linux-kernel - Re: [PATCH v3 00/15] Legacy hardware/cache events as json

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Message-ID: <055e677e-2a2f-4c56-abe0-9a437dc14d69@linaro.org>
Date: Thu, 11 Sep 2025 14:00:29 +0100
From: James Clark <james.clark@...aro.org>
To: Ian Rogers <irogers@...gle.com>
Cc: 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>, Xu Yang <xu.yang_2@....com>,
 Thomas Falcon <thomas.falcon@...el.com>, Andi Kleen <ak@...ux.intel.com>,
 linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
 bpf@...r.kernel.org, Atish Patra <atishp@...osinc.com>,
 Beeman Strong <beeman@...osinc.com>, Leo Yan <leo.yan@....com>,
 Vince Weaver <vincent.weaver@...ne.edu>
Subject: Re: [PATCH v3 00/15] Legacy hardware/cache events as json



On 10/09/2025 4:00 pm, Ian Rogers wrote:
> On Wed, Sep 10, 2025 at 4:14 AM James Clark <james.clark@...aro.org> wrote:
>>
>> On 28/08/2025 9:59 pm, Ian Rogers wrote:
>>> Mirroring similar work for software events in commit 6e9fa4131abb
>>> ("perf parse-events: Remove non-json software events"). These changes
>>> migrate the legacy hardware and cache events to json.  With no hard
>>> coded legacy hardware or cache events the wild card, case
>>> insensitivity, etc. is consistent for events. This does, however, mean
>>> events like cycles will wild card against all PMUs. A change doing the
>>> same was originally posted and merged from:
>>> https://lore.kernel.org/r/20240416061533.921723-10-irogers@google.com
>>> and reverted by Linus in commit 4f1b067359ac ("Revert "perf
>>> parse-events: Prefer sysfs/JSON hardware events over legacy"") due to
>>> his dislike for the cycles behavior on ARM with perf record. Earlier
>>> patches in this series make perf record event opening failures
>>> non-fatal and hide the cycles event's failure to open on ARM in perf
>>> record, so it is expected the behavior will now be transparent in perf
>>> record on ARM. perf stat with a cycles event will wildcard open the
>>> event on all PMUs.
>>
>> Hi Ian,
>>
>> Briefly testing perf record and perf stat seem to work now. i.e "perf
>> record -e cycles" doesn't fail and just skips the uncore cycles event.
>> And "perf stat" now includes the uncore cycles event which I think is
>> harmless.
> 
> Thanks for confirming this.
> 
>> But there are a few perf test failures. For example "test event parsing":
>>
>>     evlist after sorting/fixing: 'arm_cmn_0/cycles/,{cycles,cache-
>>       misses,branch-misses}'
>>     FAILED tests/parse-events.c:1589 wrong number of entries
>>     Event test failure: test 57 '{cycles,cache-misses,branch-
>>       misses}:e'running test 58 'cycles/name=name/'
> 
> I suspect the easiest fix for this is to change "cycles" to the
> "cpu-cycles" legacy hardware event for this test. The test has always
> had issues on ARM due to hardcoded expectations of the core PMU being
> "cpu".
> 
>> The tests "Perf time to TSC" and "Use a dummy software event to keep
>> tracking" are using libperf to open the cycles event as a sampling event
>> which now fails. It seems like we've fixed Perf record to ignore this
>> failure, but we didn't think about libperf until now.
> 
> I'm not clear on the connection here. libperf doesn't do event parsing
> and so there are no changes in tools/lib/perf. If a test has an
> expectation that "cycles" is a core event, again we can change it to
> "cpu-cycles" as a workaround for ARM. As "cycles" will wildcard now,
> we don't want that behavior in say API probing as we'll end up never
> lazily processing the PMUs. That code has been altered in these
> changes to specify the core PMU. For tests it is less of an issue and
> so the changes are more limited.
> 
> Thanks,
> Ian

Sure makes sense if there's an easy fix for the tests, we can do that. I 
suppose the main reason I mentioned it was that the tests might be 
highlighting that other genuine non-Perf and non-test users would see 
the same breakage though.