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Message-ID: <5932bbd6-afe7-416f-b590-c9f5c1c02db5@gmail.com>
Date:   Thu, 16 Nov 2023 19:33:30 +0800
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Subject: Re: [PATCH 1/1] sched/fair: Update min_vruntime in more relaxed way
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To:     Abel Wu <wuyun.abel@bytedance.com>, mingo@redhat.com,
        peterz@infradead.org
Cc:     vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        linux-kernel@vger.kernel.org
References: <20231116105425.84773-1-s921975628@gmail.com>
 <20231116105425.84773-2-s921975628@gmail.com>
 <377e0f71-80f9-4d83-a09d-e86bd61ce9ae@bytedance.com>
From:   Yiwei Lin <s921975628@gmail.com>
In-Reply-To: <377e0f71-80f9-4d83-a09d-e86bd61ce9ae@bytedance.com>
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On 11/16/23 19:16, Abel Wu wrote:
> On 11/16/23 6:54 PM, Yiwei Lin Wrote:
>> As EEVDF adopts lag-based solution which is irrespective of
>> min_vruntime like CFS before, min_vruntime is only used as
>> an offset to avoid overflow on evaluation of avg_vruntime now.
>> Rely on the fact we will always update_curr() before change
>> to cfs_rq, it seems to make sense if we just
>> update_min_vruntime() with update_curr() to reduce the cost.
>>
>> Signed-off-by: Yiwei Lin <s921975628@gmail.com>
>> ---
>>   kernel/sched/fair.c | 20 +-------------------
>>   1 file changed, 1 insertion(+), 19 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 07f555857..5c40adfae 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -3815,17 +3815,8 @@ static void reweight_entity(struct cfs_rq 
>> *cfs_rq, struct sched_entity *se,
>>       enqueue_load_avg(cfs_rq, se);
>>       if (se->on_rq) {
>>           update_load_add(&cfs_rq->load, se->load.weight);
>> -        if (!curr) {
>> -            /*
>> -             * The entity's vruntime has been adjusted, so let's check
>> -             * whether the rq-wide min_vruntime needs updated too. 
>> Since
>> -             * the calculations above require stable min_vruntime 
>> rather
>> -             * than up-to-date one, we do the update at the end of the
>> -             * reweight process.
>> -             */
>> +        if (!curr)
>>               __enqueue_entity(cfs_rq, se);
>> -            update_min_vruntime(cfs_rq);
>> -        }
>>       }
>>   }
>>   @@ -5347,15 +5338,6 @@ dequeue_entity(struct cfs_rq *cfs_rq, struct 
>> sched_entity *se, int flags)
>>         update_cfs_group(se);
>>   -    /*
>> -     * Now advance min_vruntime if @se was the entity holding it back,
>> -     * except when: DEQUEUE_SAVE && !DEQUEUE_MOVE, in this case 
>> we'll be
>> -     * put back on, and if we advance min_vruntime, we'll be placed 
>> back
>> -     * further than we started -- ie. we'll be penalized.
>> -     */
>> -    if ((flags & (DEQUEUE_SAVE | DEQUEUE_MOVE)) != DEQUEUE_SAVE)
>> -        update_min_vruntime(cfs_rq);
>> -
>>       if (cfs_rq->nr_running == 0)
>>           update_idle_cfs_rq_clock_pelt(cfs_rq);
>>   }
>
> For now, core pick of core scheduling relies on min_vruntime to be fresh,
> so please just fix commit eab03c23c2a1 to preserve its original behavior
> by moving update_min_vruntime() to proper position. And behavior change
> can be posted separated.

Sorry for not noticing the requirement on core scheduling and applying 
bad solution. I should take a closer look for the influence when 
changing the approach to update_min_vruntime().

I'll send another patch which just move update_min_vruntime() to the 
right place later on.

>
> BTW it seems unnecessary to include a cover-letter for a single patch.
>
Got it! Still learning how to work with the kernel mailing list. Thanks 
for the kind suggestion!
> Thanks,
>     Abel