[PATCH] selftests: cgroup: fix test_kmem_memcg_deletion false positives

[PATCH] selftests: cgroup: fix test_kmem_memcg_deletion false positives

[Lucas Karpinski]

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>
---
 tools/testing/selftests/cgroup/test_kmem.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/tools/testing/selftests/cgroup/test_kmem.c b/tools/testing/selftests/cgroup/test_kmem.c
index 67cc0182058d..7ac384bbfdd5 100644
--- a/tools/testing/selftests/cgroup/test_kmem.c
+++ b/tools/testing/selftests/cgroup/test_kmem.c
@@ -183,6 +183,9 @@ static int test_kmem_memcg_deletion(const char *root)
 	if (cg_run_in_subcgroups(parent, alloc_kmem_smp, NULL, 100))
 		goto cleanup;
 
+	/* wait for RCU freeing */
+	sleep(1);
+
 	current = cg_read_long(parent, "memory.current");
 	slab = cg_read_key_long(parent, "memory.stat", "slab ");
 	anon = cg_read_key_long(parent, "memory.stat", "anon ");
-- 
2.41.0

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

[Johannes Weiner]

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.

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

[Lucas Karpinski]

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

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

[Lucas Karpinski]

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