linux-kernel - Re: [PATCH v6 2/2] Sched/fair: Block nohz tick

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Message-ID: <20230731224934.GD51835@hirez.programming.kicks-ass.net>
Date:   Tue, 1 Aug 2023 00:49:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Phil Auld <pauld@...hat.com>
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>,
        Frederic Weisbecker <frederic@...nel.org>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v6 2/2] Sched/fair: Block nohz tick_stop when cfs
 bandwidth in use

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?