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Date:   Tue, 1 Aug 2023 07:13:42 -0400
From:   Phil Auld <pauld@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     linux-kernel@vger.kernel.org, Juri Lelli <juri.lelli@redhat.com>,
        Ingo Molnar <mingo@redhat.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Valentin Schneider <vschneid@redhat.com>,
        Ben Segall <bsegall@google.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mel Gorman <mgorman@suse.de>,
        Frederic Weisbecker <frederic@kernel.org>,
        Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH v6 2/2] Sched/fair: Block nohz tick_stop when cfs
 bandwidth in use
Message-ID: <20230801111342.GA268019@lorien.usersys.redhat.com>
References: <20230712133357.381137-1-pauld@redhat.com>
 <20230712133357.381137-3-pauld@redhat.com>
 <20230731224934.GD51835@hirez.programming.kicks-ass.net>
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On Tue, Aug 01, 2023 at 12:49:34AM +0200 Peter Zijlstra wrote:
> On Wed, Jul 12, 2023 at 09:33:57AM -0400, Phil Auld 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. Currently, 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. We use cfs_b->hierarchical_quota to
> > determine if the task requires the tick.
> > 
> > Add check in pick_next_task_fair() as well since that is where
> > we have a handle on the task that is actually going to be running.
> > 
> > Add check in sched_can_stop_tick() to cover some edge cases such
> > as nr_running going from 2->1 and the 1 remains the running task.
> 
> These appear fine to me, except:
>
> > Add sched_feat HZ_BW (off by default) to control the tick_stop
> > behavior.
> 
> What was the thinking here? This means nobody will be using this -- why
> would you want this default disabled?
> 

That was just a hedge in case it caused issues. I'd probably have had to
enable it in RHEL anyway. Using a feature was to make it inocuous when
disabled.  Would you prefer me to enable it or remove the sched_feat
entirely? (or do you want to just switch that to true when you apply it?)

Thanks,
Phil

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