Re: [regression] cpuset,mm: update tasks' mems_allowed in time (58568d2)

[Miao Xie <miaox@cn.fujitsu.com>]

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.