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Date:   Fri, 4 Aug 2023 14:59:28 -0400
From:   Lucas Karpinski <lkarpins@redhat.com>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Shakeel Butt <shakeelb@google.com>,
        Muchun Song <muchun.song@linux.dev>, Tejun Heo <tj@kernel.org>,
        Zefan Li <lizefan.x@bytedance.com>,
        Shuah Khan <shuah@kernel.org>, cgroups@vger.kernel.org,
        linux-mm@kvack.org, linux-kselftest@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] selftests: cgroup: fix test_kmem_memcg_deletion false
 positives
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 <20230804163716.GA337691@cmpxchg.org>
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On Fri, Aug 04, 2023 at 12:37:16PM -0400, Johannes Weiner wrote:
> On Fri, Aug 04, 2023 at 11:37:33AM -0400, Lucas Karpinski wrote:
> > The test allocates dcache inside a cgroup, then destroys the cgroups and
> > then checks the sanity of numbers on the parent level. The reason it
> > fails is because dentries are freed with an RCU delay - a debugging
> > sleep shows that usage drops as expected shortly after.
> > 
> > Insert a 1s sleep after completing the cgroup creation/deletions. This
> > should be good enough, assuming that machines running those tests are
> > otherwise not very busy. This commit is directly inspired by Johannes
> > over at the link below.
> > 
> > Link: https://lore.kernel.org/all/20230801135632.1768830-1-hannes@cmpxchg.org/
> > 
> > Signed-off-by: Lucas Karpinski <lkarpins@redhat.com>
> 
> Maybe I'm missing something, but there isn't a limit set anywhere that
> would cause the dentries to be reclaimed and freed, no? When the
> subgroups are deleted, the objects are just moved to the parent. The
> counters inside the parent (which are hierarchical) shouldn't change.
> 
> So this seems to be a different scenario than test_kmem_basic. If the
> test is failing for you, I can't quite see why.
>
You're right, the parent inherited the counters and it should behave
the same whether I'm directly removing the child or if I was moving it
under another cgroup. I do see the behaviour you described on my
x86_64 setup, but the wrong behaviour on my aarch64 dev. platform. I'll
take a closer look, but just wanted to leave an example here of what I
see.

Example of slab size pre/post sleep:
slab_pre = 18164688, slab_post = 3360000

Thanks,
Lucas