linux-kernel - Re: [PATCH -v2 15/17] sched: Fix migrate

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Message-ID: <20201006144816.GZ2628@hirez.programming.kicks-ass.net>
Date:   Tue, 6 Oct 2020 16:48:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        tglx@...utronix.de, mingo@...nel.org, linux-kernel@...r.kernel.org,
        bigeasy@...utronix.de, qais.yousef@....com, swood@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vincent.donnefort@....com, tj@...nel.org
Subject: Re: [PATCH -v2 15/17] sched: Fix migrate_disable() vs rt/dl balancing

On Tue, Oct 06, 2020 at 04:37:04PM +0200, Juri Lelli wrote:
> On 06/10/20 15:48, Peter Zijlstra wrote:
> > On Tue, Oct 06, 2020 at 12:20:43PM +0100, Valentin Schneider wrote:
> > > 
> > > On 05/10/20 15:57, Peter Zijlstra wrote:
> > > > In order to minimize the interference of migrate_disable() on lower
> > > > priority tasks, which can be deprived of runtime due to being stuck
> > > > below a higher priority task. Teach the RT/DL balancers to push away
> > > > these higher priority tasks when a lower priority task gets selected
> > > > to run on a freshly demoted CPU (pull).
> 
> Still digesting the whole lot, but can't we "simply" force push the
> higest prio (that we preempt to make space for the migrate_disabled
> lower prio) directly to the cpu that would accept the lower prio that
> cannot move?
> 
> Asking because AFAIU we are calling find_lock_rq from push_cpu_stop and
> that selects the best cpu for the high prio. I'm basically wondering if
> we could avoid moving, potentially multiple, high prio tasks around to
> make space for a lower prio task.

The intention was to do as you describe in the first paragraph, and
isn't pull also using find_lock_rq() to select the 'lowest' priority
runqueue to move the task to?

That is, both actions should end up at the same 'lowest' prio CPU.