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

[Reinette Chatre]

Hi Ilpo,

On 10/24/2023 2:26 AM, Ilpo Järvinen wrote:
> CAT selftests only cover L3 but some newer CPUs come also with L2 CAT
> support.

No need to use "new" language. L2 CAT has been available for a long time
... since Apollo Lake. Which systems actually support it is a different
topic. This is an architectural feature that has been available for a
long time. Whether a system supports it will be detected and the test
run based on that. 

> 
> 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.

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

> 
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
> ---
>  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?

Reinette