linux-kernel - Re: [RFC PATCH v10 3/8] perf stat: Fork and launch perf record when perf stat needs to get retire latency value for a metric.

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Date: Tue, 4 Jun 2024 15:41:50 -0700
From: Ian Rogers <irogers@...gle.com>
To: Namhyung Kim <namhyung@...nel.org>
Cc: "Wang, Weilin" <weilin.wang@...el.com>, Arnaldo Carvalho de Melo <acme@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	"Hunter, Adrian" <adrian.hunter@...el.com>, Kan Liang <kan.liang@...ux.intel.com>, 
	"linux-perf-users@...r.kernel.org" <linux-perf-users@...r.kernel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Taylor, Perry" <perry.taylor@...el.com>, 
	"Alt, Samantha" <samantha.alt@...el.com>, "Biggers, Caleb" <caleb.biggers@...el.com>
Subject: Re: [RFC PATCH v10 3/8] perf stat: Fork and launch perf record when
 perf stat needs to get retire latency value for a metric.

On Tue, Jun 4, 2024 at 3:32 PM Namhyung Kim <namhyung@...nel.org> wrote:
>
> On Tue, Jun 04, 2024 at 08:00:26PM +0000, Wang, Weilin wrote:
>
> > > On Thu, May 30, 2024 at 11:46 PM Wang, Weilin <weilin.wang@...el.com>
> > > > > > +       /*
> > > > > > +        * Only set retire_latency value to the first CPU and thread.
> > > > > > +        */
> > > > > > +       if (cpu_map_idx == 0 && thread == 0) {
> > > > > > +       /* Lost precision when casting from double to __u64. Any
> > > > > improvement? */
> > > > >
> > > > > As I said before I think you can set t->val * 1000 and then
> > > > > set the evsel->scale to 1e3 or 1e-3.
> > > >
> > > > Hi Namhyung,
> > > >
> > > > Sorry if this is a repeated message. I thought I replied to your suggestion
> > > > on this last time. I'm thinking we should keep it like this for now and make
> > > > this change unless we find the precision loss is critical. Because I thought
> > > > we don't want to add special code to handle the calculation and final print
> > > > to keep code simple.
> > > >
> > > > I kept this comment here so that we don't forget about it. Please let me
> > > > know if you'd like me to remove it.
> > >
> > > Please see print_counter_aggrdata().  It's the generic code to print
> > > the event value and it'll display the value multiplied by the scale
> > > (default to 1.0).  So you can keep precision as long as you set the
> > > scale properly (1e-3).
> >
> > I could see the retire_latency is printed correctly after set the evsel->scale to 1e-3
> > and multiply the t->val * 1000. However, this scale is not used in metric calculations.
> > We need to add code in metric calculation or display part to scale it as well. Is that
> > acceptable or do you have other suggestions?
>
> Hmm.. I don't know if other metric already dealt with the scale like with
> RAPL events.. If not, I think it's reasonable to add that to the metric
> calculation.
>
> Ian, what do you think?

Tbh, I don't understand the conversation and it looks like we're in
the weeds. In metrics the scale/unit from the event aren't used - that
is all events in a metric are the unscaled quantities unless something
is broken.

Thanks,
Ian

> Thanks,
> Namhyung
>