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

[Atish Kumar Patra]

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.
>
> 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(-)
>

Thanks for the series. Tested on RISC-V qemu with counter delegation enabled.
I am able to get the event encodings specified in the json file
instead of legacy event encodings.

--------------------------------
root at debian:~/host# ./perf stat -e cycles -e instructions -vvv ls
Control descriptor is not initialized
Opening: cycles
------------------------------------------------------------
perf_event_attr:
  type                             4 (cpu)
  size                             136
  config                           0x1 (cycles)
  sample_type                      IDENTIFIER
  read_format                      TOTAL_TIME_ENABLED|TOTAL_TIME_RUNNING
  disabled                         1
  inherit                          1
  enable_on_exec                   1
  { bp_len, config2 }              0x7f9
------------------------------------------------------------
sys_perf_event_open: pid 424  cpu -1  group_fd -1  flags 0x8 = 3
Opening: instructions
------------------------------------------------------------
perf_event_attr:
  type                             4 (cpu)
  size                             136
  config                           0x2 (instructions)
  sample_type                      IDENTIFIER
  read_format                      TOTAL_TIME_ENABLED|TOTAL_TIME_RUNNING
  disabled                         1
  inherit                          1
  enable_on_exec                   1
  { bp_len, config2 }              0x7fc
------------------------------------------------------------
sys_perf_event_open: pid 424  cpu -1  group_fd -1  flags 0x8 = 4
-----------------------------------------------------------------------------------

However, we may still have to provide the encodings in the driver for
vendors due to the reasons described in the
thread. Hopefully, we can get rid of legacy PMU events one day which
will allow us to remove those driver bindings in the future.

Tested-by: Atish Patra <atishp at rivosinc.com>

> --
> 2.47.0.163.g1226f6d8fa-goog
>