linux-kernel - Re: [PATCH RESEND] sched/nohz: Add HRTICK_BW for using cfs bandwidth with nohz

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Message-ID: <20230612213925.GB524810@lorien.usersys.redhat.com>
Date:   Mon, 12 Jun 2023 17:39:25 -0400
From:   Phil Auld <pauld@...hat.com>
To:     Peter Zijlstra <peterz@...radead.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>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Valentin Schneider <vschneid@...hat.com>,
        Ben Segall <bsegall@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mel Gorman <mgorman@...e.de>
Subject: Re: [PATCH RESEND] sched/nohz: Add HRTICK_BW for using cfs bandwidth
 with nohz_full

Hi Peter,

On Thu, May 18, 2023 at 10:37:18AM -0400 Phil Auld wrote:
> On Thu, May 18, 2023 at 03:47:46PM +0200 Peter Zijlstra wrote:

...

> > OMG; so because NOHZ_FULL configuration sucks, we add hacks on?
> >
>

...

>
> This seemed to be a sane way to handle what are effectively conflicting
> requirements.  Stalling a task to the point the host gets rebooted is
> pretty painful.  Maybe if we could fail the tick_stop test in this
> case that would work but that would keep all the ticks whereas this
> tries to respect the request for nohz as much as possible. 

...

Let me try to argue it differently.  Forget about the nohz_full configuration
part (I dropped that from the commit log on v2, too) since you could hit
this even if nohz_full was dynamic.

My contention is that given two conflicting requests the scheduler is making
the wrong choice. A request is being made to stop the tick if possible (which
is best effort already - there are numerous conditions to satisfy). And a
request is being made to enforce a cpu bandwidth limit (which is a hard limit
that can violate work conservation, and requires regular fine-grained
accounting).  Currently the scheduler will favor the best-effort nohz request
over the quota limit request.

I posted v2 of the HRTICK based patch but maybe a simpler one that adds a
scheduler tick dependency when we pick a bandwidth-limited task would be
more palatable. 

I have that one which I could clean up and post.

Thanks,
Phil

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