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

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

On Wed, Apr 22, 2020 at 8:54 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Wed, Apr 22, 2020 at 07:06:43AM -0400, 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.
> >
> > Setting CGRP_ROOT_MEMORY_LOCAL_EVENTS for legacy hierarchy by
> > default rather than special casing it somewhere quite deep in the code
> > would be better, per discussion with Michal.
> >
> > Fixes: 9852ae3fe529 ("mm, memcg: consider subtrees in memory.events")
> > Cc: Chris Down <chris@chrisdown.name>
> > Cc: Shakeel Butt <shakeelb@google.com>
> > Cc: Michal Hocko <mhocko@kernel.org>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> > ---
> >  mm/memcontrol.c | 14 ++++++++++++--
> >  1 file changed, 12 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 5beea03dd58a..0f7381bddcee 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -5940,10 +5940,20 @@ static void mem_cgroup_bind(struct cgroup_subsys_state *root_css)
> >        * guarantees that @root doesn't have any children, so turning it
> >        * on for the root memcg is enough.
> >        */
> > -     if (cgroup_subsys_on_dfl(memory_cgrp_subsys))
> > +     if (cgroup_subsys_on_dfl(memory_cgrp_subsys)) {
> >               root_mem_cgroup->use_hierarchy = true;
> > -     else
> > +     } else {
> >               root_mem_cgroup->use_hierarchy = false;
> > +             /*
> > +              * Set CGRP_ROOT_MEMORY_LOCAL_EVENTS for legacy hierarchy
> > +              * by default to avoid inconsistent oom_kill behavior
> > +              * between root memcg and non-root memcg.
> > +              * Regarding default hierarchy, as this flag will be set
> > +              * or cleared later, we don't need to process it in this
> > +              * function.
> > +              */
> > +             cgrp_dfl_root.flags |= CGRP_ROOT_MEMORY_LOCAL_EVENTS;
>
> That will cause problems for people trying to remount. From
> cgroup1_reconfigure():
>
>         /* Don't allow flags or name to change at remount */
>         if ((ctx->flags ^ root->flags) ||
>             (ctx->name && strcmp(ctx->name, root->name))) {
>                 errorfc(fc, "option or name mismatch, new: 0x%x \"%s\", old: 0x%x \"%s\"",
>                        ctx->flags, ctx->name ?: "", root->flags, root->name);
>                 ret = -EINVAL;
>                 goto out_unlock;
>         }
>
> These flags belong to the user, they're read-only to the cgroup
> implementation. Let's not mess with them from a controller.
>
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 4b868e5a687f..e831a90b5506 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -773,6 +773,8 @@ static inline void memcg_memory_event(struct mem_cgroup *memcg,
>                 atomic_long_inc(&memcg->memory_events[event]);
>                 cgroup_file_notify(&memcg->events_file);
>
> +               if (!cgroup_subsys_on_dfl(memory_cgrp_subsys))
> +                       break;
>                 if (cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_LOCAL_EVENTS)
>                         break;
>         } while ((memcg = parent_mem_cgroup(memcg)) &&


Hi Johnanes,

That is what I did in the previous version, see also
https://lore.kernel.org/linux-mm/20200414015952.3590-1-laoar.shao@gmail.com/
.

-- 
Thanks
Yafang