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

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Date:   Thu, 4 Feb 2021 14:29:57 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Roman Gushchin <guro@...com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, Michal Hocko <mhocko@...e.com>,
        linux-mm@...ck.org, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH 1/7] mm: memcontrol: fix cpuhotplug statistics flushing

On Tue, Feb 02, 2021 at 06:28:53PM -0800, Roman Gushchin wrote:
> 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:
> > > 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.

Good point about the selftests. I gave them a shot, and indeed this
series makes test_kmem work again:

vanilla:
ok 1 test_kmem_basic
memory.current = 8810496
slab + anon + file + kernel_stack = 17074568
slab = 6101384
anon = 946176
file = 0
kernel_stack = 10027008
not ok 2 test_kmem_memcg_deletion
ok 3 test_kmem_proc_kpagecgroup
ok 4 test_kmem_kernel_stacks
ok 5 test_kmem_dead_cgroups
ok 6 test_percpu_basic

patched:
ok 1 test_kmem_basic
ok 2 test_kmem_memcg_deletion
ok 3 test_kmem_proc_kpagecgroup
ok 4 test_kmem_kernel_stacks
ok 5 test_kmem_dead_cgroups
ok 6 test_percpu_basic

It even passes with a reduced margin in the patched kernel, since the
percpu drift - which this test already tried to account for - is now
only on the page_counter side (whereas memory.stat is always precise).

I'm going to include that data in the v2 changelog, as well as a patch
to update test_kmem.c to the more stringent error tolerances.

> So looking forward to have this patchset merged!

Thanks