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

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Message-ID: <20220423063805.GA559531@leoy-ThinkPad-X240s>
Date:   Sat, 23 Apr 2022 14:38:05 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Ali Saidi <alisaidi@...zon.com>
Cc:     kan.liang@...ux.intel.com, Nick.Forrington@....com,
        acme@...nel.org, ak@...ux.intel.com,
        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 Fri, Apr 22, 2022 at 09:22:49PM +0000, Ali Saidi wrote:
> 
> On Fri, 22 Apr 2022 21:43:28, Kan Liang wrote:
> > On 4/22/2022 2:49 PM, Ali Saidi wrote:
> > > On Wed, 20 Apr 2022 18:43:28, Kan Liang wrote:
> > >> On 4/8/2022 3:53 PM, Ali Saidi wrote:
> > >>> Add a flag to the perf mem data struct to signal that a request caused a
> > >>> cache-to-cache transfer of a line from a peer of the requestor and
> > >>> wasn't sourced from a lower cache level.
> > >>
> > >> It sounds similar to the Forward state. Why can't the
> > >> PERF_MEM_SNOOPX_FWD be reused?
> > > 
> > > Is there a definition of SNOOPX_FWD i can refer to? Happy to use this instead if
> > > the semantics align between architectures.
> > > 
> > 
> > + Andi
> > 
> > As my understanding, the SNOOPX_FWD means the Forward state, which is a 
> > non-modified (clean) cache-to-cache copy.
> > https://en.wikipedia.org/wiki/MESIF_protocol
>   
> In this case the semantics are different. We know the line was transferred from
> another peer cache, but don't know if it was clean, dirty, or if the receiving core
> now has exclusive ownership of it.

In the spec "Intel 64 and IA-32 Architectures Software Developer's Manual,
Volume 3B: System Programming Guide, Part 2", section "18.8.1.3 Off-core
Response Performance Monitoring in the Processor Core", it defines the
REMOTE_CACHE_FWD as:

"L3 Miss: local homed requests that missed the L3 cache and was serviced
by forwarded data following a cross package snoop where no modified copies
found. (Remote home requests are not counted)".

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.

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.

Thanks,
Leo