Re: [PATCH v8 0/5] perf evsel fallback changes, better s390 defaults

Thomas Richter
2026-03-19T08:03:21Z

On 3/19/26 06:39, Ian Rogers wrote:
> On Wed, Mar 18, 2026 at 4:46 PM Ian Rogers <irogers@google.com> wrote:
>>
>> Discussion with Thomas Richter in:
>> https://lore.kernel.org/lkml/20260306071002.2526085-1-tmricht@linux.ibm.com/
>> showed that the evsel__fallback wasn't working for s390. These patches
>> avoid the problematic frame pointer callchain on s390 and fix
>> evsel__fallback from a range of problems when falling back to a
>> software event. I simulated failures when developing the patches but
>> they are untested other than that.
>
> I think at this point I call it a day. Sashiko still has feedback that
> could justify a v9:
> https://sashiko.dev/#/patchset/20260318234600.730340-1-irogers%40google.com
> Specifically:
> 1) software PMUs may fail if sysfs isn't mounted. This isn't a real
> problem as "software" is a "well known" PMU that we create even if
> sysfs isn't mounted.

I totally agree with you. Patch set 8 should be it.
Honestly, if /sys (of sysfs) is not mounted, I guess perf tool is the
least of the issues that arise. I believe this is now getting
'over-engineered'.

> 2) the handling of callchain in .perfconfig files isn't right, but the
> patches aren't making it any worse. I worry there could be several
> more patches if I start fixing things wrong with .perfconfig.

Correct, you can not fix everything in one patch set :-)

> 3) the possibility to add another NULL check for safety exists, but
> the code would already crash at the same point.
> Apparently, the tendency of prompts to generate further refinements
> instead of providing all the problems at once is a known limitation of
> LLMs and the current prompting methods.
>
> Thanks,
> Ian
>
my 2 cents....
--
Thomas Richter, Dept 3303, IBM s390 Linux Development, Boeblingen, Germany
--
IBM Deutschland Research & Development GmbH

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