linux-kernel - Re: [PATCH v4 10/19] selftests/resctrl: Express span internally in bytes

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Message-ID: <ba080508-5f55-a56e-db78-299d8523a88a@intel.com>
Date:   Fri, 14 Jul 2023 10:38:00 -0700
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     "Wieczor-Retman, Maciej" <maciej.wieczor-retman@...el.com>,
        Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
        <linux-kselftest@...r.kernel.org>, Shuah Khan <shuah@...nel.org>,
        "Shaopeng Tan" <tan.shaopeng@...fujitsu.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 10/19] selftests/resctrl: Express span internally in
 bytes

Hi Maciej,

On 7/13/2023 11:43 PM, Wieczor-Retman, Maciej wrote:
> Hi Reinette!
> 
> On 14.07.2023 01:00, Reinette Chatre wrote:
>> Hi Ilpo,
>>
>> On 7/13/2023 6:19 AM, Ilpo Järvinen wrote:
>>> MBA and MBM tests to use megabytes to represent span. CMT test uses
>>> bytes. The difference requires run_benchmark() to size the buffer
>>> differently based on the test name, which in turn requires passing the
>>> test name into run_benchmark().
>>>
>>> Convert MBA and MBM tests to use internally bytes like CMT test to
>>> remove the internal inconsistency between the tests. Remove the test
>>> dependent buffer sizing from run_benchmark().
>>
>> If I understand correctly the intention is to always use bytes internally
>> and only convert to megabytes when displayed to user space. The above
>> implies that this takes care of the conversion but there still seems
>> to be places that that do not follow my understanding. For example,
>> resctrl_val.c:measure_vals() converts to megabytes before proceeding.
> 
> Doesn't the use case inside resctrl_val.c:measure_vals() satisfy
> the idea of only displaying data to the user space? From my
> understanding it reads the number of bytes and only converts to
> MB when printing the value. Or did I miss some detail there?

For MBM and MBA tests resctrl_val.c:measure_vals() converts data
to MB before it writes the raw data to the file. Compare to CMT
test that uses cache.c:measure_cache_vals() to write raw data
in bytes to file.

On a high level I see the MBA, MBM, and CMT tests in three stages:
(1) A "benchmark" is run in background, 
(2) raw data is collected at intervals and written to a file,
(3) finally, data from file is processed and results displayed to user.

>From what I can tell MBA and MBM tests continue to write data to file
in MB while CMT writes data in bytes.

The changelog claims "Convert MBA and MBM tests to use internally
bytes like CMT test to remove the internal inconsistency between
the tests.". This claim is not clear to me because from what I can tell
the three tests continue to handle data inconsistently after this change.

Reinette