Re: [PATCH v3][0/6] memcg: prevent -ENOMEM in pre_destroy()

[Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>]

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 <kamezawa.hiroyu-+CUm20s59erQFUHtdCDX3A@public.gmane.org>
> 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.

Thanks.

-- 
tejun