linux-kernel - Re: [PATCH v1] sched/cfs: Fix pick_next

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Date:   Wed, 1 Jul 2020 19:43:36 +0800
From:   Zijun Hu <zijuhu@...eaurora.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1] sched/cfs: Fix pick_next_entity() implementation error

thanks for your explanation.
you are right. @lest should be used as reference point to compare.
Please ignore this patch.

On 7/1/2020 6:47 PM, Vincent Guittot wrote:
> On Wed, 1 Jul 2020 at 12:07, Zijun Hu <zijuhu@...eaurora.org> wrote:
>>
>> sched_entity @se not static @left should be compared
>> to pick up @cfs_rq->next.
> 
> Could you elaborate why ?
> 
> left is the leftmost sched_entity and the one that should be used.
> 
> se != left means that left should be skipped after a yield and the
> next se in the rbtree is not "far" from left although it has higher
> vruntime
> 
> if we finally want to use last or next instead of se, we must ensure
> that they are still not "far" from left otherwise you can promote a
> sched entity that ends up having a really high vruntime
> 
>>
>> Signed-off-by: Zijun Hu <zijuhu@...eaurora.org>
>> ---
>>  kernel/sched/fair.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 658aa7a2ae6f..4790f2d851ad 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -4452,13 +4452,13 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
>>         /*
>>          * Prefer last buddy, try to return the CPU to a preempted task.
>>          */
>> -       if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1)
>> +       if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, se) < 1)
>>                 se = cfs_rq->last;
>>
>>         /*
>>          * Someone really wants this to run. If it's not unfair, run it.
>>          */
>> -       if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1)
>> +       if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, se) < 1)
>>                 se = cfs_rq->next;
>>
>>         clear_buddies(cfs_rq, se);
>> --
>> The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project
>>