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SubjectRe: [PATCH] selftests: cgroup: fix test_kmem_basic false positives
On Tue, Aug 01, 2023 at 09:56:32AM -0400, Johannes Weiner wrote:
> This test fails routinely in our prod testing environment, and I can
> reproduce it locally as well.
>
> The test allocates dcache inside a cgroup, then drops the memory limit
> and checks that usage drops correspondingly. 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 dropping the limit. This should be good
> enough, assuming that machines running those tests are otherwise not
> very busy.
>
> Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>

I am putting together something more formal, but this will certainly
improve things, as Johannes says, assuming the system goes mostly
idle during that one-second wait. So:

Acked-by: Paul E. McKenney <paulmck@kernel.org>

Yes, there are corner cases, such as the system having millions of
RCU callbacks queued and being unable to invoke them all during that
one-second interval. But that is a corner case, and that is exactly
why I will be putting together something more formal. ;-)

Thanx, Paul

> ---
> tools/testing/selftests/cgroup/test_kmem.c | 4 ++++
> 1 file changed, 4 insertions(+)
>
> diff --git a/tools/testing/selftests/cgroup/test_kmem.c b/tools/testing/selftests/cgroup/test_kmem.c
> index 258ddc565deb..1b2cec9d18a4 100644
> --- a/tools/testing/selftests/cgroup/test_kmem.c
> +++ b/tools/testing/selftests/cgroup/test_kmem.c
> @@ -70,6 +70,10 @@ static int test_kmem_basic(const char *root)
> goto cleanup;
>
> cg_write(cg, "memory.high", "1M");
> +
> + /* wait for RCU freeing */
> + sleep(1);
> +
> slab1 = cg_read_key_long(cg, "memory.stat", "slab ");
> if (slab1 <= 0)
> goto cleanup;
> --
> 2.41.0
>

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