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

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

On Tue, Apr 14, 2020 at 8:53 AM Shakeel Butt <shakeelb@google.com> wrote:
>
> On Mon, Apr 13, 2020 at 5:36 PM Yafang Shao <laoar.shao@gmail.com> wrote:
> >
> > 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.
> >
>
> Please note that after your patch the non-root memcg's
> memory.oom_control(oom_kill) will not include the descendant's
> oom_kill anymore. The non-root and root memcg's
> memory.oom_control(oom_kill) will not be hierarchical anymore but
> consistent. I think that was the intention of the patch, right?
>

Right. If we can't fully support it in cgroup1, then let's don't touch
its original behavior.

> > > > 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