linux-kernel - Re: [PATCH 2/2] sched: Rewrite per entity runnable load average tracking

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Message-ID: <20140707200342.GA25653@intel.com>
Date:	Tue, 8 Jul 2014 04:03:42 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...hat.com, linux-kernel@...r.kernel.org,
	rafael.j.wysocki@...el.com, arjan.van.de.ven@...el.com,
	len.brown@...el.com, alan.cox@...el.com, mark.gross@...el.com,
	pjt@...gle.com, fengguang.wu@...el.com,
	Ben Segall <bsegall@...gle.com>
Subject: Re: [PATCH 2/2] sched: Rewrite per entity runnable load average
 tracking

Thanks, Peter.

On Mon, Jul 07, 2014 at 12:46:46PM +0200, Peter Zijlstra wrote:
> 
> That tg->load_avg cacheline is already red hot glowing, and you've just
> increased the amount of updates to it.. That's not going to be pleasant.
> 

Logically, this rewrite updates tg->load_avg as soon as it is changed. But
technically, this is not necessary, as it is only needed to be updated when
it is used before update_cfs_shares() and update_cfs_rq_h_load().

Yes, I can optimize it.

> So here you add the task to the cfs_rq avg when its got migrate in,
> however:
> 
> > @@ -4552,17 +4326,9 @@ migrate_task_rq_fair(struct task_struct *p, int next_cpu)
> >  	struct sched_entity *se = &p->se;
> >  	struct cfs_rq *cfs_rq = cfs_rq_of(se);
> >  
> > +	/* Update task on old CPU, then ready to go (entity must be off the queue) */
> > +	__update_load_avg(cfs_rq_clock_task(cfs_rq), &se->avg, 0);
> > +	se->avg.last_update_time = 0;
> >  
> >  	/* We have migrated, no longer consider this task hot */
> >  	se->exec_start = 0;
> 
> there you don't remove it first..
> 

Yes, I missed it. I wonder what I was thinking. Migration is the only reason
to track task load average...

Yuyang
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