linux-kernel - Re: [PATCH 4/6 v8] sched/fair: Add push task mechanism for fair

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Message-ID: <CAKfTPtBVeOWfdSC=uOPse_6K6nWtttzSEDg3CyKEm3NtoH_UNw@mail.gmail.com>
Date: Mon, 9 Feb 2026 14:17:56 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com, 
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de, vschneid@...hat.com, 
	linux-kernel@...r.kernel.org, pierre.gondois@....com, kprateek.nayak@....com, 
	qyousef@...alina.io, christian.loehle@....com
Subject: Re: [PATCH 4/6 v8] sched/fair: Add push task mechanism for fair

On Wed, 10 Dec 2025 at 15:01, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>
> - hongyan.xia2@....com
> - luis.machado@....com
>
> On 02.12.25 19:12, Vincent Guittot wrote:
> > EAS is based on wakeup events to efficiently place tasks on the system, but
> > there are cases where a task doesn't have wakeup events anymore or at a far
> > too low pace. For such situation, we can take advantage of the task being
> > put back in the enqueued list to check if it should be pushed on another
> > CPU.
> > When the task is alone on the CPU, it's never put back in the enqueued
> > list; In this special case, we use the tick to run the check.
> >
> > Add a push task mechanism that enables fair scheduler to push runnable
> > tasks. EAS will be one user but other feature like filling idle CPUs
> > can also take advantage of it.
>
> [...]
>
> > +/*
> > + * See if the non running fair tasks on this rq can be sent on other CPUs
> > + * that fits better with their profile.
> > + */
> > +static bool push_fair_task(struct rq *rq)
> > +{
> > +     struct task_struct *next_task;
> > +     int prev_cpu, new_cpu;
> > +     struct rq *new_rq;
> > +
> > +     next_task = pick_next_pushable_fair_task(rq);
> > +     if (!next_task)
> > +             return false;
> > +
> > +     if (is_migration_disabled(next_task))
> > +             return true;
> > +
> > +     /* We might release rq lock */
> > +     get_task_struct(next_task);
> > +
> > +     prev_cpu = rq->cpu;
> > +
>
> select_task_rq_fair() requires p->pi_lock to be held. I assume
> check_pushable_task() (push single running task) has the same issue.

fair enough


>
> > +     new_cpu = select_task_rq_fair(next_task, prev_cpu, 0);
> > +
>
> [...]