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

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Date:   Mon, 6 Nov 2023 13:22:48 -0800
From:   Reinette Chatre <reinette.chatre@...el.com>
To:     Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
CC:     <linux-kselftest@...r.kernel.org>, Shuah Khan <shuah@...nel.org>,
        Shaopeng Tan <tan.shaopeng@...fujitsu.com>,
        Maciej Wieczór-Retman 
        <maciej.wieczor-retman@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 23/24] selftests/resctrl: Add L2 CAT test

Hi Ilpo,

On 11/6/2023 9:03 AM, Reinette Chatre wrote:
> On 11/6/2023 1:53 AM, Ilpo Järvinen wrote:
>> On Fri, 3 Nov 2023, Reinette Chatre wrote:
>>> On 11/3/2023 3:39 AM, Ilpo Järvinen wrote:
>>>> On Thu, 2 Nov 2023, Reinette Chatre wrote:
>>>>> On 10/24/2023 2:26 AM, Ilpo Järvinen wrote:
>>>>
>>>>>> 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.
>>>>
>>>> Okay, I was under impression that L2 misses are not available. Both based 
>>>> on what you mentioned to me half an year ago and because of what flags I 
>>>> found from the header. But I'll take another look into it.
>>>
>>> You are correct that when I did L2 testing a long time ago I used
>>> the model specific L2 miss counts. I was hoping that things have improved
>>> so that model specific counters are not needed, as you have tried here.
>>> I found the l2_rqsts symbol while looking for alternatives but I am not
>>> familiar enough with perf to know how these symbolic names are mapped.
>>> I was hoping that they could be a simple drop-in replacement to
>>> experiment with.
>>
>> According to perf_event_open() manpage, mapping those symbolic names 
>> requires libpfm so this would add a library dependency?
> 
> I do not see perf list using this library to determine the event and
> umask but I am in unfamiliar territory. I'll have to spend some more
> time here to determine options.

tools/perf/pmu-events/README cleared it up for me. The architecture specific
tables are included in the perf binary. Potentially pmu-events.h could be
included or the test could just stick with the architectural events.
A quick look at the various cache.json files created the impression that
the events of interest may actually have the same event code and umask across
platforms.
I am not familiar with libpfm. This can surely be considered if it supports
this testing. Several selftests have library dependencies.

Reinette