Re: [GIT PULL] perf tools changes for v6.10

[Arnaldo Carvalho de Melo <acme@kernel.org>]

On Fri, May 24, 2024 at 06:31:52PM -0700, Linus Torvalds wrote:
> 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.

That is a good data point, we probably could go with the revert, but I
think Ian submitted a few patches fixing this issue that came up close
to LSFMM/BPF and the merge window, so didn't have time to sit on
linux-next for a while, I'm looking those up now.

ARM64 eyes on this would also be good. Adding Mark Rutland and Leo Yan
to the CC list, maybe they can help us here with the best course of
action.

- Arnaldo