perf stat: Fix forked applications enablement of counters

I have run into the following issue:

 # perf stat -a -e new_pmu/INSTRUCTION_7/ --  mytest -c1 7

 Performance counter stats for 'system wide':

                 0      new_pmu/INSTRUCTION_7/

       0.000366428 seconds time elapsed
 #

The new PMU for s390 counts the execution of certain CPU instructions.
The root cause is the extremely small run time of the mytest program. It
just executes some assembly instructions and then exits.

In above invocation the instruction is executed exactly one time (-c1
option). The PMU is expected to report this one time execution by a
counter value of one, but fails to do so in some cases, not all.

Debugging reveals the invocation of the child process is done
*before* the counter events are installed and enabled.

Tracing reveals that sometimes the child process starts and exits before
the event is installed on all CPUs. The more CPUs the machine has, the
more often this miscount happens.

Fix this by reversing the start of the work load after the events have
been installed on the specified CPUs. Now the comment also matches the
code.

Output after:

 # perf stat -a -e new_pmu/INSTRUCTION_7/ --  mytest -c1 7

 Performance counter stats for 'system wide':

                 1      new_pmu/INSTRUCTION_7/

       0.000366428 seconds time elapsed
 #

Now the correct result is reported rock solid all the time regardless
how many CPUs are online.

Reviewers notes:

Jiri:

Right, without -a the event has enable_on_exec so the race does not
matter, but it's a problem for system wide with fork.

Namhyung:

Agreed. Also we may move the enable_counters() and the clock code out of
the if block to be shared with the else block.

Fixes: acf28922 ("perf stat: Use perf_evlist__prepare/start_workload()")
Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Link: https://lore.kernel.org/r/20220317155346.577384-1-tmricht@linux.ibm.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-stat.c

@@ -955,10 +955,10 @@ static int __run_perf_stat(int argc, const char **argv, int run_idx)
 	 * Enable counters and exec the command:
 	 */
 	if (forks) {
-		evlist__start_workload(evsel_list);
 		err = enable_counters();
 		if (err)
 			return -1;
+		evlist__start_workload(evsel_list);
 
 		t0 = rdclock();
 		clock_gettime(CLOCK_MONOTONIC, &ref_time);