Re: [PATCH v2] mm, memcg: fix inconsistent oom event behavior

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

On Tue, Apr 14, 2020 at 11:23 PM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Mon 13-04-20 21:59:52, Yafang Shao 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.
> >
> > Here's an example on why this behavior is inconsistent in cgroup1.
> >      root memcg
> >      /
> >   memcg foo
> >    /
> > memcg bar
> >
> > Suppose there's an oom_kill in memcg bar, then the oon_kill will be
> >
> >      root memcg : memory.oom_control(oom_kill)  0
> >      /
> >   memcg foo : memory.oom_control(oom_kill)  1
> >    /
> > memcg bar : memory.oom_control(oom_kill)  1
> >
> > 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. Then the user
> > has to know whether the memcg is root or not.
> >
> > If we can't fully support it in cgroup1, for example by adding
> > memory.events.local into cgroup1 as well, then let's don't touch
> > its original behavior. So let's recover the original behavior for cgroup1.
>
> Wthe localevents was mostly cgroup v2 feature. I do not think there was
> an intention to have side effects on the legacy hierarchy. I thought
> this would be the case but it is not apparently. Would it make more
> sense to have CGRP_ROOT_MEMORY_LOCAL_EVENTS for legacy hierarchy by
> default rather than special casing it somewhere quite deep in the code?
>

I had thought about setting CGRP_ROOT_MEMORY_LOCAL_EVENTS by defualt
for cgroup1, but I was not sure whether we should  also expose
memory_localevents in cgroup1_show_options().

> > Fixes: 9852ae3fe529 ("mm, memcg: consider subtrees in memory.events")
> > Cc: Chris Down <chris@chrisdown.name>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Cc: stable@vger.kernel.org
> > Reviewed-by: Shakeel Butt <shakeelb@google.com>
> > 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
>
> --
> Michal Hocko
> SUSE Labs



Thanks
Yafang