Re: [PATCH v5 0/4] Prefer sysfs/JSON events also when no PMU is provided

[Namhyung Kim <namhyung@kernel.org>]

On Thu, Jan 09, 2025 at 02:21:05PM -0800, 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.
> 
> v5: Follow Namhyung's suggestion and ignore the case where command
>     line dummy events fail to open alongside other events that all
>     fail to open. Note, the Tested-by tags are left on the series as
>     v4 and v5 were changing an error case that doesn't occur in
>     testing but was manually tested by myself.
> 
> v4: Rework the no events opening change from v3 to make it handle
>     multiple dummy events. Sadly an evlist isn't empty if it just
>     contains dummy events as the dummy event may be used with "perf
>     record -e dummy .." as a way to determine whether permission
>     issues exist. Other software events like cpu-clock would suffice
>     for this, but the using dummy genie has left the bottle.
> 
>     Another problem is that we appear to have an excessive number of
>     dummy events added, for example, we can likely avoid a dummy event
>     and add sideband data to the original event. For auxtrace more
>     dummy events may be opened too. Anyway, this has led to the
>     approach taken in patch 3 where the number of dummy parsed events
>     is computed. If the number of removed/failing-to-open non-dummy
>     events matches the number of non-dummy events then we want to
>     fail, but only if there are no parsed dummy events or if there was
>     one then it must have opened. The math here is hard to read, but
>     passes my manual testing.
> 
> v3: Make no events opening for perf record a failure as suggested by
>     James Clark and Aditya Bodkhe <Aditya.Bodkhe1@ibm.com>. Also,
>     rebase.
> 
> 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

I think the first two are quite independent.  I'll take them first.

Thanks,
Namhyung


>   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    | 47 ++++++++++++++++++---
>  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, 169 insertions(+), 74 deletions(-)
> 
> -- 
> 2.47.1.613.gc27f4b7a9f-goog
>