linux-kernel - Re: [RFC/PATCHSET 00/11] perf mem: Add new output fields for data source (v1)

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Message-ID: <33984b44-ae3d-4fbd-b918-07289a3f1d8a@amd.com>
Date: Thu, 8 May 2025 09:42:41 +0530
From: Ravi Bangoria <ravi.bangoria@....com>
To: Namhyung Kim <namhyung@...nel.org>
Cc: Arnaldo Carvalho de Melo <acme@...nel.org>,
 Ian Rogers <irogers@...gle.com>, Kan Liang <kan.liang@...ux.intel.com>,
 Jiri Olsa <jolsa@...nel.org>, Adrian Hunter <adrian.hunter@...el.com>,
 Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>,
 LKML <linux-kernel@...r.kernel.org>, linux-perf-users@...r.kernel.org,
 Leo Yan <leo.yan@....com>, Ravi Bangoria <ravi.bangoria@....com>
Subject: Re: [RFC/PATCHSET 00/11] perf mem: Add new output fields for data
 source (v1)

Hi Namhyung,

I feel the overall idea is good. Running few simple perf-mem commands
on AMD works fine too. Few general feedback below.

> The name of some new fields are the same as the corresponding sort
> keys (mem, op, snoop) so I had to change the order whether it's
> applied as an output field or a sort key.  Maybe it's better to name
> them differently but I couldn't come up with better ideas.

1) These semantic changes of the field name seems counter intuitive
   (to me). Example:

   -F mem:

     Without patch:

     $ perf mem report -F overhead,sample,mem --stdio
     # Overhead       Samples  Memory access
         39.29%             1  L3 hit
         37.50%            21  N/A
         23.21%            13  L1 hit

     With patch:

     $ perf mem report -F overhead,sample,mem --stdio
     #                          Memory
     # Overhead       Samples    Other
        100.00%            35   100.0%

   -F 'snoop':

     Without patch:

     $ perf mem report -F overhead,sample,snoop --stdio
     # Overhead       Samples  Snoop
         60.71%            34  N/A
         39.29%             1  HitM
   
     With patchset:

     $ perf mem report -F overhead,sample,snoop --stdio
     #                         --- Snoop ----
     # Overhead       Samples     HitM  Other
        100.00%            35    39.3%  60.7%

2) It was not intuitive (to me:)) that perf-mem overhead is calculated
   using sample->weight by overwriting sample->period. I also don't see
   it documented anywhere (or did I miss it?)

   perf report:

     $ perf report -F overhead,sample,period,dso --stdio
     # Overhead  Samples   Period  Shared Object
         80.00%       28  2800000  [kernel.kallsyms]
          5.71%        2   200000  ld-linux-x86-64.so.2
          5.71%        2   200000  libc.so.6
          5.71%        2   200000  ls
          2.86%        1   100000  libpcre2-8.so.0.11.2

   perf mem report:

     $ perf mem report -F overhead,sample,period,dso --stdio
     # Overhead  Samples   Period  Shared Object
         87.50%       28       49  [kernel.kallsyms]
          3.57%        2        2  ld-linux-x86-64.so.2
          3.57%        2        2  libc.so.6
          3.57%        2        2  ls
          1.79%        1        1  libpcre2-8.so.0.11.2

3) Similarly, it was not intuitive (again, to me:)) that -F op/snoop/dtlb
   percentages are calculated based on sample->weight.

4) I've similar recommended perf-mem command in perf-amd-ibs man page.
   Can you please update alternate command there.
   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/perf/Documentation/perf-amd-ibs.txt?h=v6.15-rc5#n167

Please correct me if I'm missing anything.

Thanks,
Ravi