linux-kernel - Re: [patch v4 02/18] sched: select_task_rq

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Date:	Tue, 12 Feb 2013 11:14:49 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Alex Shi <alex.shi@...el.com>
Cc:	torvalds@...ux-foundation.org, mingo@...hat.com,
	tglx@...utronix.de, akpm@...ux-foundation.org,
	arjan@...ux.intel.com, bp@...en8.de, pjt@...gle.com,
	namhyung@...nel.org, efault@....de, vincent.guittot@...aro.org,
	gregkh@...uxfoundation.org, preeti@...ux.vnet.ibm.com,
	viresh.kumar@...aro.org, linux-kernel@...r.kernel.org
Subject: Re: [patch v4 02/18] sched: select_task_rq_fair clean up

On Thu, 2013-01-24 at 11:06 +0800, Alex Shi wrote:
> It is impossible to miss a task allowed cpu in a eligible group.

I suppose your reasoning goes like: tsk->cpus_allowed is protected by
->pi_lock, we hold this, therefore it cannot change and
find_idlest_group() dtrt?

We can then state that this is due to adding proper serialization to
tsk->cpus_allowed.

> And since find_idlest_group only return a different group which
> excludes old cpu, it's also impossible to find a new cpu same as old
> cpu.

Sounds plausible, but I'm not convinced, do we have hard serialization
against hotplug?



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