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

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Date:   Mon, 14 Mar 2022 14:33:53 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Ali Saidi <alisaidi@...zon.com>
Cc:     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, yao.jin@...ux.intel.com
Subject: Re: [PATCH v2 2/2] perf mem: Support HITM for when mem_lvl_num is
 used

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.

Thanks,
Leo