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

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Message-ID: <jhjv9fnmwhg.mognet@arm.com>
Date:   Tue, 06 Oct 2020 12:20:43 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     tglx@...utronix.de, mingo@...nel.org, linux-kernel@...r.kernel.org,
        bigeasy@...utronix.de, qais.yousef@....com, swood@...hat.com,
        juri.lelli@...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 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).
>
> This adds migration interference to the higher priority task, but
> restores bandwidth to system that would otherwise be irrevocably lost.
> Without this it would be possible to have all tasks on the system
> stuck on a single CPU, each task preempted in a migrate_disable()
> section with a single high priority task running.
>
> This way we can still approximate running the M highest priority tasks
> on the system.
>

Ah, so IIUC that's the important bit that makes it we can't just say go
through the pushable_tasks list and skip migrate_disable() tasks.

Once the highest-prio task exits its migrate_disable() region, your patch
pushes it away. If we ended up with a single busy CPU, it'll spread the
tasks around one migrate_enable() at a time.

That time where the top task is migrate_disable() is still a crappy time,
and as you pointed out earlier today if it is a genuine pcpu task then the
whole thing is -EBORKED...

An alternative I could see would be to prevent those piles from forming
altogether, say by issuing a similar push_cpu_stop() on migrate_disable()
if the next pushable task is already migrate_disable(); but that's a
proactive approach whereas yours is reactive, so I'm pretty sure that's
bound to perform worse.

> Migrating the top task away is (ofcourse) still subject to
> migrate_disable() too, which means the lower task is subject to an
> interference equivalent to the worst case migrate_disable() section.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>