Re: [PATCH V2 11/13] selftests/resctrl: Use cache size to determine "fill_buf" buffer size

[Reinette Chatre]

Hi Ilpo,

On 10/4/24 7:20 AM, Ilpo Järvinen wrote:
> On Thu, 12 Sep 2024, Reinette Chatre wrote:
> 
>> By default the MBM and MBA tests use the "fill_buf" benchmark to
>> read from a buffer with the goal to measure the memory bandwidth
>> generated by this buffer access.
>>
>> Care should be taken when sizing the buffer used by the "fill_buf"
>> benchmark. If the buffer is small enough to fit in the cache then
>> it cannot be expected that the benchmark will generate much memory
>> bandwidth. For example, on a system with 320MB L3 cache the existing
>> hardcoded default of 250MB is insufficient.
>>
>> Use the measured cache size to determine a buffer size that can be
>> expected to trigger memory access while keeping the existing default
>> as minimum that has been appropriate for testing so far.
>>
>> Signed-off-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
>> ---
>> Changes since V1:
>> - Ensure buffer is at least double L3 cache size. (Ilpo)
>> - Support user override of default buffer size. (Ilpo)
>> ---
>>  tools/testing/selftests/resctrl/mba_test.c | 8 +++++++-
>>  tools/testing/selftests/resctrl/mbm_test.c | 8 +++++++-
>>  2 files changed, 14 insertions(+), 2 deletions(-)
>>
>> diff --git a/tools/testing/selftests/resctrl/mba_test.c b/tools/testing/selftests/resctrl/mba_test.c
>> index 7e43056c8737..d8d9637c1951 100644
>> --- a/tools/testing/selftests/resctrl/mba_test.c
>> +++ b/tools/testing/selftests/resctrl/mba_test.c
>> @@ -182,7 +182,13 @@ static int mba_run_test(const struct resctrl_test *test, const struct user_param
>>  		fill_buf.memflush = uparams->fill_buf->memflush;
>>  		param.fill_buf = &fill_buf;
>>  	} else if (!uparams->benchmark_cmd[0]) {
>> -		fill_buf.buf_size = DEFAULT_SPAN;
>> +		unsigned long cache_total_size = 0;
>> +
>> +		ret = get_cache_size(uparams->cpu, "L3", &cache_total_size);
>> +		if (ret)
>> +			return ret;
>> +		fill_buf.buf_size = cache_total_size * 2 > DEFAULT_SPAN ?
>> +				    cache_total_size * 2 : DEFAULT_SPAN;
>>  		fill_buf.memflush = 1;
>>  		param.fill_buf = &fill_buf;
>>  	}
>> diff --git a/tools/testing/selftests/resctrl/mbm_test.c b/tools/testing/selftests/resctrl/mbm_test.c
>> index b1f03a73333f..7635ee6b9339 100644
>> --- a/tools/testing/selftests/resctrl/mbm_test.c
>> +++ b/tools/testing/selftests/resctrl/mbm_test.c
>> @@ -149,7 +149,13 @@ static int mbm_run_test(const struct resctrl_test *test, const struct user_param
>>  		fill_buf.memflush = uparams->fill_buf->memflush;
>>  		param.fill_buf = &fill_buf;
>>  	} else if (!uparams->benchmark_cmd[0]) {
>> -		fill_buf.buf_size = DEFAULT_SPAN;
>> +		unsigned long cache_total_size = 0;
>> +
>> +		ret = get_cache_size(uparams->cpu, "L3", &cache_total_size);
>> +		if (ret)
>> +			return ret;
>> +		fill_buf.buf_size = cache_total_size * 2 > DEFAULT_SPAN ?
>> +				    cache_total_size * 2 : DEFAULT_SPAN;
>>  		fill_buf.memflush = 1;
>>  		param.fill_buf = &fill_buf;
>>  	}
>>
> 
> It has a bit of code duplication feel in it so I'd consider adding 
> something like ssize_t get_default_span() (perhaps there exists a better 
> name for it that is not "span" based). Also DEFAULT_SPAN is no longer
> truly the default span.

Good suggestion.  How about get_fill_buf_size()?

Regarding the "DEFAULT_SPAN" it seems to have some 'magic number smell' to
it. When I tried to eliminate it and only rely on cache size the tests
were failing on some earlier platforms with smaller L3. I then started
treating it as a minimum in order to keep original behavior on these earlier
systems. Perhaps renaming it to "MINIMUM_SPAN" would make this change
clear?

> 
> But neither is the end of the world as is...
> 
> Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@xxxxxxxxxxxxxxx>
> 
> 

Thank you very much.

Reinette