linux-kernel - Re: [PATCH 4/6] selftests/resctrl: Use cache size to determine "fill

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Message-ID: <094411c1-9ef9-4030-9c2a-35c78b4da00c@intel.com>
Date: Fri, 30 Aug 2024 09:00:44 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
CC: <fenghua.yu@...el.com>, <shuah@...nel.org>, <tony.luck@...el.com>,
	<peternewman@...gle.com>, <babu.moger@....com>,
	Maciej Wieczór-Retman <maciej.wieczor-retman@...el.com>,
	<linux-kselftest@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/6] selftests/resctrl: Use cache size to determine
 "fill_buf" buffer size

Hi Ilpo,

On 8/30/24 4:25 AM, Ilpo Järvinen wrote:
> On Thu, 29 Aug 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@...el.com>
>> ---
>>   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 8ad433495f61..cad473b81a64 100644
>> --- a/tools/testing/selftests/resctrl/mba_test.c
>> +++ b/tools/testing/selftests/resctrl/mba_test.c
>> @@ -170,11 +170,17 @@ static int mba_run_test(const struct resctrl_test *test, const struct user_param
>>   		.setup		= mba_setup,
>>   		.measure	= mba_measure,
>>   	};
>> +	unsigned long cache_total_size = 0;
>>   	int ret;
>>   
>>   	remove(RESULT_FILE_NAME);
>>   
>> -	param.fill_buf.buf_size = DEFAULT_SPAN;
>> +	ret = get_cache_size(uparams->cpu, "L3", &cache_total_size);
>> +	if (ret)
>> +		return ret;
>> +
>> +	param.fill_buf.buf_size = cache_total_size > DEFAULT_SPAN ?
>> +				  cache_total_size * 2 : DEFAULT_SPAN;
> 
> Should the check leave a bit of safeguard so that the buf_size is at
> least 2x (or x1.25 or some other factor)?
> 
> In here buf_size immediate jumps from 1x -> 2x when cache_total_size goes
> from DEFAULT_SPAN to DEFAULT_SPAN+1 (obviously L3 size won't be odd like
> that but I think you get my point).

Good catch. Will fix.

> 
> Also, user might want to override this as mentioned in my reply to the
> previous patch.
> 

ack.

Reinette