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.

[Wang, Weilin]

> -----Original Message-----
> From: Namhyung Kim <namhyung@xxxxxxxxxx>
> Sent: Friday, May 31, 2024 2:40 PM
> To: Wang, Weilin <weilin.wang@xxxxxxxxx>
> Cc: Ian Rogers <irogers@xxxxxxxxxx>; Arnaldo Carvalho de Melo
> <acme@xxxxxxxxxx>; Peter Zijlstra <peterz@xxxxxxxxxxxxx>; Ingo Molnar
> <mingo@xxxxxxxxxx>; Alexander Shishkin
> <alexander.shishkin@xxxxxxxxxxxxxxx>; Jiri Olsa <jolsa@xxxxxxxxxx>; Hunter,
> Adrian <adrian.hunter@xxxxxxxxx>; Kan Liang <kan.liang@xxxxxxxxxxxxxxx>;
> linux-perf-users@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Taylor, Perry
> <perry.taylor@xxxxxxxxx>; Alt, Samantha <samantha.alt@xxxxxxxxx>; Biggers,
> Caleb <caleb.biggers@xxxxxxxxx>
> 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 Thu, May 30, 2024 at 11:46 PM Wang, Weilin <weilin.wang@xxxxxxxxx>
> wrote:
> >
> >
> >
> > > -----Original Message-----
> > > From: Namhyung Kim <namhyung@xxxxxxxxxx>
> > > Sent: Thursday, May 30, 2024 11:41 PM
> > > To: Wang, Weilin <weilin.wang@xxxxxxxxx>
> > > Cc: Ian Rogers <irogers@xxxxxxxxxx>; Arnaldo Carvalho de Melo
> > > <acme@xxxxxxxxxx>; Peter Zijlstra <peterz@xxxxxxxxxxxxx>; Ingo Molnar
> > > <mingo@xxxxxxxxxx>; Alexander Shishkin
> > > <alexander.shishkin@xxxxxxxxxxxxxxx>; Jiri Olsa <jolsa@xxxxxxxxxx>; Hunter,
> > > Adrian <adrian.hunter@xxxxxxxxx>; Kan Liang <kan.liang@xxxxxxxxxxxxxxx>;
> > > linux-perf-users@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Taylor,
> Perry
> > > <perry.taylor@xxxxxxxxx>; Alt, Samantha <samantha.alt@xxxxxxxxx>;
> Biggers,
> > > Caleb <caleb.biggers@xxxxxxxxx>
> > > 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, May 28, 2024 at 11:43 PM <weilin.wang@xxxxxxxxx> wrote:
> > > >
> > > > From: Weilin Wang <weilin.wang@xxxxxxxxx>
> > > >
> > > > When retire_latency value is used in a metric formula, evsel would fork a
> perf
> > > > record process with "-e" and "-W" options. Perf record will collect
> required
> > > > retire_latency values in parallel while perf stat is collecting counting
> values.
> > > >
> > > > At the point of time that perf stat stops counting, evsel would stop perf
> > > record
> > > > by sending sigterm signal to perf record process. Sampled data will be
> > > process
> > > > to get retire latency value.
> > > >
> > > > Another thread is required to synchronize between perf stat and perf
> record
> > > > when we pass data through pipe.
> > > >
> > > > Signed-off-by: Weilin Wang <weilin.wang@xxxxxxxxx>
> > > > ---
> > > [SNIP]
> > > > +int tpebs_set_evsel(struct evsel *evsel, int cpu_map_idx, int thread)
> > > > +{
> > > > +       struct perf_counts_values *count;
> > > > +       struct tpebs_retire_lat *t;
> > > > +       bool found = false;
> > > > +       __u64 val;
> > > > +       int ret;
> > > > +
> > > > +       /* Non reitre_latency evsel should never enter this function. */
> > > > +       if (!evsel__is_retire_lat(evsel))
> > > > +               return -1;
> > > > +
> > > > +       ret = tpebs_stop();
> > > > +       if (ret)
> > > > +               return ret;
> > > > +
> > > > +       count = perf_counts(evsel->counts, cpu_map_idx, thread);
> > > > +
> > > > +       list_for_each_entry(t, &tpebs_results, nd) {
> > > > +               if (!strcmp(t->tpebs_name, evsel->name) || !strcmp(t-
> > > >tpebs_name, evsel->metric_id)) {
> > > > +                       found = true;
> > > > +                       break;
> > > > +               }
> > > > +       }
> > > > +
> > > > +       /* Set ena and run to non-zero */
> > > > +       count->ena = count->run = 1;
> > > > +       count->lost = 0;
> > > > +
> > > > +       if (!found) {
> > > > +               /*
> > > > +                * Set default value or 0 when retire_latency for this event is
> > > > +                * not found from sampling data (enable_tpebs_recording not
> set
> > > > +                * or 0 sample recorded).
> > > > +                */
> > > > +               val = 0;
> > > > +               return 0;
> > > > +       }
> > > > +
> > > > +       /*
> > > > +        * 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).

Ah, okay. I will do this. 

Thanks,
Weilin

> 
> Thanks,
> Namhyung