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

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Message-ID: <20140711084709.GF20603@laptop.programming.kicks-ass.net>
Date:	Fri, 11 Jul 2014 10:47:09 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Yuyang Du <yuyang.du@...el.com>
Cc:	bsegall@...gle.com, 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
Subject: Re: [PATCH 2/2] sched: Rewrite per entity runnable load average
 tracking

On Fri, Jul 11, 2014 at 07:22:07AM +0800, Yuyang Du wrote:
> On Thu, Jul 10, 2014 at 12:08:59PM +0200, Peter Zijlstra wrote:
> > 
> > Since clock_task is the regular clock minus some local amount, the
> > difference between two regular clock reads is always a strict upper
> > bound on clock_task differences.
> > 
> This is inspiring. Regarding the clock source in load avg tracking,
> should we simply use rq_clock_task instead of cfs_rq_clock_task.

Oh *groan* I forgot about that thing. But no, it obviously doesn't
matter for running time, because if you're throttled you're nor running
and therefore it all doesn't matter, but it can make a huge difference
for blocked time accounting I suppose.

> For the bandwidth control case, just update/increase the last_update_time when
> unthrottled by this throttled time, so the time would look like freezed. Am I
> understanding right?

Yes, it stops the clock when throttled.

> Not sure how much bandwidth control is used, but even not used, every time
> we read cfs_rq_clock_task, will burn useless cycles here.

Yep, nothing much you can do about that.

In any case, it is still the case that a normal clock difference is an
upper bound.
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