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

From: Leo Yan <leo.yan@linaro.org>
To: Ali Saidi <alisaidi@amazon.com>
Cc: kan.liang@linux.intel.com, Nick.Forrington@arm.com,
	acme@kernel.org, ak@linux.intel.com,
	alexander.shishkin@linux.intel.com, andrew.kilroy@arm.com,
	benh@kernel.crashing.org, german.gomez@arm.com,
	james.clark@arm.com, john.garry@huawei.com, jolsa@kernel.org,
	kjain@linux.ibm.com, lihuafei1@huawei.com,
	linux-arm-kernel@lists.infradead.org,
	linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org,
	mark.rutland@arm.com, mathieu.poirier@linaro.org,
	mingo@redhat.com, namhyung@kernel.org, peterz@infradead.org,
	will@kernel.org
Subject: Re: [PATCH v5 2/5] perf: Add SNOOP_PEER flag to perf mem data struct
Date: Sat, 23 Apr 2022 14:38:05 +0800	[thread overview]
Message-ID: <20220423063805.GA559531@leoy-ThinkPad-X240s> (raw)
In-Reply-To: <20220422212249.22463-1-alisaidi@amazon.com>

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

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