RE: [RFC PATCH v6 2/5] 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: Wednesday, April 24, 2024 11:50 AM
> 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 v6 2/5] perf stat: Fork and launch perf record when
> perf stat needs to get retire latency value for a metric.
> 
> On Wed, Apr 24, 2024 at 10:08 AM Wang, Weilin <weilin.wang@xxxxxxxxx>
> wrote:
> >
> >
> >
> > > -----Original Message-----
> > > From: Namhyung Kim <namhyung@xxxxxxxxxx>
> > > Sent: Tuesday, April 23, 2024 4:06 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 v6 2/5] perf stat: Fork and launch perf record
> when
> > > perf stat needs to get retire latency value for a metric.
> > >
> > > On Tue, Apr 23, 2024 at 3:16 PM Wang, Weilin <weilin.wang@xxxxxxxxx>
> > > wrote:
> > > > > > > > -static int __run_perf_record(void)
> > > > > > > > +static int __run_perf_record(const char **record_argv)
> > > > > > > >  {
> > > > > > > > +       int i = 0;
> > > > > > > > +       struct tpebs_event *e;
> > > > > > > > +
> > > > > > > >         pr_debug("Prepare perf record for retire_latency\n");
> > > > > > > > +
> > > > > > > > +       record_argv[i++] = "perf";
> > > > > > > > +       record_argv[i++] = "record";
> > > > > > > > +       record_argv[i++] = "-W";
> > > > > > > > +       record_argv[i++] = "--synth=no";
> > > > > > > > +
> > > > > > > > +       if (stat_config.user_requested_cpu_list) {
> > > > > > > > +               record_argv[i++] = "-C";
> > > > > > > > +               record_argv[i++] = stat_config.user_requested_cpu_list;
> > > > > > > > +       }
> > > > > > > > +
> > > > > > > > +       if (stat_config.system_wide)
> > > > > > > > +               record_argv[i++] = "-a";
> > > > > > > > +
> > > > > > > > +       if (!stat_config.system_wide
> > > > > && !stat_config.user_requested_cpu_list)
> > > > > > > {
> > > > > > > > +               pr_err("Require -a or -C option to run sampling.\n");
> > > > > > > > +               return -ECANCELED;
> > > > > > > > +       }
> > > > > > > > +
> > > > > > > > +       list_for_each_entry(e, &stat_config.tpebs_events, nd) {
> > > > > > > > +               record_argv[i++] = "-e";
> > > > > > > > +               record_argv[i++] = e->name;
> > > > > > > > +       }
> > > > > > > > +
> > > > > > > > +       record_argv[i++] = "-o";
> > > > > > > > +       record_argv[i++] = PERF_DATA;
> > > > > > > > +
> > > > > > > >         return 0;
> > > > > > > >  }
> > > > > > >
> > > > > > > Still I think it's weird it has 'perf record' in perf stat (despite the
> > > > > > > 'perf stat record').  If it's only Intel thing and we don't have a plan
> > > > > > > to do the same on other arches, we can move it to the arch
> > > > > > > directory and keep the perf stat code simple.
> > > > > >
> > > > > > I'm not sure what is the proper way to solve this. And Ian mentioned
> > > > > > that put code in arch directory could potentially cause other bugs.
> > > > > > So I'm wondering if we could keep this code here for now. I could
> work
> > > > > > on it later if we found it's better to be in arch directory.
> > > > >
> > > > > Maybe somewhere in the util/ and keep the main code minimal.
> > > > > IIUC it's only for very recent (or upcoming?) Intel CPUs and we
> > > > > don't have tests (hopefully can run on other arch/CPUs).
> > > > >
> > > > > So I don't think having it here would help fixing potential bugs.
> > > >
> > > > Do you mean by creating a new file in util/ to hold this code?
> > >
> > > Yeah, maybe util/intel-tpebs.c (if it's better than arch/x86/...) ?
> > >
> > > >
> > > > Yes, this feature is for very recent Intel CPUs. It should only be triggered if
> > > > a metric uses event(s) that has the R modifier in the formula.
> > >
> > > Can we have a test with a fake metric so that we can test
> > > the code on non-(or old-)Intel machines?
> >
> > All the existing metrics in non-(or old-)Intel CPUs should work as usual. So I
> think
> > existing metric tests should work for it.
> >
> > If we want to add a fake metric uses the :R modifier for those platforms, the
> metric
> > should either fail (if the fake metric uses an event not exist on the test
> platform) or
> > return all 0 retire latency data.
> >
> > So, I'm not quite sure what we want the fake metric to test for. Maybe we
> could
> > continue using existing metric tests to ensure all the defined metrics work
> correctly
> > in each machine under test?
> 
> I think it's ok to return all 0 retire latency for fake tPEBS metrics.
> It's just to verify the background record + report logic works ok.

Ok, I will work on this. We need a fake metric that uses events exist on all the 
platforms. 

Thanks,
Weilin

> 
> Thanks,
> Namhyung