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

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Message-ID: <20140709184543.GI9918@twins.programming.kicks-ass.net>
Date:	Wed, 9 Jul 2014 20:45:43 +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 Wed, Jul 09, 2014 at 09:07:53AM +0800, Yuyang Du wrote:
> That is chalenging... Can someone (Peter) grant us a lock of the remote rq? :)

Nope :-).. we got rid of that lock for a good reason.

Also, this is one area where I feel performance really trumps
correctness, we can fudge the blocked load a little. So the
sched_clock_cpu() difference is a strict upper bound on the
rq_clock_task() difference (and under 'normal' circumstances shouldn't
be much off).

So we could simply use a timestamps from dequeue and one from enqueue,
and use that.

As to the remote subtraction, a RMW on another cacheline than the
rq->lock one should be good; esp since we don't actually observe the
per-rq total often (once per tick or so) I think, no?

The thing is, we do not want to disturb scheduling on whatever cpu the
task last ran on if we wake it to another cpu. Taking rq->lock wrecks
that for sure. 

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