linux-kernel - Re: [PATCH v1 04/12] perf parse-events: Allow the cpu term to be a PMU

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Message-ID: <aHqKvOfY2QQOEcjB@google.com>
Date: Fri, 18 Jul 2025 10:56:12 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Ian Rogers <irogers@...gle.com>
Cc: Thomas Falcon <thomas.falcon@...el.com>,
	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>,
	Ben Gainey <ben.gainey@....com>,
	James Clark <james.clark@...aro.org>,
	Howard Chu <howardchu95@...il.com>,
	Weilin Wang <weilin.wang@...el.com>, Levi Yun <yeoreum.yun@....com>,
	"Dr. David Alan Gilbert" <linux@...blig.org>,
	Zhongqiu Han <quic_zhonhan@...cinc.com>,
	Blake Jones <blakejones@...gle.com>,
	Yicong Yang <yangyicong@...ilicon.com>,
	Anubhav Shelat <ashelat@...hat.com>,
	Thomas Richter <tmricht@...ux.ibm.com>,
	Jean-Philippe Romain <jean-philippe.romain@...s.st.com>,
	Song Liu <song@...nel.org>, linux-perf-users@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 04/12] perf parse-events: Allow the cpu term to be a
 PMU

Hi Ian,

On Wed, Jul 16, 2025 at 01:25:17PM -0700, Ian Rogers wrote:
> On Wed, Jul 16, 2025 at 1:09 PM Namhyung Kim <namhyung@...nel.org> wrote:
> >
> > On Fri, Jun 27, 2025 at 12:24:09PM -0700, Ian Rogers wrote:
> > > On hybrid systems, events like msr/tsc/ will aggregate counts across
> > > all CPUs. Often metrics only want a value like msr/tsc/ for the cores
> > > on which the metric is being computed. Listing each CPU with terms
> > > cpu=0,cpu=1.. is laborious and would need to be encoded for all
> > > variations of a CPU model.
> > >
> > > Allow the cpumask from a PMU to be an argument to the cpu term. For
> > > example in the following the cpumask of the cstate_pkg PMU selects the
> > > CPUs to count msr/tsc/ counter upon:
> > > ```
> > > $ cat /sys/bus/event_source/devices/cstate_pkg/cpumask
> > > 0
> > > $ perf stat -A -e 'msr/tsc,cpu=cstate_pkg/' -a sleep 0.1
> >
> > It can be confusing if 'cpu' takes a number or a PMU name.  What about
> > adding a new term (maybe 'cpu_from') to handle this case?
> 
> So it is possible for terms to be defined in sysfs in the 'format/' folder:
> ```
> $ ls /sys/bus/event_source/devices/cpu_core/format/
> cmask  edge  event  frontend  inv  ldlat  offcore_rsp  pc  umask
> ```
> By not introducing a new term we leave 'cpu_from' open for use in this
> way. When I spoke to Kan we thought using the existing term made sense
> and fits the idea of leaving things open for the kernel/drivers to
> use. It is possible to add a new term though. Let me know and I can
> update the patch and documentation accordingly.

Oh, you thought about this already.  It's true that it's possible to
clash with PMU formats in sysfs unless we have a separate namespace for
tools somehow.  But that would add (maybe unnecessary) complexity.

So I'm not against with this change.  I just wanted to ring an alarm for
potential issues.  Up to you. :)

Thanks,
Namhyung