linux-kernel - Re: [RFC PATCH v1] perf parse-events: Make legacy events lower priority than sysfs/json

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Date:   Thu, 23 Nov 2023 18:49:49 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Ian Rogers <irogers@...gle.com>,
        Mark Rutland <mark.rutland@....com>,
        Marc Zyngier <maz@...nel.org>, Hector Martin <marcan@...can.st>
Cc:     Namhyung Kim <namhyung@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        James Clark <james.clark@....com>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC PATCH v1] perf parse-events: Make legacy events lower
 priority than sysfs/json

Em Thu, Nov 23, 2023 at 07:18:57AM -0800, Ian Rogers escreveu:
> On Thu, Nov 23, 2023 at 6:37 AM Mark Rutland <mark.rutland@....com> wrote:
> > On Wed, Nov 22, 2023 at 08:29:22PM -0800, Ian Rogers wrote:
> > > This is a large behavioral change:
> > > 1) the scope of the change means it should bake on linux-next and I
> > > don't believe should be a 6.7-rc fix.

> > I'm happy for this to bake, but I do think it needs to be backported for the
> > sake of users, especially given that it *restores* the old behaviour.

> It is going to change the behavior for a far larger set of users. I'm
> also concerned that:

> ```
> $ perf list
> ...
>   cpu-cycles OR cpu/cpu-cycles/                      [Kernel PMU event]
> ...
> ```

> implies that cpu/cpu-cycles/ is a synonym for cpu-cycles, which is no
> longer true (or pick another event from sysfs whose name is the same
> as a legacy event). I'm not sure what a fix in perf list for this
> would look like.

It is a mess, indeed, cpu-cycles should be equivalent to
cpu/cpu-cycles/, map to the same HW PMU counter.

But by now I think we need to just reword the output of perf list to not
equate those events, and instead provide a more informative output, if
that is at all possible.

Something like:

Legacy events:

cpu-cycles: some nice explanation about the intent for this one (Ingo's
probably).

And then:

PMU events: (and this is even less clear :-()

cpu/cpu-cycles/: other nice explanation about the intent for this one,
if different, on this arch, for the "legacy" cpu-cycles one.

The original intent, that I called "Ingo's" was to try to somehow
generalize some concepts (CPU cycles, for instante) so that we could get
a rough idea that would allow us to somehow compare results using the
tools over different architectures (and micro-arches, etc).

I think that with these generic names we're now in damage control mode:
what matters is to keep what people got used to to continue to hold. And
that is what I think is the main argument here.

And I really think we should just head people like Hector (perf power
users) and provide a way to aggregate "cycles" over all cores, probably
not in the default output, but in a really simple way, as he seems to
want that and I would'n t be surprised that that may be something useful
after all 8-).

So back to checking why this patch isn't working in some of the
container arches I test this whole shebang, sigh.

- Arnaldo