From: Oleg Nesterov Subject: Re: [PATCH 3/7] memcg: immigrate charges only when a threadgroup leader is moved Date: Fri, 22 May 2015 18:29:00 +0200 Message-ID: <20150522162900.GA8955@redhat.com> References: <1431978595-12176-1-git-send-email-tj@kernel.org> <1431978595-12176-4-git-send-email-tj@kernel.org> <20150519121321.GB6203@dhcp22.suse.cz> <20150519212754.GO24861@htj.duckdns.org> <20150520131044.GA28678@dhcp22.suse.cz> <20150520132158.GB28678@dhcp22.suse.cz> <20150520175302.GA7287@redhat.com> <20150520202221.GD14256@dhcp22.suse.cz> <20150521192716.GA21304@redhat.com> <20150522093639.GE5109@dhcp22.suse.cz> Mime-Version: 1.0 Return-path: Content-Disposition: inline In-Reply-To: <20150522093639.GE5109-2MMpYkNvuYDjFM9bn6wA6Q@public.gmane.org> Sender: cgroups-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org List-ID: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Michal Hocko Cc: Tejun Heo , lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org On 05/22, Michal Hocko wrote: > > On Thu 21-05-15 21:27:16, Oleg Nesterov wrote: > > On 05/20, Michal Hocko wrote: > > > > > > On Wed 20-05-15 19:53:02, Oleg Nesterov wrote: > > > > > > > > Yes, yes, the group leader can't go away until the whole thread-group dies. > > > > > > OK, then we should have a guarantee that mm->owner is always thread > > > group leader, right? > > > > No, please note that the exiting leader does exit_mm()->mm_update_next_owner() > > and this changes mm->owner. > > I am confused now. Yeah it changes the owner but the new one will be > again the thread group leader, right? Why? In the likely case (if CLONE_VM without CLONE_THREAD was not used) the last for_each_process() in mm_update_next_owner() will find another thread from the same group. Oh. I think mm_update_next_owner() needs some cleanups. Perhaps I'll send the patch today. > > Btw, this connects to other potential cleanups... task_struct->mm looks > > a bit strange, we probably want to move it into signal_struct->mm and > > make exit_mm/etc per-process. But this is not trivial, and off-topic. > > I am not sure this is a good idea but I would have to think about this > some more. Let's not distract this email thread and discuss it in a > separate thread please. Yes, yes. I mentioned this to explain that we can't keep the exited leader as mm->owner in any case. And report that get_mem_cgroup_from_mm() looks racy. Oleg.