[PATCH v1 0/4] Prefer sysfs/JSON events also when no PMU is provided

[James Clark]

On 07/11/2024 18:51, Ian Rogers wrote:
> On Sat, Oct 26, 2024 at 5:18 AM Ian Rogers <irogers at google.com> wrote:
>>
>> At the RISC-V summit the topic of avoiding event data being in the
>> RISC-V PMU kernel driver came up. There is a preference for sysfs/JSON
>> events being the priority when no PMU is provided so that legacy
>> events maybe supported via json. Originally Mark Rutland also
>> expressed at LPC 2023 that doing this would resolve bugs on ARM Apple
>> M? processors, but James Clark more recently tested this and believes
>> the driver issues there may not have existed or have been resolved. In
>> any case, it is inconsistent that with a PMU event names avoid legacy
>> encodings, but when wildcarding PMUs (ie without a PMU with the event
>> name) the legacy encodings have priority.
>>
>> The patch doing this work was reverted in a v6.10 release candidate
>> as, even though the patch was posted for weeks and had been on
>> linux-next for weeks without issue, Linus was in the habit of using
>> explicit legacy events with unsupported precision options on his
>> Neoverse-N1. This machine has SLC PMU events for bus and CPU cycles
>> where ARM decided to call the events bus_cycles and cycles, the latter
>> being also a legacy event name. ARM haven't renamed the cycles event
>> to a more consistent cpu_cycles and avoided the problem. With these
>> changes the problematic event will now be skipped, a large warning
>> produced, and perf record will continue for the other PMU events. This
>> solution was proposed by Arnaldo.
>>
>> Two minor changes have been added to help with the error message and
>> to work around issues occurring with "perf stat metrics (shadow stat)
>> test".
>>
>> The patches have only been tested on my x86 non-hybrid laptop.
> 
> Hi Atish and James,
> 
> Could I get your tags for this series?
> 
> The patches were originally motivated by wanting to make the behavior
> of events parsed like "cycles" match that of "cpu/cycles/", the PMU is
> wildcarded to "cpu" in the first case. This was divergent because of
> ARM we switched from preferring legacy (type = PERF_TYPE_HARDWARE,
> config = PERF_COUNT_HW_CPU_CYCLES) to sysfs/json (type=<core PMU's
> type>, config=<encoding from event>) when a PMU name was given. This
> aligns with RISC-V wanting to use json encodings to avoid complexity
> in the PMU driver.
> 

I couldn't find the thread, but I remember fairly recently it was 
mentioned that RISC-V would be supporting the legacy events after all, 
maybe it was a comment from Atish? I'm not sure if that changes the 
requirements for this or not?

I still can't really imagine how tooling would work if every tool has to 
maintain the mappings of basic events like instructions and branches. 
For example all the perf_event_open tests in ltp use the legacy events.

And wouldn't porting existing software to RISC-V would be an issue if it 
doesn't behave in a similar way to what's there already?

> James, could you show the neoverse with the cmn PMU behavior for perf
> record of "cycles:pp" due to sensitivities there.
> 

Yep I can check this on Monday.

> Thanks,
> Ian
> 


> 
> 
> 
>> Ian Rogers (4):
>>    perf evsel: Add pmu_name helper
>>    perf stat: Fix find_stat for mixed legacy/non-legacy events
>>    perf record: Skip don't fail for events that don't open
>>    perf parse-events: Reapply "Prefer sysfs/JSON hardware events over
>>      legacy"
>>
>>   tools/perf/builtin-record.c    | 22 +++++++---
>>   tools/perf/util/evsel.c        | 10 +++++
>>   tools/perf/util/evsel.h        |  1 +
>>   tools/perf/util/parse-events.c | 26 +++++++++---
>>   tools/perf/util/parse-events.l | 76 +++++++++++++++++-----------------
>>   tools/perf/util/parse-events.y | 60 ++++++++++++++++++---------
>>   tools/perf/util/pmus.c         | 20 +++++++--
>>   tools/perf/util/stat-shadow.c  |  3 +-
>>   8 files changed, 145 insertions(+), 73 deletions(-)
>>
>> --
>> 2.47.0.163.g1226f6d8fa-goog
>>