linux-kernel - Re: [RFC] cpuset update_cgroup_cpus

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Date:	Mon, 15 Oct 2007 23:21:08 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Paul Jackson <pj@....com>
cc:	menage@...gle.com, nickpiggin@...oo.com.au, a.p.zijlstra@...llo.nl,
	balbir@...ux.vnet.ibm.com, linux-kernel@...r.kernel.org,
	clg@...ibm.com, ebiederm@...ssion.com, containers@...ts.osdl.org,
	serue@...ibm.com, svaidy@...ux.vnet.ibm.com,
	akpm@...ux-foundation.org, xemul@...nvz.org
Subject: Re: [RFC] cpuset update_cgroup_cpus_allowed

On Mon, 15 Oct 2007, Paul Jackson wrote:

> My solution may be worse than that.  Because set_cpus_allowed() will
> fail if asked to set a non-overlapping cpumask, my solution could never
> terminate.  If asked to set a cpusets cpus to something that went off
> line right then, this I'd guess this code could keep looping forever,
> looking for cpumasks that didn't match, and then not noticing that it
> was failing to set them so as they would match.
> 

Why can't you just add a helper function to sched.c:

	void set_hotcpus_allowed(struct task_struct *task,
				 cpumask_t cpumask)
	{
		mutex_lock(&sched_hotcpu_mutex);
		set_cpus_allowed(task, cpumask);
		mutex_unlock(&sched_hotcpu_mutex);
	}

And then change each task's cpus_allowed via that function instead of 
set_cpus_allowed() directly?

You don't need to worry about making the task->cpuset->cpus_allowed 
assignment a critical section because common_cpu_mem_hotplug_unplug() will 
remove any hot-unplugged cpus from each cpuset's cpus_allowed in the 
hierarchy.

Your loop will still need to be reworked so that cgroup_iter_{start,end}() 
are not reinvoked unnecessarily and you rely only on cgroup_iter_next() 
returning NULL to determine when you've gone through the entire list.  
There's no need to go back and check the cpus_allowed of tasks you've 
already called set_cpus_allowed() on either directly or indirectly via my 
helper function above.

		David
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