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Date:   Fri, 9 Sep 2022 18:09:13 +0800
From:   Chen Yu <yu.c.chen@intel.com>
To:     Abel Wu <wuyun.abel@bytedance.com>
CC:     Peter Zijlstra <peterz@infradead.org>,
        Mel Gorman <mgorman@suse.de>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Josh Don <joshdon@google.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        K Prateek Nayak <kprateek.nayak@amd.com>,
        "Gautham R . Shenoy" <gautham.shenoy@amd.com>,
        <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v5 3/5] sched/fair: Skip core update if task pending
Message-ID: <YxsLXunUjY2vBeDX@chenyu5-mobl1>
References: <20220909055304.25171-1-wuyun.abel@bytedance.com>
 <20220909055304.25171-4-wuyun.abel@bytedance.com>
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On 2022-09-09 at 13:53:02 +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.
> 
> Signed-off-by: Abel Wu <wuyun.abel@bytedance.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;
> +
Is it to deal with the race condition? I'm thinking of the
following scenario: task p1 on rq1 is about to switch to idle.
However when p1 reaches __update_idle_core(), someone on other
CPU tries to wake up p2, and leverages rq1 to queue p2
thus set the ttwu_pending flag on rq1. It is likely that
rq1 becomes idle but soon finds that TF_NEED_RESCHED is set, thus
quits the idle loop. As a result rq will not be idle and we will
get false positive here.

thanks,
Chenyu
>  	rcu_read_lock();
>  	if (test_idle_cores(core, true))
>  		goto unlock;
> -- 
> 2.37.3
>