linux-kernel - Re: [PATCH v4 2/4] perf arm-spe: Use SPE data source for neoverse cores

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Date:   Thu, 31 Mar 2022 20:44:25 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     German Gomez <german.gomez@....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,
        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 v4 2/4] perf arm-spe: Use SPE data source for neoverse
 cores

On Thu, Mar 31, 2022 at 01:28:58PM +0100, German Gomez wrote:
> Hi all,
> 
> It seems I gave the Review tags a bit too early this time. Apologies for
> the inconvenience. Indeed there was more interesting discussions to be
> had :)
> 
> (Probably best to remove by tags for the next re-spin)

Now worries, German.  Your review and testing are very helpful :)

> On 29/03/2022 15:32, Ali Saidi wrote:
> > [...]
> >
> >> I still think we should consider to extend the memory levels to
> >> demonstrate clear momory hierarchy on Arm archs, I personally like the
> >> definitions for "PEER_CORE", "LCL_CLSTR", "PEER_CLSTR" and "SYS_CACHE",
> >> though these cache levels are not precise like L1/L2/L3 levels, they can
> >> help us to map very well for the cache topology on Arm archs and without
> >> any confusion.  We could take this as an enhancement if you don't want
> >> to bother the current patch set's upstreaming.
> > I'd like to do this in a separate patch, but I have one other proposal. The
> > Neoverse cores L2 is strictly inclusive of the L1, so even if it's in the L1,
> > it's also in the L2. Given that the Graviton systems and afaik the Ampere
> > systems don't have any cache between the L2 and the SLC, thus anything from
> > PEER_CORE, LCL_CLSTR, or PEER_CLSTR would hit in the L2, perhaps we
> > should just set L2 for these cases? German, are you good with this for now? 
> 
> Sorry for the delay. I'd like to also check this with someone. I'll try
> to get back asap. In the meantime, if this approach is also OK with Leo,
> I think it would be fine by me.

Thanks for the checking internally.  Let me just bring up my another
thinking (sorry that my suggestion is float): another choice is we set
ANY_CACHE as cache level if we are not certain the cache level, and
extend snoop field to indicate the snooping logics, like:

  PERF_MEM_SNOOP_PEER_CORE
  PERF_MEM_SNOOP_LCL_CLSTR
  PERF_MEM_SNOOP_PEER_CLSTR

Seems to me, we doing this is not only for cache level, it's more
important for users to know the variant cost for involving different
snooping logics.

Thanks,
Leo