linux-kernel - Re: [PATCH] Sched/fair: Block nohz tick

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Message-ID: <20230622154458.GD727646@lorien.usersys.redhat.com>
Date:   Thu, 22 Jun 2023 11:44:58 -0400
From:   Phil Auld <pauld@...hat.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, Juri Lelli <juri.lelli@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Valentin Schneider <vschneid@...hat.com>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH] Sched/fair: Block nohz tick_stop when cfs bandwidth in
 use

On Thu, Jun 22, 2023 at 10:22:16AM -0400 Steven Rostedt wrote:
> On Thu, 22 Jun 2023 09:27:51 -0400
> Phil Auld <pauld@...hat.com> wrote:
> 
> > CFS bandwidth limits and NOHZ full don't play well together.  Tasks
> > can easily run well past their quotas before a remote tick does
> > accounting.  This leads to long, multi-period stalls before such
> > tasks can run again. Currentlyi, when presented with these conflicting
> > requirements the scheduler is favoring nohz_full and letting the tick
> > be stopped. However, nohz tick stopping is already best-effort, there
> > are a number of conditions that can prevent it, whereas cfs runtime
> > bandwidth is expected to be enforced.
> > 
> > Make the scheduler favor bandwidth over stopping the tick by setting
> > TICK_DEP_BIT_SCHED when the only running task is a cfs task with
> > runtime limit enabled.
> > 
> > Add sched_feat HZ_BW (off by default) to control this behavior.
> 
> So the tl;dr; is: "If the current task has a bandwidth limit, do not disable
> the tick" ?
>

Yes.   W/o the tick we can't reliably support/enforce the bandwidth limit.


Cheers,
Phil

> -- Steve
> 

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