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

[Lucas Karpinski]

On Tue, Aug 01, 2023 at 09:39:28AM -0700, Paul E. McKenney wrote:
> 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@xxxxxxxxxxx>
> 
> 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@xxxxxxxxxx>
> 
> 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
> >

The same issue exists in the test case test_kmem_memcg_deletion. I
wouldn't mind posting the patch, but it seems you want to propose
something more formal. Let me know your opinion.

Thanks,
Lucas