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

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Message-ID: <20251205125606.GA2528459@noisy.programming.kicks-ass.net>
Date: Fri, 5 Dec 2025 13:56:06 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: K Prateek Nayak <kprateek.nayak@....com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>, mingo@...hat.com,
	juri.lelli@...hat.com, dietmar.eggemann@....com,
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
	vschneid@...hat.com, linux-kernel@...r.kernel.org,
	pierre.gondois@....com, qyousef@...alina.io, hongyan.xia2@....com,
	christian.loehle@....com, luis.machado@....com
Subject: Re: [PATCH 4/6 v8] sched/fair: Add push task mechanism for fair

On Fri, Dec 05, 2025 at 06:19:07PM +0530, K Prateek Nayak wrote:
> On 12/5/2025 2:29 PM, Peter Zijlstra wrote:
> >>> Why not use move_queued_task() ?
> >>
> >> double_lock_balance() can fail and prevent being blocked waiting for
> >> new rq whereas move_queued_task() will wait, won't it ?
> >>
> >> Do you think move_queued_task() would be better ?
> > 
> > No, double_lock_balance() never fails, the return value indicates if the
> > currently held rq-lock, (the first argument) was unlocked while
> > attaining both -- this is required when the first rq is a higher address
> > than the second.
> > 
> > double_lock_balance() also puts the wait-time and hold time of the
> > second inside the hold time of the first, which gets you a quadric term
> > in the rq hold times IIRC. Something that's best avoided.
> > 
> > move_queued_task() OTOH takes the task off the runqueue you already hold
> > locked, drops this lock, acquires the second, puts the task there, and
> > returns with the dst rq locked.
> 
> So I was experimenting with:
> 
>     deactivate_task(rq, p, 0);
>     set_task_cpu(p, target_cpu);
>     __ttwu_queue_wakelist(p, target_cpu, 0);
> 
> and nothing has screamed at me yet during the benchmark runs.
> Would this be any good instead of the whole lock juggling?

This will get schedstats and any class with ->task_woken confused I
think. The IPI handler (sched_ttwu_pending / ttwu_do_activate) is
currently only geared towards doing the remote bit of ttwu.

This is fixable of course.

> Since this CPU is found to be going overloaded, pushing via an
> IPI vs taking the overhead ourselves seems to make more sense
> to me from EAS standpoint.

The performance characteristics here are very platform dependent.
Sometimes raising an IPI can be very expensive in and of itself.

> Given TTWU_QUEUE is disabled for PREEMPT_RT, I'm assuming this
> might be problematic too?

Yes, you have to check TTWU_QUEUE and have a fallback.