From: Kamezawa Hiroyuki Subject: Re: [PATCH v3][0/6] memcg: prevent -ENOMEM in pre_destroy() Date: Thu, 28 Jun 2012 17:33:51 +0900 Message-ID: <4FEC16EF.40408@jp.fujitsu.com> References: <4FACDED0.3020400@jp.fujitsu.com> <20120621202043.GD4642@google.com> <4FE3ADDD.9060908@jp.fujitsu.com> <20120627175818.GM15811@google.com> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20120627175818.GM15811-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii"; format="flowed" To: Tejun Heo Cc: "linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org" , "cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , Michal Hocko , Johannes Weiner , Frederic Weisbecker , Han Ying , Glauber Costa , "Aneesh Kumar K.V" , Andrew Morton , Hiroyuki Kamezawa , Linux Kernel (2012/06/28 2:58), Tejun Heo wrote: > Hello, KAME. > > On Fri, Jun 22, 2012 at 08:27:25AM +0900, Kamezawa Hiroyuki wrote: >> Remaining 20% of work is based on a modification to cgroup layer >> >> How do you think this patch ? (This patch is not tested yet...so >> may have troubles...) I think callers of pre_destory() is not so many... >> >> == >> From a28db946f91f3509d25779e8c5db249506cc4b07 Mon Sep 17 00:00:00 2001 >> From: KAMEZAWA Hiroyuki >> Date: Fri, 22 Jun 2012 08:38:38 +0900 >> Subject: [PATCH] cgroup: keep cgroup_mutex() while calling ->pre_destroy() >> >> In past, memcg's pre_destroy() was verrry slow because of the possibility >> of page reclaiming in it. So, cgroup_mutex() was released before calling >> pre_destroy() callbacks. Now, it's enough fast. memcg just scans the list >> and move pages to other cgroup, no memory reclaim happens. >> Then, we can keep cgroup_mutex() there. >> >> By holding looks, we can avoid following cases >> 1. new task is attached while rmdir(). >> 2. new child cgroup is created while rmdir() >> 3. new task is attached to cgroup and removed from cgroup before >> checking css's count. So, ->destroy() will be called even if >> some trashes by the task remains >> >> (3. is terrible case...even if I think it will not happen in real world..) > > Ooh, once memcg drops the __DEPRECATED_clear_css_refs, cgroup_rmdir() > will mark the cgroup dead before start calling pre_destroy() and none > of the above will happen. > Hm, threads which touches memcg should hold memcg's reference count rather than css. Right ? IIUC, one of reason is a reference from kswapd etc...hm. I'll check it. Thanks, -Kame