This simply adds the field to 'struct perf_evsel' and allows setting
it via the event parser, to test it lets trace trace:

First look at where in a function that receives an evsel we can put a probe
to read how evsel->max_events was setup:

  # perf probe -x ~/bin/perf -L trace__event_handler
  <trace__event_handler@/home/acme/git/perf/tools/perf/builtin-trace.c:0>
        0  static int trace__event_handler(struct trace *trace, struct perf_evsel *evsel,
                                          union perf_event *event __maybe_unused,
                                          struct perf_sample *sample)
        3  {
        4         struct thread *thread = machine__findnew_thread(trace->host, sample->pid, sample->tid);
        5         int callchain_ret = 0;

        7         if (sample->callchain) {
        8                 callchain_ret = trace__resolve_callchain(trace, evsel, sample, &callchain_cursor);
        9                 if (callchain_ret == 0) {
       10                         if (callchain_cursor.nr < trace->min_stack)
       11                                 goto out;
       12                         callchain_ret = 1;
                          }
                  }

See what variables we can probe at line 7:

  # perf probe -x ~/bin/perf -V trace__event_handler:7
  Available variables at trace__event_handler:7
          @<trace__event_handler+89>
                  int     callchain_ret
                  struct perf_evsel*      evsel
                  struct perf_sample*     sample
                  struct thread*  thread
                  struct trace*   trace
                  union perf_event*       event

Add a probe at that line asking for evsel->max_events to be collected and named
as "max_events":

  # perf probe -x ~/bin/perf trace__event_handler:7 'max_events=evsel->max_events'
  Added new event:
    probe_perf:trace__event_handler (on trace__event_handler:7 in /home/acme/bin/perf with max_events=evsel->max_events)

  You can now use it in all perf tools, such as:

  	perf record -e probe_perf:trace__event_handler -aR sleep 1

Now use 'perf trace', here aliased to just 'trace' and trace trace, i.e.
the first 'trace' is tracing just that 'probe_perf:trace__event_handler' event,
while the traced trace is tracing all scheduler tracepoints, will stop at two
events (--max-events 2) and will just set evsel->max_events for all the sched
tracepoints to 9, we will see the output of both traces intermixed:

  # trace -e *perf:*event_handler trace --max-events 2 -e sched:*/nr=9/
       0.000 :0/0 sched:sched_waking:comm=rcu_sched pid=10 prio=120 target_cpu=000
       0.009 :0/0 sched:sched_wakeup:comm=rcu_sched pid=10 prio=120 target_cpu=000
       0.000 trace/23949 probe_perf:trace__event_handler:(48c34a) max_events=0x9
       0.046 trace/23949 probe_perf:trace__event_handler:(48c34a) max_events=0x9
  #

Now, if the traced trace sends its output to /dev/null, we'll see just
what the first level trace outputs: that evsel->max_events is indeed
being set to 9:

  # trace -e *perf:*event_handler trace -o /dev/null --max-events 2 -e sched:*/nr=9/
       0.000 trace/23961 probe_perf:trace__event_handler:(48c34a) max_events=0x9
       0.030 trace/23961 probe_perf:trace__event_handler:(48c34a) max_events=0x9
  #

Now that we can set evsel->max_events, we can go to the next step, honour that
per-event property in 'perf trace'.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Milian Wolff <milian.wolff@kdab.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-og00yasj276joem6e14l1eas@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>