linux-kernel - Re: [PATCH v2 2/2] perf mem: Support HITM for when mem_lvl

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Date:   Wed, 16 Mar 2022 11:43:52 +0000
From:   German Gomez <german.gomez@....com>
To:     Ali Saidi <alisaidi@...zon.com>, leo.yan@...aro.org
Cc:     acme@...nel.org, alexander.shishkin@...ux.intel.com,
        andrew.kilroy@....com, benh@...nel.crashing.org,
        james.clark@....com, john.garry@...wei.com, jolsa@...nel.org,
        kjain@...ux.ibm.com, lihuafei1@...wei.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-perf-users@...r.kernel.org, mark.rutland@....com,
        mathieu.poirier@...aro.org, mingo@...hat.com, namhyung@...nel.org,
        peterz@...radead.org, will@...nel.org, yao.jin@...ux.intel.com,
        Nick.Forrington@....com
Subject: Re: [PATCH v2 2/2] perf mem: Support HITM for when mem_lvl_num is
 used


On 15/03/2022 18:44, German Gomez wrote:
> On 14/03/2022 18:37, Ali Saidi wrote:
>> Hi German and Leo,
>>
>> On   Mon, 14 Mar 2022 18:00:13 +0000, German Gomez wrote:
>>> Hi Leo, Ali
>>>
>>> On 14/03/2022 06:33, Leo Yan wrote:
>>>> On Sun, Mar 13, 2022 at 07:19:33PM +0000, Ali Saidi wrote:
>>>>
>>>> [...]
>>>>
>>>>>>>> +			if (lvl & P(LVL, L3) || lnum == P(LVLNUM, L4)) {
>>>>>>> According to a comment in the previous patch, using L4 is specific to Neoverse, right?
>>>>>>>
>>>>>>> Maybe we need to distinguish the Neoverse case from the generic one here as well
>>>>>>>
>>>>>>> if (is_neoverse)
>>>>>>> // treat L4 as llc
>>>>>>> else
>>>>>>> // treat L3 as llc
>>>>>> I personally think it's not good idea to distinguish platforms in the decoding code.
>>>>> I agree here. The more we talk about this, the more I'm wondering if we're
>>>>> spending too much code solving a problem that doesn't exist. I know of no
>>>>> Neoverse systems that actually have 4 cache levels, they all actually have three
>>>>> even though it's technically possible to have four.  I have some doubts anyone
>>>>> will actually build four levels of cache and perhaps the most prudent path here
>>>>> is to assume only three levels (and adjust the previous patch) until someone 
>>>>> actually produces a system with four levels instead of a lot of code that is
>>>>> never actually exercised?
>>>> I am not right person to say L4 cache is not implemented in Neoverse
>>>> platforms; my guess for a "System cache" data source might be L3 or
>>>> L4 and it is a implementation dependent.  Maybe German or Arm mates
>>>> could confirm for this.
>>> I had a look at the TRMs for the N1[1], V1[2] and N2[3] Neoverse cores
>>> (specifically the LL_CACHE_RD pmu events). If we were to assign a number
>>> to the system cache (assuming all caches are implemented):
>>>
>>> *For N1*, if L2 and L3 are implemented, system cache would follow at *L4*
>> To date no one has built 4 level though. Everyone has only built three.
> The N1SDP board advertises 4 levels (we use it regularly for testing perf patches)

That said, it's probably the odd one out.

I'm not against assuming 3 levels. Later if there's is a strong need for L4, indeed we can go back and change it.

Thanks,
German

>
> | $ cat /sys/devices/system/cpu/cpu0/cache/index4/{level,shared_cpu_list}
> | 4
> | 0-3
>
> Would it be a good idea to obtain the system cache level# from sysfs?
>
>>> *For V1 and N2*, if L2 is implemented, system cache would follow at *L3*
>>> (these don't seem to have the same/similar per-cluster L3 cache from the N1)
>> And in the future they're not able to build >3. German and Leo if there aren't
>> strong objections I think the best path forward is for me to respin these
>> assuming only 3 levels and if someone builds 4 in a far-off-future we can always
>> change the implementation then. Agreed?
>>
>> Thanks,
>> Ali
>>