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

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Date:   Mon, 6 Nov 2023 09:03:24 -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 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.

> 
>>>>> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
>>>>> ---
>>>>>  tools/testing/selftests/resctrl/cat_test.c    | 68 +++++++++++++++++--
>>>>>  tools/testing/selftests/resctrl/resctrl.h     |  1 +
>>>>>  .../testing/selftests/resctrl/resctrl_tests.c |  1 +
>>>>>  3 files changed, 63 insertions(+), 7 deletions(-)
>>>>>
>>>>> diff --git a/tools/testing/selftests/resctrl/cat_test.c b/tools/testing/selftests/resctrl/cat_test.c
>>>>> index 48a96acd9e31..a9c72022bb5a 100644
>>>>> --- a/tools/testing/selftests/resctrl/cat_test.c
>>>>> +++ b/tools/testing/selftests/resctrl/cat_test.c
>>>>> @@ -131,8 +131,47 @@ void cat_test_cleanup(void)
>>>>>  	remove(RESULT_FILE_NAME);
>>>>>  }
>>>>>  
>>>>> +/*
>>>>> + * L2 CAT test measures L2 misses indirectly using L3 accesses as a proxy
>>>>> + * because perf cannot directly provide the number of L2 misses (there are
>>>>> + * only platform specific ways to get the number of L2 misses).
>>>>> + *
>>>>> + * This function sets up L3 CAT to reduce noise from other processes during
>>>>> + * L2 CAT test.
>>>>
>>>> This motivation is not clear to me. Does the same isolation used during 
>>>> L3 CAT testing not work? I expected it to follow the same idea with the 
>>>> L2 cache split in two, the test using one part and the rest of the 
>>>> system using the other. Is that not enough isolation?
>>>
>>> Isolation for L2 is done very same way as with L3 and I think it itself 
>>> works just fine.
>>>
>>> However, because L2 CAT selftest as is measures L3 accesses that in ideal 
>>> world equals to L2 misses, isolating selftest related L3 accesses from the 
>>> rest of the system should reduce noise in the # of L3 accesses. It's not 
>>> mandatory though so if L3 CAT is not available the function just prints a 
>>> warning about the potential noise and does setup nothing for L3.
>>
>> This is not clear to me. If the read misses L2 and then accesses L3 then
>> it should not matter which part of L3 cache the work is isolated to. 
>> What noise do you have in mind?
> 
> The way it is currently done is to measure L3 accesses. If something else 
> runs at the same time as the CAT selftest, it can do mem accesses that 
> cause L3 accesses which is noise in the # of L3 accesses number since 
> those accesses were unrelated to the L2 CAT selftest.
> 

Creating a CAT allocation sets aside a portion of cache where a task/cpu
can allocation into cache, it does not prevent one task from accessing
the cache concurrently with another.

Reinette