linux-kernel - Re: [PATCH] sched: sync with the cfs

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Date:	Thu, 13 Aug 2015 16:19:04 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	Yuyang Du <yuyang.du@...el.com>
Cc:	mingo@...nel.org, peterz@...radead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: sync with the cfs_rq when changing sched class

On Thu, Aug 13, 2015 at 06:41:45AM +0800, Yuyang Du wrote:
> On Thu, Aug 13, 2015 at 02:55:55PM +0900, byungchul.park@....com wrote:
> > 
> > currently, a task load is synced with its cfs_rq, only when it
> > leaves from fair class. we also need to sync it with cfs_rq when
> > it returns back to fair class, too.
>  
> Syncing it at the time it is switched to fair is not necessary, because
> since last_update_time if it has ever been updated, the load has become
> random, IOW, useless. So we simply leave it unattended, and let itself
> merge in the system.

hello,

i agree with you almost. it would have a meaningless value over time,
while it has a meaningful value as soon as it leaved that cfs_rq.

however, IMHO, nobody know when a task is switched between sched classes.
i think it would be better that we consider that load rather than leave
it unattended, even though, of course, in both of cases __update_load_avg()
will dacay and fix it over time.

shouldn't we consider it?

1. case returning back to fair class very soon:
original code cannot reflect the task load to cfs_rq, while
patched code can reflect the task load to cfs_rq.

2. case returning back to fair class after long:
original code adds 0 to cfs_rq and let __update_load_avg() fix it, while
patched code adds a meaningless value to cfs_rq and let
__update_load_avg() fix it, afterall these become same.

> 
> >  
> >  #ifdef CONFIG_SMP
> >  	/* synchronize task with its prev cfs_rq */
> > -	if (!queued)
> > -		__update_load_avg(cfs_rq->avg.last_update_time, cpu_of(rq_of(cfs_rq)),
> > -				&se->avg, se->on_rq * scale_load_down(se->load.weight),
> > -				cfs_rq->curr == se, NULL);
> > -
> > -	/* remove our load when we leave */
> > -	cfs_rq->avg.load_avg = max_t(long, cfs_rq->avg.load_avg - se->avg.load_avg, 0);
> > -	cfs_rq->avg.load_sum = max_t(s64, cfs_rq->avg.load_sum - se->avg.load_sum, 0);
> > -	cfs_rq->avg.util_avg = max_t(long, cfs_rq->avg.util_avg - se->avg.util_avg, 0);
> > -	cfs_rq->avg.util_sum = max_t(s32, cfs_rq->avg.util_sum - se->avg.util_sum, 0);
> > +	detach_entity_load_avg(cfs_rq, se);
> >  #endif
> 
> You changed the logic.

yes, i changed it. but i think that calling __update_load_avg() is not
a problem even in case of "queued == 1". so i didn't think that change
seriously.

wrong? :(

thanks,
byungchul

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