linux-kernel - Re: [PATCH v3 00/15] Legacy hardware/cache events as json

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Message-ID: <aMNEP5rMBkq2ODAW@google.com>
Date: Thu, 11 Sep 2025 14:50:55 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Ian Rogers <irogers@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Mark Rutland <mark.rutland@....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@...aro.org>, Xu Yang <xu.yang_2@....com>,
	Thomas Falcon <thomas.falcon@...el.com>,
	Andi Kleen <ak@...ux.intel.com>, linux-kernel@...r.kernel.org,
	linux-perf-users@...r.kernel.org, bpf@...r.kernel.org,
	Atish Patra <atishp@...osinc.com>,
	Beeman Strong <beeman@...osinc.com>, Leo Yan <leo.yan@....com>,
	Vince Weaver <vincent.weaver@...ne.edu>
Subject: Re: [PATCH v3 00/15] Legacy hardware/cache events as json

On Wed, Sep 10, 2025 at 02:58:05PM -0700, Ian Rogers wrote:
> On Wed, Sep 10, 2025 at 1:10 PM Namhyung Kim <namhyung@...nel.org> wrote:
> > A nit.  Can we have one actual event and an alias of it?
> >
> > I think 'branch-instructions' will be the actual event and 'branches'
> > will be the alias.  Then the description will be like
> >
> >   branch-instructions
> >       [Retired branch instructions.  Unit: cpu_atom]
> >   ...
> >
> >   branches
> >       [This event is an alias of branch-instructions.]
> >
> > The same goes to 'cycles' and 'cpu-cycles'.
> 
> Similar 'cs' and 'context-switches' in
> tools/perf/pmu-events/arch/common/common/software.json.
> 
> So there are a few different ways to do this:
> 
> 1) In perf list detect two events have the same encoding and list them together.
> 2) In the json have a new aliases list then either:
> 2.1) gets expanded in jevents.py as part of the build,
> 2.2) passes into the pmu-events.c and the C code is updated to use an
> alias list associated with each event.
> 
> Option (1) will have something like quadratic complexity, but a fast
> perf list isn't a particular goal I've heard of.
> Option (2.2) will mean the existing binary searches for events will
> become a binary search for an event and then linear searches through
> the aliases. To make this not a slowdown we'd likely need more lookup
> tables to avoid the linear searches.
> Option (2.1) feels the most plausible. I was hoping the json and the
> sysfs layout would kind of match, this would be true after the
> jevents.py expands the aliases. This option is already kind of already
> done in the legacy cache case as the
> tools/perf/pmu-events/make_legacy_cache.py is making this. We'd still
> need option (1) with this.
> 
> Anyway, I'm not sure these downsides are countered by a slightly
> smaller hardware.json and software.json, or maybe we should just go
> with option 1 if the perf list output is all you care about. Let me
> know if you see a different way of making it happen. I don't think the
> vendors will be particularly happy for their upstream formats to
> change given other tools will rely on them.

Well, I was asking just to update the description in JSON.  I'm not sure
if it's a common problem we need to solve.  Updating a few known aliases
in the hardware and software description would be fine.

Thanks,
Namhyung