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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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