linux-kernel - Re: [PATCH v5 3/5] sched/fair: Skip core update if task pending

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Date:   Wed, 14 Sep 2022 15:37:34 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Abel Wu <wuyun.abel@...edance.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Josh Don <joshdon@...gle.com>, Chen Yu <yu.c.chen@...el.com>,
        K Prateek Nayak <kprateek.nayak@....com>,
        "Gautham R . Shenoy" <gautham.shenoy@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 3/5] sched/fair: Skip core update if task pending

On Fri, 2022-09-09 at 13:53 +0800, Abel Wu wrote:
> The function __update_idle_core() considers this cpu is idle so
> only checks its siblings to decide whether the resident core is
> idle or not and update has_idle_cores hint if necessary. But the
> problem is that this cpu might not be idle at that moment any
> more, resulting in the hint being misleading.
> 
> It's not proper to make this check everywhere in the idle path,
> but checking just before core updating can make the has_idle_core
> hint more reliable with negligible cost.
> 

Reviewed-by: Tim Chen <tim.c.chen@...ux.intel.com>

> Signed-off-by: Abel Wu <wuyun.abel@...edance.com>
> ---
>  kernel/sched/fair.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 7abe188a1533..fad289530e07 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6294,6 +6294,9 @@ void __update_idle_core(struct rq *rq)
>  	int core = cpu_of(rq);
>  	int cpu;
>  
> +	if (rq->ttwu_pending)
> +		return;
> +
>  	rcu_read_lock();
>  	if (test_idle_cores(core, true))
>  		goto unlock;