linux-kernel - Re: [PATCH 2/2 v4] sched: Rewrite per entity runnable load average tracking

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Date:	Tue, 29 Jul 2014 09:13:42 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	bsegall@...gle.com, mingo@...hat.com, linux-kernel@...r.kernel.org,
	pjt@...gle.com, arjan.van.de.ven@...el.com, len.brown@...el.com,
	rafael.j.wysocki@...el.com, alan.cox@...el.com,
	mark.gross@...el.com, fengguang.wu@...el.com
Subject: Re: [PATCH 2/2 v4] sched: Rewrite per entity runnable load average
 tracking

On Mon, Jul 28, 2014 at 07:19:09PM +0200, Peter Zijlstra wrote:
> > > And here we try and make good on that assumption. The thing I worry
> > > about is what happens if the machine is entirely idle...
> > >
> > > What guarantees an semi up-to-date cfs_rq->avg.last_update_time.
> > 
> > update_blocked_averages I think should do just as good a job as the old
> > code, which isn't perfect but is about as good as you can get worst case.
> 
> Right, that's called from rebalance_domains() which should more or less
> update this value on tick boundaries or thereabouts for most 'active'
> cpus.
> 
> But if the entire machine is idle, the first wakeup (if its a x-cpu one)
> might see a very stale timestamp.
> 
> If we can fix that, that would be good I suppose, but I'm not
> immediately seeing something pretty there, but you're right, it'd not be
> worse than the current situation.

It matters time is up-to-date before load_avg is actually used. So yes, we should
have already achieved that.
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