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Date: Tue, 23 Feb 2010 14:31:51 -0800 (PST)
From: David Rientjes <rientjes@google.com>
To: Miao Xie <miaox@cn.fujitsu.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)
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On Tue, 23 Feb 2010, Miao Xie 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.
> 

That is terribly inconsistent between top_cpuset and all descendants; all 
other cpusets require that task->cpus_allowed be a subset of 
cpu_online_mask, including those descendants that allow all cpus (and all 
mems).
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