Re: [PATCH 1/7] mm: memcontrol: fix cpuhotplug statistics flushing

On Tue, Feb 02, 2021 at 03:07:47PM -0800, Roman Gushchin wrote:

On Tue, Feb 02, 2021 at 01:47:40PM -0500, Johannes Weiner wrote:

quoted

The memcg hotunplug callback erroneously flushes counts on the local
CPU, not the counts of the CPU going away; those counts will be lost.

Flush the CPU that is actually going away.

Also simplify the code a bit by using mod_memcg_state() and
count_memcg_events() instead of open-coding the upward flush - this is
comparable to how vmstat.c handles hotunplug flushing.

To the whole series: it's really nice to have an accurate stats at
non-leaf levels. Just as an illustration: if there are 32 CPUs and
1000 sub-cgroups (which is an absolutely realistic number, because
often there are many dying generations of each cgroup), the error
margin is 3.9GB. It makes all numbers pretty much random and all
possible tests extremely flaky.

Btw, I was just looking into kmem kselftests failures/flakiness,
which is caused by exactly this problem: without waiting for the
finish of dying cgroups reclaim, we can't make any reliable assumptions
about what to expect from memcg stats.

So looking forward to have this patchset merged!