Re: [PATCH] selftests: cgroup: fix test_kmem_memcg_deletion false positives

[Lucas Karpinski <lkarpins@redhat.com>]

On Fri, Aug 04, 2023 at 02:59:28PM -0400, Lucas Karpinski wrote:
> 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
Looked into the failures and I do have a proposed solution, just want
some feedback first. With how the kernel entry in memory.stat is 
updated, it takes into account all charged / uncharged pages, it looks 
like it makes more sense to use that single entry rather than `slab + 
anon + file + kernel_stack + pagetables + percpu + sock' as it would
cover all utilization.

Thanks,
Lucas