Re: [PATCH] mm, memcg: fix inconsistent oom event behavior

[Yafang Shao <laoar.shao@gmail.com>]

On Tue, Apr 14, 2020 at 1:06 AM Shakeel Butt <shakeelb@google.com> wrote:
>
> On Sun, Apr 12, 2020 at 7:04 AM Yafang Shao <laoar.shao@gmail.com> wrote:
> >
> > A recent commit 9852ae3fe529 ("mm, memcg: consider subtrees in
> > memory.events") changes the behavior of memcg events, which will
> > consider subtrees in memory.events. But oom_kill event is a special one
> > as it is used in both cgroup1 and cgroup2. In cgroup1, it is displayed
> > in memory.oom_control. The file memory.oom_control is in both root memcg
> > and non root memcg, that is different with memory.event as it only in
> > non-root memcg. That commit is okay for cgroup2, but it is not okay for
> > cgroup1 as it will cause inconsistent behavior between root memcg and
> > non-root memcg.
>
> I still couldn't understand the cgroup v1's root vs non_root behavior
> change. The behavior change I see is the hierarchical one i.e.
> MEMCG_OOM_KILL event in the descendant will cause the notification and
> count increment in the ancestors even in the cgroup v1.

For the non-root memcg, its memory.oom_control(oom_kill) includes its
descendants' oom_kill, but for root memcg, it doesn't include its
descendants' oom_kill. That means, memory.oom_control(oom_kill) has
different meanings in different memcgs. That is inconsistent.

[snip]
> I suppose we
> don't want that behavior change in v1.
>

That is another topic. I think this feature is welcomed to cgroup1, if
we can fully support it, for example by adding memory.events.local
into cgroup1 as well, but as far as I know the cgroup1 is frozen.

> > Let's recover the original behavior for cgroup1.
> >
> > Fixes: 9852ae3fe529 ("mm, memcg: consider subtrees in memory.events")
> > Cc: Chris Down <chris@chrisdown.name>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Cc: Shakeel Butt <shakeelb@google.com>
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> > ---
> >  include/linux/memcontrol.h | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> > index 8c340e6b347f..a0ae080a67d1 100644
> > --- a/include/linux/memcontrol.h
> > +++ b/include/linux/memcontrol.h
> > @@ -798,7 +798,8 @@ static inline void memcg_memory_event(struct mem_cgroup *memcg,
> >                 atomic_long_inc(&memcg->memory_events[event]);
> >                 cgroup_file_notify(&memcg->events_file);
> >
> > -               if (cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_LOCAL_EVENTS)
> > +               if (cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_LOCAL_EVENTS ||
> > +                   !cgroup_subsys_on_dfl(memory_cgrp_subsys))
> >                         break;
> >         } while ((memcg = parent_mem_cgroup(memcg)) &&
> >                  !mem_cgroup_is_root(memcg));
> > --
> > 2.18.2
> >


Thanks
Yafang