From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx187.postini.com [74.125.245.187]) by kanga.kvack.org (Postfix) with SMTP id 039E26B004D for ; Fri, 30 Nov 2012 04:42:36 -0500 (EST) Message-ID: <50B87F84.7040206@parallels.com> Date: Fri, 30 Nov 2012 13:42:28 +0400 From: Glauber Costa MIME-Version: 1.0 Subject: Re: [PATCHSET cgroup/for-3.8] cpuset: decouple cpuset locking from cgroup core References: <1354138460-19286-1-git-send-email-tj@kernel.org> <50B8263C.7060908@jp.fujitsu.com> <50B875B4.2020507@parallels.com> <20121130092435.GD29317@dhcp22.suse.cz> In-Reply-To: <20121130092435.GD29317@dhcp22.suse.cz> Content-Type: text/plain; charset="ISO-8859-1" Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Michal Hocko Cc: Kamezawa Hiroyuki , Tejun Heo , lizefan@huawei.com, paul@paulmenage.org, containers@lists.linux-foundation.org, cgroups@vger.kernel.org, peterz@infradead.org, bsingharora@gmail.com, hannes@cmpxchg.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org On 11/30/2012 01:24 PM, Michal Hocko wrote: > On Fri 30-11-12 13:00:36, Glauber Costa wrote: >> On 11/30/2012 07:21 AM, Kamezawa Hiroyuki wrote: >>> (2012/11/29 6:34), Tejun Heo wrote: >>>> Hello, guys. >>>> >>>> Depending on cgroup core locking - cgroup_mutex - is messy and makes >>>> cgroup prone to locking dependency problems. The current code already >>>> has lock dependency loop - memcg nests get_online_cpus() inside >>>> cgroup_mutex. cpuset the other way around. >>>> >>>> Regardless of the locking details, whatever is protecting cgroup has >>>> inherently to be something outer to most other locking constructs. >>>> cgroup calls into a lot of major subsystems which in turn have to >>>> perform subsystem-specific locking. Trying to nest cgroup >>>> synchronization inside other locks isn't something which can work >>>> well. >>>> >>>> cgroup now has enough API to allow subsystems to implement their own >>>> locking and cgroup_mutex is scheduled to be made private to cgroup >>>> core. This patchset makes cpuset implement its own locking instead of >>>> relying on cgroup_mutex. >>>> >>>> cpuset is rather nasty in this respect. Some of it seems to have come >>>> from the implementation history - cgroup core grew out of cpuset - but >>>> big part stems from cpuset's need to migrate tasks to an ancestor >>>> cgroup when an hotunplug event makes a cpuset empty (w/o any cpu or >>>> memory). >>>> >>>> This patchset decouples cpuset locking from cgroup_mutex. After the >>>> patchset, cpuset uses cpuset-specific cpuset_mutex instead of >>>> cgroup_mutex. This also removes the lockdep warning triggered during >>>> cpu offlining (see 0009). >>>> >>>> Note that this leaves memcg as the only external user of cgroup_mutex. >>>> Michal, Kame, can you guys please convert memcg to use its own locking >>>> too? >>>> >>> >>> Hmm. let me see....at quick glance cgroup_lock() is used at >>> hierarchy policy change >>> kmem_limit >>> migration policy change >>> swapiness change >>> oom control >>> >>> Because all aboves takes care of changes in hierarchy, >>> Having a new memcg's mutex in ->create() may be a way. >>> >>> Ah, hm, Costa is mentioning task-attach. is the task-attach problem in memcg ? >>> >> >> We disallow the kmem limit to be set if a task already exists in the >> cgroup. So we can't allow a new task to attach if we are setting the limit. > > This is racy without additional locking, isn't it? > Speaking of it: Tejun's tree still lacks the kmem bits. How hard would it be for you to merge his branch into a temporary branch of your tree? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org