Re: [PATCH v3 0/4] Prefer sysfs/JSON events also when no PMU is provided

[Ian Rogers]

On Mon, Dec 23, 2024 at 7:09 AM James Clark <james.clark@xxxxxxxxxx> wrote:
>
>
>
> On 21/12/2024 7:26 pm, Ian Rogers 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.
> >
> > v3: Make no events opening for perf record a failure as suggested by
> >      James Clark and Aditya Bodkhe <Aditya.Bodkhe1@xxxxxxx>. Also,
> >      rebase.
>
> Looks like this could be interacting with the dummy event for itrace
> events which I must have missed before. Now it "fails" but the exit code
> is 0 which some of the tests rely on. I noticed "Miscellaneous Intel PT
> testing" is failing because its skip mechanism is broken:
>
> $ sudo perf record -e intel_pt/aux-action=start-paused/u
> Error:
> Failure to open event 'intel_pt/aux-action=start-paused/u' on PMU
> 'intel_pt' which will be removed.
>
> $ echo $?
> 0
>
> So the test thinks it has the aux-action feature but it doesn't.

Thanks James, there is also discussion of this here:
https://lore.kernel.org/lkml/CAP-5=fX8hWF-PaAE2VKHW3fk1W19xd0hyBVsP3653J9xw-U7VQ@xxxxxxxxxxxxxx/
I think in v3 I need to switch from detecting adding dummy events, to
detecting parsing of dummy events. Then if there is nothing but dummy
events and none were parsed we exit.

Thanks,
Ian

> > v2: Rebase and add tested-by tags from James Clark, Leo Yan and Atish
> >      Patra who have tested on RISC-V and ARM CPUs, including the
> >      problem case from before.
> >
> > 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    | 34 ++++++++++++---
> >   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, 156 insertions(+), 74 deletions(-)
> >
>