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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Fri, 24 May 2024 18:31:52 -0700
Message-ID: <CAHk-=wiWvtFyedDNpoV7a8Fq_FpbB+F5KmWK2xPY3QoYseOf_A@mail.gmail.com>
Subject: Re: [GIT PULL] perf tools changes for v6.10
To: Arnaldo Carvalho de Melo <acme@kernel.org>, Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>, Thomas Gleixner <tglx@linutronix.de>, Jiri Olsa <jolsa@kernel.org>, 
	Namhyung Kim <namhyung@kernel.org>, Adrian Hunter <adrian.hunter@intel.com>, 
	Clark Williams <williams@redhat.com>, Kate Carcia <kcarcia@redhat.com>, linux-kernel@vger.kernel.org, 
	linux-perf-users@vger.kernel.org, Anne Macedo <retpolanne@posteo.net>, 
	Bhaskar Chowdhury <unixbhaskar@gmail.com>, Ethan Adams <j.ethan.adams@gmail.com>, 
	James Clark <james.clark@arm.com>, Kan Liang <kan.liang@linux.intel.com>, 
	Thomas Richter <tmricht@linux.ibm.com>, Tycho Andersen <tycho@tycho.pizza>, 
	Yang Jihong <yangjihong@bytedance.com>, Arnaldo Carvalho de Melo <acme@redhat.com>
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On Tue, 21 May 2024 at 12:26, Arnaldo Carvalho de Melo <acme@kernel.org> wrote:
>
> perf tools fixes and improvements for v6.10:

This actually broke 'perf' completely for me on arm64.

With a 6.9 version of 'perf', I can do this:

    perf record -e cycles:pp make -j199

and it all works fine.

With the current -git version, when I do the same, I instead get

  Error:
  cycles:pp: PMU Hardware doesn't support
sampling/overflow-interrupts. Try 'perf stat'

and after trying desperately to chase down what went wrong on the
kernel side, I finally figured out that it wasn't a kernel change at
all, it was the tooling that had changed.

I did a 'git bisect', and it says

  617824a7f0f73e4de325cf8add58e55b28c12493 is the first bad commit
  commit 617824a7f0f73e4de325cf8add58e55b28c12493
  Author: Ian Rogers <irogers@google.com>
  Date:   Mon Apr 15 23:15:25 2024 -0700

      perf parse-events: Prefer sysfs/JSON hardware events over legacy

and very clearly this does *NOT* work at all for me.

I didn't notice until now, simply because I had been busy with the
merge window, so I hadn't been doing any profiles, but the merge
window is calming down and the end is nigh, and I just wasted more
time than I care to admit trying to figure out what went wrong in the
kernel.

And no, I don't speak JSON, and I have *no* idea what the legacy
events are. Plus I'm not very familiar with the arm64 profiling etc
anyway, so I'm just a clueless user here.

I *can* confirm that just reverting that commit makes that trivial
"perf record" work for me. So the bisect was good, and it reverts
cleanly, but I don't know _why_ my arm64 setup hates it so much.

             Linus