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From:   Benjamin Segall <bsegall@google.com>
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>,
        Peter Zijlstra <peterz@infradead.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Valentin Schneider <vschneid@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mel Gorman <mgorman@suse.de>,
        Frederic Weisbecker <frederic@kernel.org>,
        Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH v5 2/2] Sched/fair: Block nohz tick_stop when cfs
 bandwidth in use
References: <20230707195748.2918490-1-pauld@redhat.com>
        <20230707195748.2918490-3-pauld@redhat.com>
Date:   Mon, 10 Jul 2023 16:54:58 -0700
In-Reply-To: <20230707195748.2918490-3-pauld@redhat.com> (Phil Auld's message
        of "Fri, 7 Jul 2023 15:57:48 -0400")
Message-ID: <xm26lefnfhkd.fsf@google.com>
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Phil Auld <pauld@redhat.com> writes:

> 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.
>
> Add sched_feat HZ_BW (off by default) to control the tick_stop
> behavior.
>
> Signed-off-by: Phil Auld <pauld@redhat.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Cc: Valentin Schneider <vschneid@redhat.com>
> Cc: Ben Segall <bsegall@google.com>
> Cc: Frederic Weisbecker <frederic@kernel.org>
> ---
>  kernel/sched/core.c     | 12 ++++++++++
>  kernel/sched/fair.c     | 49 +++++++++++++++++++++++++++++++++++++++++
>  kernel/sched/features.h |  2 ++
>  kernel/sched/sched.h    |  1 +
>  4 files changed, 64 insertions(+)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 1b214e10c25d..4b8534abdf4f 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1229,6 +1229,18 @@ bool sched_can_stop_tick(struct rq *rq)
>  	if (rq->nr_running > 1)
>  		return false;
>  
> +	/*
> +	 * If there is one task and it has CFS runtime bandwidth constraints
> +	 * and it's on the cpu now we don't want to stop the tick.
> +	 * This check prevents clearing the bit if a newly enqueued task here is
> +	 * dequeued by migrating while the constrained task continues to run.
> +	 * E.g. going from 2->1 without going through pick_next_task().
> +	 */
> +	if (sched_feat(HZ_BW) && rq->nr_running == 1 && task_on_rq_queued(rq->curr)) {
> +		if (cfs_task_bw_constrained(rq->curr))
> +			return false;
> +	}
> +

I think we still need the fair_sched_class check with the bit being on
cfs_rq/tg rather than task.