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

[Ian Rogers]

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.

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

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
>