perf annotate: Add comment about annotated_src->nr_histograms

Commit-ID:  9132d3d92d8953ee79690408a9a24d938be22cd8
Gitweb:     https://git.kernel.org/tip/9132d3d92d8953ee79690408a9a24d938be22cd8
Author:     Arnaldo Carvalho de Melo <[email protected]>
AuthorDate: Fri, 25 May 2018 11:27:38 -0300
Committer:  Arnaldo Carvalho de Melo <[email protected]>
CommitDate: Mon, 4 Jun 2018 10:28:52 -0300

perf annotate: Add comment about annotated_src->nr_histograms

When we have multiple groups in an evlist, say:

  $ perf stat -e '{cycles,instructions},{cache-references,cache-misses}' sleep 1

Performance counter stats for 'sleep 1':

           343,134      cycles:u
           249,292      instructions:u            #    0.73  insn per cycle
            15,556      cache-references:u
             8,925      cache-misses:u            #   57.373 % of all cache refs

       1.000957550 seconds time elapsed

$

Then the perf_evsel instances for the two group leaders ("cycles" and "cache-references") will have evsel->nr_members set to 2, while all the evsel->evlist->nr_entries will be set to 4, so we can't use evsel->evlist->nr_entries everywhere, as event groups need to be taken into account.

But this probably requires us to audit at least the forced-group code, where we want all of the events to be in a "group", to see them all in the screen, one column for each, even knowing that they were not necessarily scheduled to count at the same time by the kernel perf subsystem.

---
 tools/perf/util/annotate.h | 4 ++++
 1 file changed, 4 insertions(+)

index 7ad503fbff74..3dc4ca1d6c08 100644
@@ -202,6 +202,10 @@ struct cyc_hist {
 /** struct annotated_source - symbols with hits have this attached as in sannotation
  *
  * @histograms: Array of addr hit histograms per event being monitored
+ * nr_histograms: This may not be the same as evsel->evlist->nr_entries if
+ * 		  we have more than a group in a evlist, where we will want
+ * 		  to see each group separately, that is why symbol__annotate2()
+ * 		  sets src->nr_histograms to evsel->nr_members.
  * @lines: If 'print_lines' is specified, per source code line percentages
  * @source: source parsed from a disassembler like objdump -dS
  * @cyc_hist: Average cycles per basic block