linux-kernel - Re: [PATCH 0/3] perf vendor events amd: Address event errata

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Message-ID: <CAP-5=fUEeFb3jh-MtxEEH0Z+HFAD0oxSc4uE66Rfg+BRzYRB5Q@mail.gmail.com>
Date: Wed, 7 May 2025 08:56:48 -0700
From: Ian Rogers <irogers@...gle.com>
To: Sandipan Das <sandipan.das@....com>
Cc: linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Adrian Hunter <adrian.hunter@...el.com>, Kan Liang <kan.liang@...ux.intel.com>, 
	Stephane Eranian <eranian@...gle.com>, Ravi Bangoria <ravi.bangoria@....com>, 
	Ananth Narayan <ananth.narayan@....com>
Subject: Re: [PATCH 0/3] perf vendor events amd: Address event errata

On Wed, May 7, 2025 at 7:28 AM Sandipan Das <sandipan.das@....com> wrote:
>
> Remove unreliable Zen 5 events and metrics. The following errata from
> the Revision Guide for AMD Family 1Ah Models 00h-0Fh Processors have
> been addressed.
> #1569 PMCx078 Counts Incorrectly in Unpredictable Ways
> #1583 PMCx18E May Overcount Instruction Cache Accesses
> #1587 PMCx188 May Undercount IBS (Instruction Based Sampling) Fetch Events
>
> The document can be downloaded from
> https://bugzilla.kernel.org/attachment.cgi?id=308095

Hi Sandipan,

the document is somewhat brief, for example:
```
1583 PMCx18E May Overcount Instruction Cache Accesses

Description
If PMCx18E[IcAccessTypes] is programmed to 18x (Instruction Cache
Miss) or 1Fx (All Instruction Cache Accesses) then the performance
counter may overcount.

Potential Effect on System
Inaccuracies in performance monitoring software may be experienced.

Suggested Workaround
None

Fix Planned
No fix planned
```
Given being able to count instruction cache accesses (for example) is
a useful feature, would it be possible to change:
```
-  {
-    "EventName": "ic_tag_hit_miss.instruction_cache_hit",
-    "EventCode": "0x18e",
-    "BriefDescription": "Instruction cache hits.",
-    "UMask": "0x07"
-  },
...
```
to be say:
```
  {
    "EventName": "ic_tag_hit_miss.instruction_cache_hit",
    "EventCode": "0x18e",
    "BriefDescription": "Instruction cache hits. Note, this counter is
affected by errata 1583.",
    "UMask": "0x07",
    "Experimental": "1"
  },
```
That is rather than remove the event, the event is tagged as
experimental (taken to mean accuracy isn't guaranteed) and the errata
is explicitly noted in the description. Currently the Experimental tag
has no impact on what happens in the perf tool, for example, the
"Deprecated" tag hides events in the `perf list` command and is
commonly used when an event is renamed.

Thanks,
Ian
> Sandipan Das (3):
>   perf vendor events amd: Remove Zen 5 instruction cache events
>   perf vendor events amd: Remove Zen 5 TLB flush event
>   perf vendor events amd: Remove Zen 5 IBS fetch event
>
>  .../arch/x86/amdzen5/inst-cache.json          | 24 -------------------
>  .../arch/x86/amdzen5/load-store.json          |  6 -----
>  .../arch/x86/amdzen5/recommended.json         | 13 ----------
>  3 files changed, 43 deletions(-)
>
> --
> 2.43.0
>