linux-kernel - Re: [PATCH v2] sched/fair: Make SCHED_IDLE entity be preempted in strict hierarchy

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Message-ID: <8fea90cc-8a57-4cd7-8b60-6179a8eb9cfd@linux.alibaba.com>
Date: Thu, 4 Jul 2024 16:15:45 +0800
From: Tianchen Ding <dtcccc@...ux.alibaba.com>
To: linux-kernel@...r.kernel.org
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
 Mel Gorman <mgorman@...e.de>, Daniel Bristot de Oliveira
 <bristot@...hat.com>, Valentin Schneider <vschneid@...hat.com>,
 Josh Don <joshdon@...gle.com>
Subject: Re: [PATCH v2] sched/fair: Make SCHED_IDLE entity be preempted in
 strict hierarchy

gentle ping...

On 2024/6/26 10:35, Tianchen Ding wrote:
> Consider the following cgroup:
> 
>                         root
>                          |
>               ------------------------
>               |                      |
>         normal_cgroup            idle_cgroup
>               |                      |
>     SCHED_IDLE task_A           SCHED_NORMAL task_B
> 
> According to the cgroup hierarchy, A should preempt B. But current
> check_preempt_wakeup_fair() treats cgroup se and task separately, so B
> will preempt A unexpectedly.
> Unify the wakeup logic by {c,p}se_is_idle only. This makes SCHED_IDLE of
> a task a relative policy that is effective only within its own cgroup,
> similar to the behavior of NICE.
> 
> Also fix se_is_idle() definition when !CONFIG_FAIR_GROUP_SCHED.
> 
> Fixes: 304000390f88 ("sched: Cgroup SCHED_IDLE support")
> Signed-off-by: Tianchen Ding <dtcccc@...ux.alibaba.com>
> Reviewed-by: Josh Don <joshdon@...gle.com>
> Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
> v2:
> Use entity_is_task() to check whether pse is a task.
> Improve comments and commit log.
> 
> v1: https://lore.kernel.org/all/20240624073900.10343-1-dtcccc@linux.alibaba.com/
> ---
>   kernel/sched/fair.c | 24 ++++++++++++------------
>   1 file changed, 12 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 41b58387023d..f0b038de99ce 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -511,7 +511,7 @@ static int cfs_rq_is_idle(struct cfs_rq *cfs_rq)
>   
>   static int se_is_idle(struct sched_entity *se)
>   {
> -	return 0;
> +	return task_has_idle_policy(task_of(se));
>   }
>   
>   #endif	/* CONFIG_FAIR_GROUP_SCHED */
> @@ -8382,16 +8382,7 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
>   	if (test_tsk_need_resched(curr))
>   		return;
>   
> -	/* Idle tasks are by definition preempted by non-idle tasks. */
> -	if (unlikely(task_has_idle_policy(curr)) &&
> -	    likely(!task_has_idle_policy(p)))
> -		goto preempt;
> -
> -	/*
> -	 * Batch and idle tasks do not preempt non-idle tasks (their preemption
> -	 * is driven by the tick):
> -	 */
> -	if (unlikely(p->policy != SCHED_NORMAL) || !sched_feat(WAKEUP_PREEMPTION))
> +	if (!sched_feat(WAKEUP_PREEMPTION))
>   		return;
>   
>   	find_matching_se(&se, &pse);
> @@ -8401,7 +8392,7 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
>   	pse_is_idle = se_is_idle(pse);
>   
>   	/*
> -	 * Preempt an idle group in favor of a non-idle group (and don't preempt
> +	 * Preempt an idle entity in favor of a non-idle entity (and don't preempt
>   	 * in the inverse case).
>   	 */
>   	if (cse_is_idle && !pse_is_idle)
> @@ -8409,6 +8400,15 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
>   	if (cse_is_idle != pse_is_idle)
>   		return;
>   
> +	/*
> +	 * Batch tasks do not preempt non-idle tasks (their preemption
> +	 * is driven by the tick).
> +	 * We've done the check about "only one of the entities is idle",
> +	 * so cse must be non-idle if p is a batch task.
> +	 */
> +	if (unlikely(entity_is_task(pse) && p->policy == SCHED_BATCH))
> +		return;
> +
>   	cfs_rq = cfs_rq_of(se);
>   	update_curr(cfs_rq);
>