linux-kernel - Re: volanoMark regression with kernel 2.6.26-rc1

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Date:	Thu, 15 May 2008 10:41:38 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	vatsa@...ux.vnet.ibm.com
Cc:	Mike Galbraith <efault@....de>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Aneesh Kumar KV <aneesh.kumar@...ux.vnet.ibm.com>
Subject: Re: volanoMark regression with kernel 2.6.26-rc1

On Thu, 2008-05-15 at 13:50 +0530, Srivatsa Vaddagiri wrote:
> On Wed, May 14, 2008 at 05:12:47PM +0200, Peter Zijlstra wrote:
> > A few other things I found that make a significant difference:
> 
> One more observation: access to aggregate()->rq_weight etc arent
> correctly synchronized i.e while a cpu is doing a aggregate_walk_tree()
> in a domain, and thus possibly modifying rq_weight, load etc, other cpus could 
> be concurrently accessing the same data. As a result, its possible to
> see inconsistent rq_weight, load, task_weight combination?

Yes - and that should not be too big an issue as long as we can deal
with it.

Any number we'll put to it will be based on a snapshot of the state so
we're wrong no matter what we do. The trick is trying to keep sane.

My current stack on top of sched-devel:

http://programming.kicks-ass.net/kernel-patches/sched-smp-group-fixes/

I've found that:
http://programming.kicks-ass.net/kernel-patches/sched-smp-group-fixes/sched-agg-update-move_tasks.patch

was sufficient to deal with all the anomalities I've found so far.

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