linux-kernel - Re: [PATCH 2/3] perf metricgroup: Fix uncore metric expressions

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Message-ID: <CAM9d7cg+tzp=5QDF4ZA+NPrv-NY9LntRPinCi98yu-90C=Vfiw@mail.gmail.com>
Date:   Thu, 10 Sep 2020 14:55:34 +0900
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@...hat.com>, Kajol Jain <kjain@...ux.ibm.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Jin Yao <yao.jin@...ux.intel.com>,
        Hongbo Yao <yaohongbo@...wei.com>,
        Thomas Richter <tmricht@...ux.ibm.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>, Jin@...gle.com
Subject: Re: [PATCH 2/3] perf metricgroup: Fix uncore metric expressions

On Thu, Sep 10, 2020 at 12:41 PM Ian Rogers <irogers@...gle.com> wrote:
>
> On Wed, Sep 9, 2020 at 10:52 AM Ian Rogers <irogers@...gle.com> wrote:
> >
> > On Wed, Sep 9, 2020 at 2:53 AM Namhyung Kim <namhyung@...nel.org> wrote:
> > >
> > > Hi Ian,
> > >
> > > On Wed, Sep 9, 2020 at 5:02 PM Ian Rogers <irogers@...gle.com> wrote:
> > > >
> > > > A metric like DRAM_BW_Use has on SkylakeX events uncore_imc/cas_count_read/
> > > > and uncore_imc/case_count_write/. These events open 6 events per socket
> > > > with pmu names of uncore_imc_[0-5]. The current metric setup code in
> > > > find_evsel_group assumes one ID will map to 1 event to be recorded in
> > > > metric_events. For events with multiple matches, the first event is
> > > > recorded in metric_events (avoiding matching >1 event with the same
> > > > name) and the evlist_used updated so that duplicate events aren't
> > > > removed when the evlist has unused events removed.
> > > >
> > > > Before this change:
> > > > $ /tmp/perf/perf stat -M DRAM_BW_Use -a -- sleep 1
> > > >
> > > >  Performance counter stats for 'system wide':
> > > >
> > > >              41.14 MiB  uncore_imc/cas_count_read/
> > > >      1,002,614,251 ns   duration_time
> > > >
> > > >        1.002614251 seconds time elapsed
> > > >
> > > > After this change:
> > > > $ /tmp/perf/perf stat -M DRAM_BW_Use -a -- sleep 1
> > > >
> > > >  Performance counter stats for 'system wide':
> > > >
> > > >             157.47 MiB  uncore_imc/cas_count_read/ #     0.00 DRAM_BW_Use
> > >
> > > Hmm.. I guess the 0.00 result is incorrect, no?
> >
> > Agreed. There are a number of pre-existing bugs in this code. I'll try
> > to look into this one.
>
> There was a bug where the metric_leader wasn't being set correctly and
> so the accumulation of values wasn't working as expected. There's a
> small fix in:
> https://lore.kernel.org/lkml/20200910032632.511566-3-irogers@google.com/T/#u
> that also does the change I mentioned below.
>
> The metric still remains at 0.0 following this change as I believe
> there is a bug in the metric. The metric expression is:
> "( 64 * ( uncore_imc@..._count_read@ + uncore_imc@..._count_write@ ) /
> 1000000000 ) / duration_time"
> ie the counts of uncore_imc/cas_count_read/ and
> uncore_imc/cas_count_write/ are being first being scaled up by 64,
> that is to turn a count of cache lines into bytes, the count is then
> divided by 1000000000 or a GB to supposedly give GB/s. However, the
> counts are read and scaled:
>
> ...
> void perf_stat__update_shadow_stats(struct evsel *counter, u64 count,
> ...
>   count *= counter->scale;
> ...
>
> The scale value from
> /sys/devices/uncore_imc_0/events/cas_count_read.scale is
> 6.103515625e-5 or 64/(1024*1024). This means the result of the
> expression is 64 times too large but in PB/s and so too small to
> display. I don't rule out there being other issues but this appears to
> be a significant one. Printing using intervals also looks suspicious
> as the count appears to just increase rather than vary up and down.

You're right!

Maybe we can change it to simply like below and change the unit to MiB/s?

( uncore_imc@..._count_read@ + uncore_imc@..._count_write@ ) / duration_time

Thanks
Namhyung