthread_create_on_cpu()
> > > -> kthread_bind()
> > > -> __kthread_bind_mask()
> > > ->wait_task_inactive()
> > >
> > > Where in wait_task_inactive() we were regularly hitting the
> > > queued case, which sets a 1 tick timeout, which when called
> > > multiple times in a row, accumulates quickly into a long
> > > delay.
> >
> > Argh, this is all stupid :-(
> >
> > The whole __kthread_bind_*() thing is a bit weird, but fundamentally it
> > tries to avoid a race vs current. Notably task_state::flags is only ever
> > modified by current, except here.
> >
> > delayed_dequeue is fine, except wait_task_inactive() hasn't been
> > told about it (I hate that function, murder death kill etc.).
> >
> > But more fundamentally, we've put so much crap into struct kthread and
> > kthread() itself by now, why not also pass down the whole per-cpu-ness
> > thing and simply do it there. Heck, Frederic already made it do affinity
> > crud.
> >
> > On that, Frederic, *why* do you do that after started=1, that seems like
> > a weird place, should this not be done before complete() ?, like next to
> > sched_setscheduler_nocheck() or so?
>
> You mean the call to kthread_affine_node() ? Because it is a default behaviour
> that only happens if no call to kthread_bind() or kthread_affine_preferred()
> has been issued before the first wake up to the kthread.
>
> If kthread_affine_node() was called before everything by default instead
> then we would get its unconditional overhead for all started kthreads. Plus
> kthread_bind() and kthread_affine_preferred() would need to undo
> kthread_affine_node().

Urgh, I see. Perhaps we should put a comment on, because I'm sure I'll
have this same question again next time (probably in another few years)
when I look at this code.