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

[James Clark <james.clark@linaro.org>]

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@ibm.com>. 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.

> 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(-)
>