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

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Message-ID: <ae9f49dc-b300-4827-9cb6-be40354f5d57@arm.com>
Date: Tue, 8 Jul 2025 14:44:56 +0200
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Peter Zijlstra <peterz@...radead.org>, 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
Cc: linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 12/12] sched: Add ttwu_queue support for delayed tasks

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()?

> +	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?