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

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Date:   Fri, 9 Sep 2022 18:13:38 +0800
From:   Abel Wu <wuyun.abel@...edance.com>
To:     Chen Yu <yu.c.chen@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Josh Don <joshdon@...gle.com>,
        Tim Chen <tim.c.chen@...ux.intel.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 9/9/22 6:09 PM, Chen Yu wrote:
> 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@...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;
>> +
> 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.

Yes, exactly as you said.