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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
              126.97 MiB  uncore_imc/cas_count_write/
       1,003,019,728 ns   duration_time

Erroneous duplication introduced in:
commit 2440689d ("perf metricgroup: Remove duped metric group events").

Fixes: ded80bda

 ("perf expr: Migrate expr ids table to a hashmap").
Reported-by: Jin Yao <yao.jin@linux.intel.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Andrii Nakryiko <andriin@fb.com>
Cc: Athira Jajeev <atrajeev@linux.vnet.ibm.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: KP Singh <kpsingh@chromium.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: Stephane Eranian <eranian@google.com>
Cc: Yonghong Song <yhs@fb.com>
Cc: bpf@vger.kernel.org
Cc: netdev@vger.kernel.org
Link: http://lore.kernel.org/lkml/20200917201807.4090224-1-irogers@google.com

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>