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

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Message-ID: <20220427161908.GE562576@leoy-ThinkPad-X240s>
Date:   Thu, 28 Apr 2022 00:19:08 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>
Cc:     Andi Kleen <ak@...ux.intel.com>, 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

Hi Kan,

On Mon, Apr 25, 2022 at 01:01:40PM -0400, Liang, Kan wrote:
> 
> 
> 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.

Thanks a lot for the info.

> > > > 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.

It's a bit difficult for me to make decision is because now SNOOPX_FWD
is not used in the file util/mem-events.c, so I am not very sure if
SNOOPX_FWD has the consistent usage across different arches.

On the other hand, I sent a patch for 'peer' flag statistics [1], you
could review it and it only stats for L2 and L3 cache level for local
node.

The main purpose for my sending this email is if you think the FWD can
be the consistent for both arches, and even the new added display mode
is also useful for x86 arch (we can rename it as 'fwd' display mode),
then I am very glad to unify the flag.

Thanks,
Leo

[1] https://lore.kernel.org/lkml/20220427155013.1833222-5-leo.yan@linaro.org/