linux-kernel - Re: [PATCH v3 4/5] sched: sync a se with its cfs_rq when switching sched class to fair class

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Date:	Thu, 20 Aug 2015 15:12:27 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	Yuyang Du <yuyang.du@...el.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 4/5] sched: sync a se with its cfs_rq when switching
 sched class to fair class

On Thu, Aug 20, 2015 at 04:11:06AM +0800, Yuyang Du wrote:
> On Wed, Aug 19, 2015 at 07:12:41PM +0200, Peter Zijlstra wrote:
> > On Wed, Aug 19, 2015 at 03:47:15PM +0900, byungchul.park@....com wrote:
> > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > > index 1be042a..3419f6c 100644
> > > --- a/kernel/sched/fair.c
> > > +++ b/kernel/sched/fair.c
> > > @@ -2711,6 +2711,17 @@ static inline void update_load_avg(struct sched_entity *se, int update_tg)
> > >  
> > >  static void attach_entity_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
> > >  {
> > > +	/*
> > > +	 * in case of migration and cgroup-change, more care should be taken
> > > +	 * because se's cfs_rq was changed, that means calling __update_load_avg
> > > +	 * with new cfs_rq->avg.last_update_time is meaningless. so we skip the
> > > +	 * update here. we have to update it with prev cfs_rq just before changing
> > > +	 * se's cfs_rq, and get here soon.
> > > +	 */
> > > +	if (se->avg.last_update_time)
> > > +		__update_load_avg(cfs_rq->avg.last_update_time, cpu_of(rq_of(cfs_rq)),
> > > +				&se->avg, 0, 0, NULL);
> > > +
> > >  	se->avg.last_update_time = cfs_rq->avg.last_update_time;
> > >  	cfs_rq->avg.load_avg += se->avg.load_avg;
> > >  	cfs_rq->avg.load_sum += se->avg.load_sum;
> > 
> > you seem to have forgotten to remove the same logic from
> > enqueue_entity_load_avg(), which will now call __update_load_avg()
> > twice.
> 
> In case of enqueue_entity_load_avg(), that seems to be ok.
> 
> However, the problem is that he made it "entangled":
> 
> In enqueue_entity_load_avg():
> 
> 	if (migrated)
> 		attach_entity_load_avg();
> 
> while in attach_entity_load_avg():
> 
> 	if (!migrated)
> 		__update_load_avg();
> 
> so, if attach() is called from enqueue(), that if() is never true.

in case of migration, we should not call __update_load_avg() when attaching,
because se->avg.last_update_time is not meaningless on changed cfs_rq. that
is why attach_entity_load_avg() conditionally calls __update_load_avg().

enqueue() calls attach() only when migrating, so the if() should be never
true. i think it is normal. is it a problem?

> 
> To Byungchul,
> 
> 1) I suggest you not entangle the entire series by mixing problem

i think i didn't mix any problems. i solved each problem indivisually.

>    sovling with code manipulating. That said, it is better you
>    first solve the "move between task group" problem and the
>    switch_to/from problem (if it is a problem, either way, comment

patch 2, 3 solve the former, and patch 4 solves the latter.

>    with your explanation to how you deal with the lost record and why).

how i deal with the lost record, is the "decaying" as you can see easily.
i think it is the most natural way to deal with it.

> 2) After that, make the code cleaner, without change to logic, especially
>    avoid entangling the logic in order to do the code manipulation.

i think i should modify patches if i entangled code, and could you tell me
where i did it?

> 3) If you don't hate upper case letter, use it properly.

i will try it.

thanks,
byungchul

> 
> If it helps.
> 
> Thanks,
> Yuyang
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