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Date:   Tue, 1 Aug 2023 09:39:28 -0700
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Michal Hocko <mhocko@suse.com>,
        linux-kselftest@vger.kernel.org, linux-mm@kvack.org,
        cgroups@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] selftests: cgroup: fix test_kmem_basic false positives
Message-ID: <c40ca485-f52e-411a-9f33-3adabc53c0fc@paulmck-laptop>
Reply-To: paulmck@kernel.org
References: <20230801135632.1768830-1-hannes@cmpxchg.org>
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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
>