linux-kernel - Re: [PATCH v5 2/5] perf: Add SNOOP_PEER flag to perf mem data struct

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Date:   Mon, 25 Apr 2022 13:01:40 -0400
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Leo Yan <leo.yan@...aro.org>, Andi Kleen <ak@...ux.intel.com>
Cc:     Ali Saidi <alisaidi@...zon.com>, Nick.Forrington@....com,
        acme@...nel.org, alexander.shishkin@...ux.intel.com,
        andrew.kilroy@....com, benh@...nel.crashing.org,
        german.gomez@....com, 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
Subject: Re: [PATCH v5 2/5] perf: Add SNOOP_PEER flag to perf mem data struct



On 4/24/2022 7:43 AM, Leo Yan wrote:
> On Sat, Apr 23, 2022 at 05:53:28AM -0700, Andi Kleen wrote:
>>
>>> Except SNOOPX_FWD means a no modified cache snooping, it also means it's
>>> a cache conherency from *remote* socket.  This is quite different from we
>>> define SNOOPX_PEER, which only snoop from peer CPU or clusters.
>>>

The FWD doesn't have to be *remote*. The definition you quoted is just 
for the "L3 Miss", which is indeed a remote forward. But we still have 
cross-core FWD. See Table 19-101.

Actually, X86 uses the PERF_MEM_REMOTE_REMOTE + PERF_MEM_SNOOPX_FWD to 
indicate the remote FWD, not just SNOOPX_FWD.

>>> If no objection, I prefer we could keep the new snoop type SNOOPX_PEER,
>>> this would be easier for us to distinguish the semantics and support the
>>> statistics for SNOOPX_FWD and SNOOPX_PEER separately.
>>>
>>> I overlooked the flag SNOOPX_FWD, thanks a lot for Kan's reminding.
>>
>> Yes seems better to keep using a separate flag if they don't exactly match.
>>

Yes, I agree with Andi. If you still think the existing flag combination 
doesn't match your requirement, a new separate flag should be 
introduced. I'm not familiar with ARM. I think I will leave it to you 
and the maintainer to decide.

Thanks,
Kan