linux-kernel - Re: [PATCH v2 12/12] sched: Add ttwu

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Message-ID: <20250708185710.GD477119@noisy.programming.kicks-ass.net>
Date: Tue, 8 Jul 2025 20:57:10 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: mingo@...hat.com, juri.lelli@...hat.com, vincent.guittot@...aro.org,
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
	vschneid@...hat.com, clm@...a.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 12/12] sched: Add ttwu_queue support for delayed tasks

On Tue, Jul 08, 2025 at 02:44:56PM +0200, Dietmar Eggemann wrote:
> On 02/07/2025 13:49, Peter Zijlstra wrote:
> 
> [...]
> 
> > @@ -3801,6 +3839,52 @@ static inline bool ttwu_do_migrate(struc
> >  	return true;
> >  }
> >  
> > +static int ttwu_delayed(struct rq *rq, struct task_struct *p, int wake_flags,
> > +			struct rq_flags *rf)
> > +{
> > +	struct rq *p_rq = task_rq(p);
> > +	int cpu;
> > +
> > +	/*
> > +	 * Notably it is possible for on-rq entities to get migrated -- even
> > +	 * sched_delayed ones. This should be rare though, so flip the locks
> > +	 * rather than IPI chase after it.
> > +	 */
> > +	if (unlikely(rq != p_rq)) {
> > +		rq_unlock(rq, rf);
> > +		p_rq = __task_rq_lock(p, rf);
> > +		update_rq_clock(p_rq);
> > +	}
> > +
> > +	if (task_on_rq_queued(p))
> > +		dequeue_task(p_rq, p, DEQUEUE_NOCLOCK | DEQUEUE_SLEEP | DEQUEUE_DELAYED);
> > +
> > +	/*
> > +	 * NOTE: unlike the regular try_to_wake_up() path, this runs both
> > +	 * select_task_rq() and ttwu_do_migrate() while holding rq->lock
> > +	 * rather than p->pi_lock.
> > +	 */
> > +	cpu = select_task_rq(p, p->wake_cpu, &wake_flags);
> 
> There are 'lockdep_assert_held(&p->pi_lock)'s in select_task_rq() and
> select_task_rq_fair() which should trigger IMHO? Can they be changed the
> same way like  __task_rq_lock()?

And not a single robot has yet reported this :-(.. Yeah, let me go look.
Seeing how this was performance stuff, I clearly did not run enough
lockdep builds :/

> > +	if (ttwu_do_migrate(rq, p, cpu))
> > +		wake_flags |= WF_MIGRATED;
> 
> [...]
> 
> >  /*
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -2280,6 +2280,7 @@ static inline int task_on_rq_migrating(s
> >  #define WF_RQ_SELECTED		0x80 /* ->select_task_rq() was called */
> >  
> >  #define WF_ON_CPU		0x0100
> 
> Looks like this is still not used. Not sure whether it can be removed or
> you wanted to add a condition for this as well?

Bah, I'm sure I deleted that at some point. Let me try killing it again
:-)