Re: [PATCH v5 4/4] perf parse-events: Reapply "Prefer sysfs/JSON hardware events over legacy"

[Ian Rogers]

On Wed, Jan 29, 2025 at 1:55 PM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> On Wed, Jan 15, 2025 at 01:20:32PM -0800, Ian Rogers wrote:
> > On Wed, Jan 15, 2025 at 9:59 AM Namhyung Kim <namhyung@xxxxxxxxxx> wrote:
> > > I think the behavior should be:
> > >
> > >   cycles -> PERF_COUNT_HW_CPU_CYCLES
> > >   cpu-cycles -> PERF_COUNT_HW_CPU_CYCLES
> > >   cpu_cycles -> no legacy -> sysfs or json
> > >   cpu/cycles/ -> sysfs or json
> > >   cpu/cpu-cycles/ -> sysfs or json
> >
> > So I disagree as if you add a PMU to an event name the encoding
> > shouldn't change:
> > 1) This historically was perf's behavior.
>
> Well.. I'm not sure about the history.  I believe the logic I said above
> is the historic and (I think) right behavior.

You're wrong as you are describing the behavior post:
https://lore.kernel.org/r/20231123042922.834425-1-irogers@xxxxxxxxxx
commit a24d9d9dc096fc0d0bd85302c9a4fe4fe3b1107b from Nov 2022, but
somehow without legacy event fall backs which Intel added with a PMU
for hybrid.

The behavior in this patch series is best for RISC-V, presumably ARM
(particularly for Apple M? CPUs), carries ARM and Intel's tags,
implements the behavior Arnaldo asked for, and solves the
inconsistency that I think is fundamentally wrong in the tool that PMU
names shouldn't matter on an event name (an inconsistency my past
fixes introduced). It is also part of solving other problems:
https://lore.kernel.org/linux-perf-users/20250127-counter_delegation-v3-0-64894d7e16d5@xxxxxxxxxxxx/

You've not pointed at anything wrong in the scheme that these patches
introduce, and are supported by vendors, except that it is a behavior
change. I can, and have, pointed at many issues with your proposal
above and the current behavior. The behavior change came about to work
around PMU bugs over 2 years ago but only partially did so. It makes
sense to remedy this and for the clean, consistent behavior this
series achieves. It is unfortunate that it is a behavior change, but
the first step for that was made 2 years ago. I think it also makes
sense that something self described as legacy is a lower priority and
of the past (wrt event naming moving forward).

Thanks,
Ian