On Wed 22-04-20 07:06:43, 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.
OK, this makes sense to me. Cgroup v1 really had local semantic and
9852ae3fe529 changed it unintentionally. I think it is reasonable to use
the CGRP_ROOT_MEMORY_LOCAL_EVENTS which denotes this mode but I will
defer to cgroup maintainers. Maybe there are some other side effects
which I am not aware of that would make this more awkward than a special
case for cgroup v1
> 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>
Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> 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.
> + */
I do not think the comment has to be so specific about oom events
behavior. I would just go with
/*
* Cgroup v1 has traditionally had local semantic for
* event counters. Cgroup v2 changed that to a
* hierarchical behavior. This is expressed by
* CGRP_ROOT_MEMORY_LOCAL_EVENTS in the cgroup core.
*/
> + cgrp_dfl_root.flags |= CGRP_ROOT_MEMORY_LOCAL_EVENTS;
> + }
> }
>
> static int seq_puts_memcg_tunable(struct seq_file *m, unsigned long value)
> --
> 2.18.2
--
Michal Hocko
SUSE Labs
