From mboxrd@z Thu Jan 1 00:00:00 1970 From: Balbir Singh Subject: Re: [RFC][PATCH] cgroup_threadgroup_rwsem - affects scalability and OOM Date: Tue, 9 Aug 2016 23:57:03 +1000 Message-ID: <20160809135703.GA11823@350D> References: <4717ef90-ca86-4a34-c63a-94b8b4bfaaec@gmail.com> <57A99BCB.6070905@huawei.com> Reply-To: bsingharora@gmail.com Mime-Version: 1.0 Return-path: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=date:from:to:cc:subject:message-id:reply-to:references:mime-version :content-disposition:in-reply-to:user-agent; bh=D1HEFcVskDkHt7YGuNz0uW43PYL+FatSWyyJXzhA964=; b=X/Q9m7iT2fy3WzoEsHthrn1h/gguEKfWvw5V2QLMC4CWOPzVZT/LW/dAsUewvBN6k6 YhzQ50KfbJWKF09nO2TelelrfUVWZGk2h+qFeKe7tIYYCrjQTdvW3GnXHQy+Tbnma3Cm /Luy+6TdbfT4zEGZSXyHozBizrVFn7Ii9PTuk+KBq1g/frKlthR61M1Rbs/5+7Ew9hhD BZhVtdMcg1E73VcCtz0G9OnbSLkGt1fCZCey5a7krml8cCo5ZyinaFFv7gnCIRSYaLGA eaB07ZuuLTMUckobS1WV6hsChbLfyt2dsGh7hAxxY2FBEk/q0KesUVaT+TwVK/4Vmm4e Phug== Content-Disposition: inline In-Reply-To: <57A99BCB.6070905@huawei.com> Sender: owner-linux-mm@kvack.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Zefan Li Cc: Balbir Singh , cgroups@vger.kernel.org, Oleg Nesterov , Andrew Morton , Tejun Heo , "linux-mm@kvack.org" On Tue, Aug 09, 2016 at 05:00:59PM +0800, Zefan Li wrote: > > This almost stalls the system, this patch moves the threadgroup_change_begin > > from before cgroup_fork() to just before cgroup_canfork(). Ideally we shouldn't > > have to worry about threadgroup changes till the task is actually added to > > the threadgroup. This avoids having to call reclaim with cgroup_threadgroup_rwsem > > held. > > > > There are other theoretical issues with this semaphore > > > > systemd can do > > > > 1. cgroup_mutex (cgroup_kn_lock_live) > > 2. cgroup_threadgroup_rwsem (W) (__cgroup_procs_write) > > > > and other threads can go > > > > 1. cgroup_threadgroup_rwsem (R) (copy_process) > > 2. mem_cgroup_iter (as a part of reclaim) (cgroup_mutex -- rcu lock or cgroup_mutex) > > > > However, I've not examined them in too much detail or looked at lockdep > > wait chains for those paths. > > > > I am sure there is a good reason for placing cgroup_threadgroup_rwsem > > where it is today and I might be missing something. I am also surprised > > no-one else has run into it so far. > > > > Comments? > > > > We used to use cgroup_threadgroup_rwsem for syncronization between threads > in the same threadgroup, but now it has evolved to ensure atomic operations > across multi processes. > Yes and it seems incorrect > For example, I'm trying to fix a race. See https://lkml.org/lkml/2016/8/8/900 > > And the fix kind of relies on the fact that cgroup_post_fork() is placed > inside the read section of cgroup_threadgroup_rwsem, so that cpuset_fork() > won't race with cgroup migration. > My patch retains that behaviour, before ss->fork() is called we hold the cgroup_threadgroup_rwsem, in fact it is held prior to ss->can_fork() Balbir -- 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