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Date:   Fri, 30 Jun 2023 17:06:41 +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>,
        Frederic Weisbecker <frederic@kernel.org>
Subject: Re: [PATCH v4] Sched/fair: Block nohz tick_stop when cfs bandwidth
 in use
Message-ID: <20230630150641.GH2533791@hirez.programming.kicks-ass.net>
References: <20230630135714.1018434-1-pauld@redhat.com>
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On Fri, Jun 30, 2023 at 09:57:14AM -0400, Phil Auld wrote:

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index a68d1276bab0..2685373e12f1 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1229,6 +1229,16 @@ 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.
> +	 */
> +	if (sched_feat(HZ_BW) && rq->nr_running == 1 && rq->curr
> +	    && rq->curr->sched_class == &fair_sched_class && task_on_rq_queued(rq->curr)) {

&& goes at the end of the previous line

rq->curr is never NULL

But surely you can find a saner way to write this?

> +		if (sched_cfs_bandwidth_active(rq->curr))
> +			return false;
> +	}
> +
>  	return true;
>  }
>  #endif /* CONFIG_NO_HZ_FULL */
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 373ff5f55884..125b1ec4476f 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6139,6 +6139,50 @@ static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq)
>  	rcu_read_unlock();
>  }
>  
> +#ifdef CONFIG_NO_HZ_FULL
> +static inline bool cfs_se_bandwidth_enabled(struct sched_entity *se)
> +{
> +	int ret = 0;
> +
> +	for_each_sched_entity(se)
> +		ret += cfs_rq_of(se)->runtime_enabled;
> +
> +	return ret != 0;
> +}
> +
> +bool sched_cfs_bandwidth_active(struct task_struct *p)
> +{
> +	if (cfs_bandwidth_used() && cfs_se_bandwidth_enabled(&p->se))
> +		return true;
> +
> +	return false;
> +}
> +
> +/* called from pick_next_task_fair() */
> +static void sched_fair_update_stop_tick(struct rq *rq, struct task_struct *p)
> +{
> +	int cpu = cpu_of(rq);
> +
> +	if (!sched_feat(HZ_BW) || !cfs_bandwidth_used())
> +		return;
> +
> +	if (!tick_nohz_full_cpu(cpu))
> +		return;
> +
> +	if (rq->nr_running != 1)
> +		return;
> +
> +	/*
> +	 *  We know there is only one task runnable and we've just picked it. The
> +	 *  normal enqueue path will have cleared TICK_DEP_BIT_SCHED if we will
> +	 *  be otherwise able to stop the tick. Just need to check if we are using
> +	 *  bandwidth control.
> +	 */
> +	if (cfs_se_bandwidth_enabled(&p->se))
> +		tick_nohz_dep_set_cpu(cpu, TICK_DEP_BIT_SCHED);
> +}

Yeah, I think not; pick_next_task_fair() just walked the cgroup
hierarchy and now you do it again.

Also, why does this code exist at all? Both enqueue/dequeue already end
up in sched_update_tick_depenency() and should be able to handle the
nr_running==1 with bandwidth crap, no?