Re: [BUGFIX][PATCH] memcg: fix oom killer kills a task in other cgroup

[Minchan Kim <minchan.kim@gmail.com>]

Hi, Kame.

On Fri, Feb 5, 2010 at 9:39 AM, KAMEZAWA Hiroyuki
<kamezawa.hiroyu@jp.fujitsu.com> wrote:
> Please take this patch in different context with recent discussion.
> This is a quick-fix for a terrible bug.
>
> This patch itself is against mmotm but can be easily applied to mainline or
> stable tree, I think. (But I don't CC stable tree until I get ack.)
>
> ==
> Now, oom-killer kills process's chidlren at first. But this means
> a child in other cgroup can be killed. But it's not checked now.
>
> This patch fixes that.
>
> CC: Balbir Singh <balbir@linux.vnet.ibm.com>
> CC: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> ---
>  mm/oom_kill.c |    3 +++
>  1 file changed, 3 insertions(+)
>
> Index: mmotm-2.6.33-Feb03/mm/oom_kill.c
> ===================================================================
> --- mmotm-2.6.33-Feb03.orig/mm/oom_kill.c
> +++ mmotm-2.6.33-Feb03/mm/oom_kill.c
> @@ -459,6 +459,9 @@ static int oom_kill_process(struct task_
>        list_for_each_entry(c, &p->children, sibling) {
>                if (c->mm == p->mm)
>                        continue;
> +               /* Children may be in other cgroup */
> +               if (mem && !task_in_mem_cgroup(c, mem))
> +                       continue;
>                if (!oom_kill_task(c))
>                        return 0;
>        }
>
> --

I am worried about latency of OOM at worst case.
I mean that task_in_mem_cgroup calls task_lock of child.
We have used task_lock in many place.
Some place task_lock hold and then other locks.
For example, exit_fs held task_lock and try to hold write_lock of fs->lock.
If child already hold task_lock and wait to write_lock of fs->lock, OOM latency
is dependent of fs->lock.

I am not sure how many usecase is also dependent of other locks.
If it is not as is, we can't make sure in future.

So How about try_task_in_mem_cgroup?
If we can't hold task_lock, let's continue next child.


-- 
Kind regards,
Minchan Kim

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