Re: [PATCH v2 1/1] selftests/resctrl: Fix MBM test failure when MBA unavailable

[Ilpo Järvinen]

On Wed, 18 Oct 2023, Ilpo Järvinen wrote:

> Commit 20d96b25cc4c ("selftests/resctrl: Fix schemata write error
> check") exposed a problem in feature detection logic in MBM selftest.
> If schemata does not support MB:x=x entries, the schemata write to
> initialize 100% memory bandwidth allocation in mbm_setup() will now
> fail with -EINVAL due to the error handling corrected by the commit
> 20d96b25cc4c ("selftests/resctrl: Fix schemata write error check").
> That commit just uncovers the failed write, it is not wrong itself.
> 
> If MB:x=x is not supported by schemata, it is safe to assume 100%
> memory bandwidth is always set. Therefore, the previously ignored error
> does not make the MBM test itself wrong.
> 
> Restore the previous behavior of MBM test by checking MB support before
> attempting to write it into schemata which results in behavior
> equivalent to ignoring the write error.
> 
> Fixes: 20d96b25cc4c ("selftests/resctrl: Fix schemata write error check")
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
> Reviewed-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
> ---

It seems it's too early morning for me so I entirely forgot to describe 
the changes I made... doh. So here it is (nothing earth-shattering here 
really):

v2:
- Make checkpatch --strict happy.
  - Remove () that --strict called "extra".
  - Duplicate the referenced commit description in the changelog
    to follow the commit formatting guidelines.
  - Alter the wording at one of the references into the commit 
    20d96b25cc4c in order to avoid having to copy the description
    there too.
- Added Reinette's rev-by.

-- 
 i.


>  tools/testing/selftests/resctrl/mbm_test.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/resctrl/mbm_test.c b/tools/testing/selftests/resctrl/mbm_test.c
> index d3c0d30c676a..741533f2b075 100644
> --- a/tools/testing/selftests/resctrl/mbm_test.c
> +++ b/tools/testing/selftests/resctrl/mbm_test.c
> @@ -95,7 +95,7 @@ static int mbm_setup(struct resctrl_val_param *p)
>  		return END_OF_TESTS;
>  
>  	/* Set up shemata with 100% allocation on the first run. */
> -	if (p->num_of_runs == 0)
> +	if (p->num_of_runs == 0 && validate_resctrl_feature_request("MB", NULL))
>  		ret = write_schemata(p->ctrlgrp, "100", p->cpu_no,
>  				     p->resctrl_val);
>  
>