linux-kernel - Re: [patch] sched: fix uneven per-cpu task

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Date:	Thu, 18 Dec 2008 14:42:00 +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 uneven per-cpu task_group share distribution


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

> On Mon, 2008-12-15 at 23:37 -0800, Ken Chen wrote:
> > While testing CFS scheduler on linux-2.6-tip tree
> > git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip
> > 
> > We found that task which is pinned to a CPU could be starved relative to its
> > allocated fair share.
> > 
> > The per-cpu sched_enetity load share calculation in tg_shares_up /
> > update_group_shares_cpu distributes total task_group's share among all CPUs
> > for a given SD domain, this would dilute task_group's per-cpu share because
> > it distributes share to CPU that even has no load.  The trapped share is now
> > un-consumable and it leads to fair share starvation on the runnable CPU.
> > Peter was right that it is still required for the low level function to make
> > distinction between a boosted share that don't have any load and actual tg
> > share that should be distributed among CPUs in which the tg is running.
> > 
> > Patch to add that boost and we think the scheduler should only boost one
> > times of tg shares over all empty CPU that don't have any load for the
> > specific task_group in order to bound maximum temporary boost that a given
> > task_group can have.
> 
> Yeah I was worried about you removing that boost, but since you seemed
> to be doing serious testing on the thing.. A well, this sounds good.
> 
> > Signed-off-by: Ken Chen <kenchen@...gle.com>
> 
> OK, patch looks good too, thanks!
> 
> Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>

applied to tip/cpus4096. [not in the tip/sched/core branch, because 
cpumask_t changes conflict with Ken's fix so this is the proper ordering.]

	Ingo
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