linux-kernel - Re: [patch] cpusets: allow PF_THREAD_BOUND kworkers to escape from a cpuset

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Date:	Fri, 23 Sep 2011 06:27:39 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Mike Galbraith <efault@....de>
cc:	Tejun Heo <htejun@...il.com>, Li Zefan <lizf@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Paul Menage <paul@...lmenage.org>
Subject: Re: [patch] cpusets: allow PF_THREAD_BOUND kworkers to escape from
 a cpuset

On Fri, 23 Sep 2011, Mike Galbraith wrote:

> cpusets: allow PF_THREAD_BOUND kworkers to escape from a cpuset
> 
> kworkers can be born in a cpuset, leaving them adrift on an unsinkable ship.
> Allow them to be moved to the root cpuset so the cpuset can be destroyed
> 
> Signed-off-by: Mike Galbraith <efault@....de>
> Acked-by: Li Zefan <lizf@...fujitsu.com>

Did Li ack this version?

> 
> diff --git a/kernel/cpuset.c b/kernel/cpuset.c
> index 10131fd..3769f9e 100644
> --- a/kernel/cpuset.c
> +++ b/kernel/cpuset.c
> @@ -1384,7 +1384,7 @@ static int cpuset_can_attach(struct cgroup_subsys *ss, struct cgroup *cont,
>  	 * set_cpus_allowed_ptr() on all attached tasks before cpus_allowed may
>  	 * be changed.
>  	 */
> -	if (tsk->flags & PF_THREAD_BOUND)
> +	if (tsk->flags & PF_THREAD_BOUND && cont != cont->top_cgroup)
>  		return -EINVAL;
>  
>  	return 0;
> @@ -1426,9 +1426,14 @@ static void cpuset_attach_task(struct cgroup *cont, struct task_struct *tsk)
>  	/*
>  	 * can_attach beforehand should guarantee that this doesn't fail.
>  	 * TODO: have a better way to handle failure here
> +	 * 
> +	 * Special case: bound kthreads born in a cpuset may be moved to
> +	 * the top level cpuset without attempting to diddle their mask.
>  	 */
> -	err = set_cpus_allowed_ptr(tsk, cpus_attach);
> -	WARN_ON_ONCE(err);
> +	if (!(tsk->flags & PF_THREAD_BOUND && cont == cont->top_cgroup)) {
> +		err = set_cpus_allowed_ptr(tsk, cpus_attach);
> +		WARN_ON_ONCE(err);
> +	}
>  
>  	cpuset_change_task_nodemask(tsk, &cpuset_attach_nodemask_to);
>  	cpuset_update_task_spread_flag(cs, tsk);

This doesn't make any sense, the user can now change the cpumask of the 
kworker with cpusets but not with sched_setaffinity().

PF_THREAD_BOUND is set specifically so threads cannot move from the cpu 
that they are bound to, that's why the cpuset code and sched_setaffinity() 
reject such a configuration.  So the problem isn't in the cpuset code or 
scheduler at all, you would need to deal with this in the kworker code by 
either not setting PF_THREAD_BOUND (which, according to the comment, Tejun 
thinks is pretty important) or manage the worker threads in a way such 
that the new cpumask (all cpus, since it's in the root cpuset) actually 
make sense for that kworker.  The cpuset code won't care if the kworker 
has a cpumask of all online cpus.
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