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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Message-ID: <20150820235630.GK24261@byungchulpark-X58A-UD3R>
Date:	Fri, 21 Aug 2015 08:56:30 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	Peter Zijlstra <peterz@...radead.org>, yuyang.du@...el.com
Cc:	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 11:13:23PM +0200, Peter Zijlstra wrote:
> 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.
> 
> Right, I noticed the same yesterday when I took a second look at that
> stuff. It was a little confusing indeed.

don't couple functions to each other between functions. i believe it's
acceptable if you look at each function itself.

in other words, attach_entity_load_avg() must call __update_load_avg()
with the condition, whoever calls attach_entity_load_avg(). and
enqueue_entity_load_avg(), too.

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