Message-ID: <4B839E9D.8020604@cn.fujitsu.com>
Date: Tue, 23 Feb 2010 17:23:41 +0800
From: Miao Xie <miaox@cn.fujitsu.com>
Reply-To: miaox@cn.fujitsu.com
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To: David Rientjes <rientjes@google.com>
CC: Nick Piggin <npiggin@suse.de>, linux-kernel@vger.kernel.org,
       linux-mm@kvack.org, Lee Schermerhorn <lee.schermerhorn@hp.com>
Subject: Re: [regression] cpuset,mm: update tasks' mems_allowed in time (58568d2)
References: <20100218134921.GF9738@laptop> <alpine.DEB.2.00.1002181302430.13707@chino.kir.corp.google.com> <20100219033126.GI9738@laptop> <alpine.DEB.2.00.1002190143040.6293@chino.kir.corp.google.com> <4B827043.3060305@cn.fujitsu.com> <alpine.DEB.2.00.1002221339160.14426@chino.kir.corp.google.com> <4B838490.1050908@cn.fujitsu.com> <alpine.DEB.2.00.1002230046160.12015@chino.kir.corp.google.com>
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on 2010-2-23 16:55, David Rientjes wrote:
> Cpu hotplug sets top_cpuset's cpus_allowed to cpu_active_mask by default, 
> regardless of what was onlined or offlined.  cpus_attach in the context of 
> your patch (in cpuset_attach()) passes cpu_possible_mask to 
> set_cpus_allowed_ptr() if the task is being attached to top_cpuset, my 
> question was why don't we pass cpu_active_mask instead?  In other words, I 
> think we should do
> 
> 	cpumask_copy(cpus_attach, cpu_active_mask);
> 
> when attached to top_cpuset like my patch did.

If we pass cpu_active_mask to set_cpus_allowed_ptr(), task->cpus_allowed just contains
the online cpus. In this way, if we do cpu hotplug(such as: online some cpu), we must
update cpus_allowed of all tasks in the top cpuset.

But if we pass cpu_possible_mask, we needn't update cpus_allowed of all tasks in the
top cpuset. And when the kernel looks for a cpu for task to run, the kernel will use
cpu_active_mask to filter out offline cpus in task->cpus_allowed. Thus, it is safe.

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