Re: [PATCH 23/24] selftests/resctrl: Add L2 CAT test

["Ilpo Järvinen" <ilpo.jarvinen@linux.intel.com>]

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On Thu, 2 Nov 2023, Reinette Chatre wrote:
> On 10/24/2023 2:26 AM, Ilpo Järvinen wrote:

> > Add L2 CAT selftest. As measuring L2 misses is not easily available
> > with perf, use L3 accesses as a proxy for L2 CAT working or not.
> 
> I understand the exact measurement is not available but I do notice some
> L2 related symbolic counters when I run "perf list". l2_rqsts.all_demand_miss
> looks promising.

Okay, I was under impression that L2 misses are not available. Both based 
on what you mentioned to me half an year ago and because of what flags I 
found from the header. But I'll take another look into it.

> L3 cannot be relied on for those systems, like Apollo lake, that do
> not have an L3.

Do you happen know what perf will report for such CPUs, will it return 
L2 as LLC?

> > Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
> > ---
> >  tools/testing/selftests/resctrl/cat_test.c    | 68 +++++++++++++++++--
> >  tools/testing/selftests/resctrl/resctrl.h     |  1 +
> >  .../testing/selftests/resctrl/resctrl_tests.c |  1 +
> >  3 files changed, 63 insertions(+), 7 deletions(-)
> > 
> > diff --git a/tools/testing/selftests/resctrl/cat_test.c b/tools/testing/selftests/resctrl/cat_test.c
> > index 48a96acd9e31..a9c72022bb5a 100644
> > --- a/tools/testing/selftests/resctrl/cat_test.c
> > +++ b/tools/testing/selftests/resctrl/cat_test.c
> > @@ -131,8 +131,47 @@ void cat_test_cleanup(void)
> >  	remove(RESULT_FILE_NAME);
> >  }
> >  
> > +/*
> > + * L2 CAT test measures L2 misses indirectly using L3 accesses as a proxy
> > + * because perf cannot directly provide the number of L2 misses (there are
> > + * only platform specific ways to get the number of L2 misses).
> > + *
> > + * This function sets up L3 CAT to reduce noise from other processes during
> > + * L2 CAT test.
> 
> This motivation is not clear to me. Does the same isolation used during 
> L3 CAT testing not work? I expected it to follow the same idea with the 
> L2 cache split in two, the test using one part and the rest of the 
> system using the other. Is that not enough isolation?

Isolation for L2 is done very same way as with L3 and I think it itself 
works just fine.

However, because L2 CAT selftest as is measures L3 accesses that in ideal 
world equals to L2 misses, isolating selftest related L3 accesses from the 
rest of the system should reduce noise in the # of L3 accesses. It's not 
mandatory though so if L3 CAT is not available the function just prints a 
warning about the potential noise and does setup nothing for L3.

But I'll see if I can make it use L2 misses directly so this wouldn't 
matter.

-- 
 i.