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Date: Mon, 24 Mar 2008 23:03:04 +0530 From: Balbir Singh <balbir@...ux.vnet.ibm.com> To: Paul Menage <menage@...gle.com> CC: linux-mm@...ck.org, Hugh Dickins <hugh@...itas.com>, Sudhir Kumar <skumar@...ux.vnet.ibm.com>, YAMAMOTO Takashi <yamamoto@...inux.co.jp>, lizf@...fujitsu.com, linux-kernel@...r.kernel.org, taka@...inux.co.jp, David Rientjes <rientjes@...gle.com>, Pavel Emelianov <xemul@...nvz.org>, Andrew Morton <akpm@...ux-foundation.org>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> Subject: Re: [RFC][-mm] Memory controller add mm->owner Paul Menage wrote: > On Mon, Mar 24, 2008 at 9:21 AM, Balbir Singh <balbir@...ux.vnet.ibm.com> wrote: >> > Also, if mm->owner exits but mm is still alive (unlikely, but could >> > happen with weird custom threading libraries?) then we need to >> > reassign mm->owner to one of the other users of the mm (by looking >> > first in the thread group, then among the parents/siblings/children, >> > and then among all processes as a last resort?) >> > >> >> The comment in __exit_signal states that >> >> "The group leader stays around as a zombie as long >> as there are other threads. When it gets reaped, >> the exit.c code will add its counts into these totals." > > Ah, that's useful to know. > >> Given that the thread group leader stays around, do we need to reassign >> mm->owner? Do you do anything special in cgroups like cleanup the >> task_struct->css->subsys_state on exit? >> > > OK, so we don't need to handle this for NPTL apps - but for anything > still using LinuxThreads or manually constructed clone() calls that > use CLONE_VM without CLONE_PID, this could still be an issue. CLONE_PID?? Do you mean CLONE_THREAD? For the case you mentioned, mm->owner is a moving target and we don't want to spend time finding the successor, that can be expensive when threads start exiting one-by-one quickly and when the number of threads are high. I wonder if there is an efficient way to find mm->owner in that case. (Also I > guess there's the case of someone holding a reference to the mm via a > /proc file?) > Yes, but in that case we'll not be charging/uncharging anything to that mm or the cgroup to which the mm belongs. >> >> - rcu_read_lock(); >> >> - mem = rcu_dereference(mm->mem_cgroup); >> >> + mem = mem_cgroup_from_task(mm->owner); >> > >> > I think we still need the rcu_read_lock(), since mm->owner can move >> > cgroups any time. >> > >> >> OK, so cgroup task movement is protected by RCU, right? I'll check for all >> mm->owner uses. >> > > Yes - cgroup_attach() uses synchronize_rcu() before release the cgroup > mutex. So although you can't guarantee that the cgroup set won't > change if you're just using RCU, you can't guarantee that you're > addressing a still-valid non-destroyed (and of course non-freed) > cgroup set. > Yes, I understand that part of RCU. -- Warm Regards, Balbir Singh Linux Technology Center IBM, ISTL -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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