Re: [PATCH 1/2 v2] memcg: make oom_lock 0 and 1 based rather than coutner

Michal Hocko
2011-08-09T15:22:56Z

On Tue 09-08-11 16:03:12, Johannes Weiner wrote:
> On Wed, Jul 13, 2011 at 01:05:49PM +0200, Michal Hocko wrote:
> > @@ -1803,37 +1806,83 @@ static int mem_cgroup_hierarchical_reclaim(struct mem_cgroup *root_mem,
> > /*
> > * Check OOM-Killer is already running under our hierarchy.
> > * If someone is running, return false.
> > + * Has to be called with memcg_oom_mutex
> > */
> > static bool mem_cgroup_oom_lock(struct mem_cgroup *mem)
> > {
> > - int x, lock_count = 0;
> > - struct mem_cgroup *iter;
> > + int lock_count = -1;
> > + struct mem_cgroup *iter, *failed = NULL;
> > + bool cond = true;
> >
> > - for_each_mem_cgroup_tree(iter, mem) {
> > - x = atomic_inc_return(&iter->oom_lock);
> > - lock_count = max(x, lock_count);
> > + for_each_mem_cgroup_tree_cond(iter, mem, cond) {
> > + bool locked = iter->oom_lock;
> > +
> > + iter->oom_lock = true;
> > + if (lock_count == -1)
> > + lock_count = iter->oom_lock;
> > + else if (lock_count != locked) {
> > + /*
> > + * this subtree of our hierarchy is already locked
> > + * so we cannot give a lock.
> > + */
> > + lock_count = 0;
> > + failed = iter;
> > + cond = false;
> > + }
>
> I noticed system-wide hangs during a parallel/hierarchical memcg test
> and found that a single task with a central i_mutex held was sleeping
> on the memcg oom waitqueue, stalling everyone else contending for that
> same inode.

Nasty. Thanks for reporting and fixing this. The condition is screwed
totally :/

>
> The problem is the above code, which never succeeds in hierarchies
> with more than one member. The first task going OOM tries to oom lock
> the hierarchy, fails, goes to sleep on the OOM waitqueue with the
> mutex held, without anybody actually OOM killing anything to make
> progress.
>
> Here is a patch that rectified things for me.
>
> ---
> From c4b52cbe01ed67d6487a96850400cdf5a9de91aa Mon Sep 17 00:00:00 2001
> From: Johannes Weiner <jweiner@redhat.com>
> Date: Tue, 9 Aug 2011 15:31:30 +0200
> Subject: [patch] memcg: fix hierarchical oom locking
>
> Commit "79dfdac memcg: make oom_lock 0 and 1 based rather than
> counter" tried to oom lock the hierarchy and roll back upon
> encountering an already locked memcg.
>
> The code is pretty confused when it comes to detecting a locked memcg,
> though, so it would fail and rollback after locking one memcg and
> encountering an unlocked second one.
>
> The result is that oom-locking hierarchies fails unconditionally and
> that every oom killer invocation simply goes to sleep on the oom
> waitqueue forever. The tasks practically hang forever without anyone
> intervening, possibly holding locks that trip up unrelated tasks, too.
>
> Signed-off-by: Johannes Weiner <jweiner@redhat.com>

Looks good. Thanks!
Just a minor nit about done label bellow.

Acked-by: Michal Hocko <mhocko@suse.cz>
> ---
> mm/memcontrol.c | 14 ++++----------
> 1 files changed, 4 insertions(+), 10 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 930de94..649c568 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1841,25 +1841,19 @@ static int mem_cgroup_hierarchical_reclaim(struct mem_cgroup *root_mem,
> */
> static bool mem_cgroup_oom_lock(struct mem_cgroup *mem)
> {
> - int lock_count = -1;

Yes, the whole lock_count thingy is just stupid. We care just about all
or nothing and state of the first is really not important.

> struct mem_cgroup *iter, *failed = NULL;
> bool cond = true;
>
> for_each_mem_cgroup_tree_cond(iter, mem, cond) {
> - bool locked = iter->oom_lock;
> -
> - iter->oom_lock = true;
> - if (lock_count == -1)
> - lock_count = iter->oom_lock;
> - else if (lock_count != locked) {
> + if (iter->oom_lock) {
> /*
> * this subtree of our hierarchy is already locked
> * so we cannot give a lock.
> */
> - lock_count = 0;
> failed = iter;
> cond = false;
> - }
> + } else
> + iter->oom_lock = true;
> }
>
> if (!failed)

We can return here and get rid of done label.

> @@ -1878,7 +1872,7 @@ static bool mem_cgroup_oom_lock(struct mem_cgroup *mem)
> iter->oom_lock = false;
> }
> done:
> - return lock_count;
> + return failed == NULL;
> }
>
> /*
> --
> 1.7.6

--
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9
Czech Republic