Re: [PATCH v2 2/2] mm: Consider subtrees in memory.events

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, Feb 08, 2019 at 10:44:19PM +0000, Chris Down wrote:
> memory.stat and other files already consider subtrees in their output,
> and we should too in order to not present an inconsistent interface.
> 
> The current situation is fairly confusing, because people interacting
> with cgroups expect hierarchical behaviour in the vein of memory.stat,
> cgroup.events, and other files. For example, this causes confusion when
> debugging reclaim events under low, as currently these always read "0"
> at non-leaf memcg nodes, which frequently causes people to misdiagnose
> breach behaviour. The same confusion applies to other counters in this
> file when debugging issues.
> 
> Aggregation is done at write time instead of at read-time since these
> counters aren't hot (unlike memory.stat which is per-page, so it does it
> at read time), and it makes sense to bundle this with the file
> notifications.
> 
> After this patch, events are propagated up the hierarchy:
> 
>     [root@ktst ~]# cat /sys/fs/cgroup/system.slice/memory.events
>     low 0
>     high 0
>     max 0
>     oom 0
>     oom_kill 0
>     [root@ktst ~]# systemd-run -p MemoryMax=1 true
>     Running as unit: run-r251162a189fb4562b9dabfdc9b0422f5.service
>     [root@ktst ~]# cat /sys/fs/cgroup/system.slice/memory.events
>     low 0
>     high 0
>     max 7
>     oom 1
>     oom_kill 1
> 
> As this is a change in behaviour, this can be reverted to the old
> behaviour by mounting with the `memory_localevents` flag set. However,
> we use the new behaviour by default as there's a lack of evidence that
> there are any current users of memory.events that would find this change
> undesirable.
> 
> Signed-off-by: Chris Down <chris@chrisdown.name>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@kernel.org>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Roman Gushchin <guro@fb.com>
> Cc: Dennis Zhou <dennis@kernel.org>
> Cc: linux-kernel@vger.kernel.org
> Cc: cgroups@vger.kernel.org
> Cc: linux-mm@kvack.org
> Cc: kernel-team@fb.com

Acked-by: Johannes Weiner <hannes@cmpxchg.org>