linux-kernel - Re: [PATCH v2 07/12] psi: Split psi_ttwu

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Message-ID: <94ea5721-2d32-416c-919c-a0d9e5eadf99@meta.com>
Date: Thu, 17 Jul 2025 19:59:52 -0400
From: Chris Mason <clm@...a.com>
To: Peter Zijlstra <peterz@...radead.org>, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, vschneid@...hat.com
Cc: linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 07/12] psi: Split psi_ttwu_dequeue()

On 7/2/25 7:49 AM, Peter Zijlstra wrote:
> Currently psi_ttwu_dequeue() is called while holding p->pi_lock and
> takes rq->lock. Split the function in preparation for calling
> ttwu_do_migration() while already holding rq->lock.
> 

[ ... ]


This patch regresses schbench -L -m 4 -M auto -t 256 -n 0 -r 0 -s 0 from
5.2M RPS to 4.5M RPS, and profiles show that CPU 0-3 are spending more
time in __task_rq_lock()

> -static inline void psi_ttwu_dequeue(struct task_struct *p)
> +static inline bool psi_ttwu_need_dequeue(struct task_struct *p)
>  {
>  	if (static_branch_likely(&psi_disabled))
> -		return;
> +		return false;
>  	/*
>  	 * Is the task being migrated during a wakeup? Make sure to
>  	 * deregister its sleep-persistent psi states from the old
>  	 * queue, and let psi_enqueue() know it has to requeue.
>  	 */
> -	if (unlikely(p->psi_flags)) {
> -		struct rq_flags rf;
> -		struct rq *rq;
> -
> -		rq = __task_rq_lock(p, &rf);
> -		psi_task_change(p, p->psi_flags, 0);
> -		__task_rq_unlock(rq, &rf);
> -	}
> +	if (!likely(!p->psi_flags))
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^

I think we need roughly one less bang?

-chris