perf machine: Fix the value used for unknown pids

The value used for unknown pids cannot be zero because that is used by
the "idle" task.

Use -1 instead.  Also handle the unknown pid case when creating map
groups.

Note that, threads with an unknown pid should not occur because fork (or
synthesized) events precede the thread's existence.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1405332185-4050-2-git-send-email-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
index 64a186e..c2f4ca9 100644
--- a/tools/perf/util/session.c
+++ b/tools/perf/util/session.c
@@ -1083,13 +1083,14 @@
 
 struct thread *perf_session__findnew(struct perf_session *session, pid_t pid)
 {
-	return machine__findnew_thread(&session->machines.host, 0, pid);
+	return machine__findnew_thread(&session->machines.host, -1, pid);
 }
 
 static struct thread *perf_session__register_idle_thread(struct perf_session *session)
 {
-	struct thread *thread = perf_session__findnew(session, 0);
+	struct thread *thread;
 
+	thread = machine__findnew_thread(&session->machines.host, 0, 0);
 	if (thread == NULL || thread__set_comm(thread, "swapper", 0)) {
 		pr_err("problem inserting idle task.\n");
 		thread = NULL;