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Date:   Thu, 18 May 2023 15:47:46 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Phil Auld <pauld@redhat.com>
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>
Subject: Re: [PATCH RESEND] sched/nohz: Add HRTICK_BW for using cfs bandwidth
 with nohz_full
Message-ID: <20230518134746.GB2860939@hirez.programming.kicks-ass.net>
References: <20230518132038.3534728-1-pauld@redhat.com>
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On Thu, May 18, 2023 at 09:20:38AM -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.  Use the hrtick mechanism to set a sched
> tick to fire at remaining_runtime in the future if we are on
> a nohz full cpu, if the task has quota and if we are likely to
> disable the tick (nr_running == 1).  This allows for bandwidth
> accounting before tasks go too far over quota.
> 
> A number of container workloads use a dynamic number of real
> nohz tasks but also have other work that is limited which ends
> up running on the "spare" nohz cpus.  This is an artifact of
> having to specify nohz_full cpus at boot. Adding this hrtick
> resolves the issue of long stalls on these tasks.
> 
> Add the sched_feat HRTICK_BW off by default to allow users to
> enable this only when needed.

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