linux-kernel - Re: [PATCH] sched/pelt: sync util/runnable

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Date:   Fri, 24 Apr 2020 10:41:44 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.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] sched/pelt: sync util/runnable_sum with PELT window when
 propagating

On 24/04/2020 09:37, Vincent Guittot wrote:
> On Thu, 23 Apr 2020 at 21:29, Peter Zijlstra <peterz@...radead.org> wrote:
>>
>> On Wed, Apr 22, 2020 at 05:14:01PM +0200, Vincent Guittot wrote:
>>> update_tg_cfs_util/runnable() propagate the impact of the attach/detach of
>>> an entity down into the cfs_rq hierarchy which must keep the sync with
>>> the current pelt window.
>>>
>>> Even if we can't sync child rq and its group se, we can sync the group se
>>> and parent cfs_rq with current PELT window. In fact, we must keep them sync
>>> in order to stay also synced with others se and group se that are already
>>> attached to the cfs_rq.
>>>
>>> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
>>> ---
>>>  kernel/sched/fair.c | 26 ++++++--------------------
>>>  1 file changed, 6 insertions(+), 20 deletions(-)
>>>
>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>> index 02f323b85b6d..ca6aa89c88f2 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -3441,52 +3441,38 @@ static inline void
>>>  update_tg_cfs_util(struct cfs_rq *cfs_rq, struct sched_entity *se, struct cfs_rq *gcfs_rq)
>>>  {
>>>       long delta = gcfs_rq->avg.util_avg - se->avg.util_avg;
>>> +     u32 divider = LOAD_AVG_MAX - 1024 + cfs_rq->avg.period_contrib;
>>>
>>>       /* Nothing to update */
>>>       if (!delta)
>>>               return;
>>>
>>> -     /*
>>> -      * The relation between sum and avg is:
>>> -      *
>>> -      *   LOAD_AVG_MAX - 1024 + sa->period_contrib
>>> -      *
>>> -      * however, the PELT windows are not aligned between grq and gse.
>>> -      */
>>
>> Instead of deleting this, could we perhaps extend it?
> 
> In fact, this is not the only place in fair.c that uses this rule to
> align _avg and _sum but others don't have any special comment.
> 
> I can add a more detailed description of this relation for
> ___update_load_avg() in pelt.c and make a ref to this in all places in
> fair.c that use this rule which are :
> - update_tg_cfs_util
> - update_tg_cfs_runnable
> - update_cfs_rq_load_avg
> - attach_entity_load_avg
> - reweight_entity

But IMHO the

"* however, the PELT windows are not aligned between grq and gse."

should only apply to update_tg_cfs_util() and update_tg_cfs_runnable().
And attach_entity_load_avg() (for cfs_rq and se).

They seem to be special since we derive divider from a cfs_rq PELT value
and use it for a se PELT value.

I assume this fact is specifically worth highlighting with a comment. I
mean the fact we can do this because the decay windows are actually aligned.