Messages in this thread | | | Date | Thu, 15 Jan 2009 10:47:17 +0530 | From | Balbir Singh <> | Subject | Re: [RFC][PATCH 5/4] memcg: don't call res_counter_uncharge when obsolete |
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* KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-01-15 13:52:23]:
> On Thu, 15 Jan 2009 09:47:50 +0530 > Balbir Singh <balbir@linux.vnet.ibm.com> wrote: > > > * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-01-15 12:24:16]: > > > > > On Wed, 14 Jan 2009 19:25:39 +0530 > > > Balbir Singh <balbir@linux.vnet.ibm.com> wrote: > > > > > > > * Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> [2009-01-14 17:51:21]: > > > > > > > > > This is a new one. Please review. > > > > > > > > > > === > > > > > From: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> > > > > > > > > > > mem_cgroup_get ensures that the memcg that has been got can be accessed > > > > > even after the directory has been removed, but it doesn't ensure that parents > > > > > of it can be accessed: parents might have been freed already by rmdir. > > > > > > > > > > This causes a bug in case of use_hierarchy==1, because res_counter_uncharge > > > > > climb up the tree. > > > > > > > > > > Check if the memcg is obsolete, and don't call res_counter_uncharge when obsole. > > > > > > > > > > Signed-off-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> > > > > > > > > I liked the earlier, EBUSY approach that ensured that parents could > > > > not go away if children exist. IMHO, the code has gotten too complex > > > > and has too many corner cases. Time to revisit it. > > > > > > > > > > But I don't like -EBUSY ;) > > > > > > When rmdir() returns -EBUSY even if there are no (visible) children and tasks, > > > our customer will take kdump and send it to me "please explain this kernel bug" > > > > > > I'm sure it will happen ;) > > > > > > > OK, but memory.stat can show why the group is busy and with > > move_to_parent() such issues should not occur right? I'll relook at > > the code. Thanks for your input. > > > Write a shell script as following ? > == > TASKS=`cat /xxx/xxx/xxx/tasks` > if [ -n $TASKS ]; then > echo "there is alive tasks in group /xxx/xxx/xxx/" > fi > > rmdir /xxx/xxx/xxx/ > CODE=$? > if [ $CODE = EBUSY ]; then > investigate why.... > fi > == > I don't want. >
I agree with that.
> I think rmdir() should succeed everywhen "there are no tasks and children". > And that can be done. >
All I am saying is that let rmdir() fail if there are tasks or children, which I suspect cgroup takes care of. The second thing to do would be to add a mem_cgroup_get_hierarchical() and _put_hierarchical() API's so that we can get references all the way up to the parents. My concern is that not calling res_counter_uncharge() can lead to dangling references and bad behaviour.
> With Paul's suggestion, I'll add wait_queue for rmdir of cgroup. >
That might be a good idea and also a good idea to maintain the hierarchy (since we will walk up when we do uncharge) until we know that css reference count is down to 0.
-- Balbir
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