linux-kernel - Re: [patch] sched: fix inconsistency when redistribute per-cpu tg->cfs

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Date:	Wed, 19 Nov 2008 18:39:46 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Ken Chen <kenchen@...gle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [patch] sched: fix inconsistency when redistribute per-cpu
	tg->cfs_rq  shares.


* Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> On Tue, 2008-11-18 at 22:41 -0800, Ken Chen wrote:
> > In the update_shares() path leading to tg_shares_up(), the calculation of
> > per-cpu cfs_rq shares is rather erratic even under moderate task wake up
> > rate.  The problem is that the per-cpu tg->cfs_rq load weight used in the
> > sd_rq_weight aggregation and actual redistribution of the cfs_rq->shares
> > are collected at different time.  Under moderate system load, we've seen
> > quite a bit of variation on the cfs_rq->shares and ultimately wildly
> > affects sched_entity's load weight.
> 
> Another thing we could possibly do is put a low-pass filter on the
> per-cpu load values so that we smooth out the fluctuations, hmm?
> 
> > This patch caches the result of initial per-cpu load weight when doing the
> > sum calculation, and then pass it down to update_group_shares_cpu() for
> > redistributing per-cpu cfs_rq shares.  This allows consistent total cfs_rq
> > shares across all CPUs. It also simplifies the rounding and zero load
> > weight check.
> > 
> > Signed-off-by: Ken Chen <kenchen@...gle.com>
> 
> This does indeed look much better, the cleanup factor alone makes it 
> a worthwhile patch, he fact that is improves behaviour makes it even 
> better :-)
> 
> Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> 
> Thanks Ken!

applied to tip/sched/core (for v2.6.29), thanks guys!

	Ingo
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