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Subject: Re: [PATCH] sched/fair: Optimize dequeue_task_fair()
To:     Vincent Guittot <vincent.guittot@linaro.org>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        linux-kernel <linux-kernel@vger.kernel.org>
References: <6f2f195aea48bc50187dfb064aa530ba132be01b.1592230286.git.rocking@linux.alibaba.com>
 <CAKfTPtACgUutP86nM6HvgQ-Hj7RdBnK+yxqg3Dr+cOZCPSoeyg@mail.gmail.com>
From:   Peng Wang <rocking@linux.alibaba.com>
Message-ID: <60d6eec1-4406-655c-c329-fc1c915a7721@linux.alibaba.com>
Date:   Tue, 16 Jun 2020 14:09:39 +0800
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On 6/15/20 11:09 PM, Vincent Guittot wrote:
> On Mon, 15 Jun 2020 at 16:20, Peng Wang <rocking@linux.alibaba.com> wrote:
>>
>> While looking at enqueue_task_fair and dequeue_task_fair, it occurred
>> to me that dequeue_task_fair can also be optimized as Vincent described
>> in commit 7d148be69e3a ("sched/fair: Optimize enqueue_task_fair()").
> 
> good point
> 
>>
>> dequeue_throttle label can ensure se not to be NULL, and se is
>> always NULL when reaching root level.
>>
>> Signed-off-by: Peng Wang <rocking@linux.alibaba.com>
>> ---
>>   kernel/sched/fair.c | 6 +++---
>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index cbcb2f7..e6be952 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -5614,10 +5614,10 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>>
>>          }
>>
>> -dequeue_throttle:
>> -       if (!se)
>> -               sub_nr_running(rq, 1);
>> +       /* At this point se is NULL and we are at root level*/
>> +       sub_nr_running(rq, 1);
>>
>> +dequeue_throttle:
>>          /* balance early to pull high priority tasks */
>>          if (unlikely(!was_sched_idle && sched_idle_rq(rq)))
>>                  rq->next_balance = jiffies;
> 
> You can move the label here because sched_idle_rq() uses
> rq->nr-running and rq->cfs.idle_h_nr_running so they will not change
> if we jump to the label before reaching root level

Yes, then we can also skip the early balance check.

> 
>> --
>> 2.9.5
>>