linux-kernel - Re: [patch] sched: add locking when update the task_group's cfs

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Date:	Wed, 19 Nov 2008 09:21:46 -0800
From:	Ken Chen <kenchen@...gle.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [patch] sched: add locking when update the task_group's cfs_rq[] 
	array.

On Wed, Nov 19, 2008 at 8:54 AM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> On Tue, 2008-11-18 at 22:48 -0800, Ken Chen wrote:
>> add locking when update the task_group's cfs_rq[] array.  tg_shares_up()
>> can be potentially executed concurrently on multiple CPUs with overlaping
>> cpu mask depending on where task_cpu() was when a task got woken up.  Lack
>> of any locking while redistribute tg->shares over cfs_rq[] array opens up
>> a large window for conflict updates and utimately cause corruptions to the
>> integrity of per cpu cfs_rq shares. Add a tg_lock to protect the operations.
>
> I see why you want to do this, but introducing a global lock makes me
> sad :/

I wholly agree on the scalability.  The bigger the system, the more it
needs to protect the integrity of cfs_rq[]->shares that the sum still
adds up to tg->shares.  Otherwise, the share distributed on each CPU's
cfs_rq might go wildly and indirectly leads to fluctuation of
effective total tg->shares.  However, I have the same doubt that this
will scale on large CPU system.  Does CFS really have to iterate the
whole task_group tree?

- Ken
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