perf tools: Add hint for 'Too many events are opened.' error message

Enhancing the 'Too many events are opened.' error message with hint to
use use 'ulimit -n <limit>' command.

Before:

  $ perf record -e 'sched:*,syscalls:*' ls
  Error:
  Too many events are opened.
  Try again after reducing the number of events.

Now:

  $ perf record -e 'sched:*,syscalls:*' ls
  Error:
  Too many events are opened.
  Probably the maximum number of open file descriptors has been reached.
  Hint: Try again after reducing the number of events.
  Hint: Try increasing the limit with 'ulimit -n <limit>'
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1432587114-14924-1-git-send-email-jolsa@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/evsel.c

@@ -2149,7 +2149,9 @@ int perf_evsel__open_strerror(struct perf_evsel *evsel, struct target *target,
 	case EMFILE:
 		return scnprintf(msg, size, "%s",
 			 "Too many events are opened.\n"
-			 "Try again after reducing the number of events.");
+			 "Probably the maximum number of open file descriptors has been reached.\n"
+			 "Hint: Try again after reducing the number of events.\n"
+			 "Hint: Try increasing the limit with 'ulimit -n <limit>'");
 	case ENODEV:
 		if (target->cpu_list)
 			return scnprintf(msg, size, "%s",