On Wed 22-04-20 08:54:26, Johannes Weiner 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@xxxxxxxxxxxxxx>
> > Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> > Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
> > ---
> > 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;
> }
OK, I was not aware of this restriction. Under these circumstances
special casing in memcg_memory_event is the right approach.
> 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)) &&
--
Michal Hocko
SUSE Labs
