perf evsel: Fix inverted test for fixing up attr.inherit flag

The kernel refuses mmapping an event with the inherit flag set for
something that is systemwide (cpu == -1), and the evsel layer got this
reversed at some point, fix it.

The symtom was that the --pid and --tid parameters for 'perf record' and
'perf top' returned with -EINVAL, like:

 # /tmp/build-perf/perf record -v -fo/tmp/perf.data -p 1042
   Warning:  ... trying to fall back to cpu-clock-ticks

   Fatal: failed to mmap with 22 (Invalid argument)
Reported-by: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Tom Zanussi <tzanussi@gmail.com>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/evsel.c

@@ -179,8 +179,19 @@ static int __perf_evsel__open(struct perf_evsel *evsel, struct cpu_map *cpus,
 
 	for (cpu = 0; cpu < cpus->nr; cpu++) {
 		int group_fd = -1;
-
-		evsel->attr.inherit = (cpus->map[cpu] < 0) && inherit;
+		/*
+		 * Don't allow mmap() of inherited per-task counters. This
+		 * would create a performance issue due to all children writing
+		 * to the same buffer.
+		 *
+		 * FIXME:
+		 * Proper fix is not to pass 'inherit' to perf_evsel__open*,
+		 * but a 'flags' parameter, with 'group' folded there as well,
+		 * then introduce a PERF_O_{MMAP,GROUP,INHERIT} enum, and if
+		 * O_MMAP is set, emit a warning if cpu < 0 and O_INHERIT is
+		 * set. Lets go for the minimal fix first tho.
+		 */
+		evsel->attr.inherit = (cpus->map[cpu] >= 0) && inherit;
 
 		for (thread = 0; thread < threads->nr; thread++) {