linux-kernel - Re: [PATCH cgroup/for-4.5-fixes] cpuset: make mm migration asynchronous

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Date:	Fri, 22 Jan 2016 10:23:09 -0500
From:	Tejun Heo <tj@...nel.org>
To:	Li Zefan <lizefan@...wei.com>, Johannes Weiner <hannes@...xchg.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	linux-s390 <linux-s390@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	cgroups@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH cgroup/for-4.5-fixes] cpuset: make mm migration
 asynchronous

On Tue, Jan 19, 2016 at 12:18:41PM -0500, Tejun Heo wrote:
> If "cpuset.memory_migrate" is set, when a process is moved from one
> cpuset to another with a different memory node mask, pages in used by
> the process are migrated to the new set of nodes.  This was performed
> synchronously in the ->attach() callback, which is synchronized
> against process management.  Recently, the synchronization was changed
> from per-process rwsem to global percpu rwsem for simplicity and
> optimization.
> 
> Combined with the synchronous mm migration, this led to deadlocks
> because mm migration could schedule a work item which may in turn try
> to create a new worker blocking on the process management lock held
> from cgroup process migration path.
> 
> This heavy an operation shouldn't be performed synchronously from that
> deep inside cgroup migration in the first place.  This patch punts the
> actual migration to an ordered workqueue and updates cgroup process
> migration and cpuset config update paths to flush the workqueue after
> all locks are released.  This way, the operations still seem
> synchronous to userland without entangling mm migration with process
> management synchronization.  CPU hotplug can also invoke mm migration
> but there's no reason for it to wait for mm migrations and thus
> doesn't synchronize against their completions.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Reported-and-tested-by: Christian Borntraeger <borntraeger@...ibm.com>
> Cc: stable@...r.kernel.org # v4.4+

Applied to cgroup/for-4.5-fixes.

Thanks.

-- 
tejun