Re: [PATCH perf/core 16/22] perf: Fix __cmd_top and perf_session__process_events to put the idle thread

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Wed, Dec 09, 2015 at 11:11:23AM +0900, Masami Hiramatsu escreveu:
> Since perf_session__register_idle_thread() got the idle thread,
> caller functions have to put it afterwards.
> Note that since the thread was already inserted to the session
> list, it will be released when the session is released.
> Also, in perf_session__register_idle_thread() failure path,
> the thread should be put before returning.

Wouldn't this be better by making perf_session__register_idle_thread()
return -1 if it fails? I.e. that way its callers won't have to
immediately put the idle thread, as they are doing nothing with it.

In the future, if someone needs a handle for that thread, a lookup can
be done:

  idle = machine__find_thread(&session->machines.host, 0, 0);

I.e. this would be the resulting patch, please let me know if you are ok
with this approach:

diff --git a/tools/perf/builtin-top.c b/tools/perf/builtin-top.c
index 7e2e72e6d9d1..f26b08e72f74 100644
--- a/tools/perf/builtin-top.c
+++ b/tools/perf/builtin-top.c
@@ -964,7 +964,7 @@ static int __cmd_top(struct perf_top *top)
 	if (ret)
 		goto out_delete;
 
-	if (perf_session__register_idle_thread(top->session) == NULL)
+	if (perf_session__register_idle_thread(top->session) < 0)
 		goto out_delete;
 
 	machine__synthesize_threads(&top->session->machines.host, &opts->target,
diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
index c35ffdd360fe..9774686525b4 100644
--- a/tools/perf/util/session.c
+++ b/tools/perf/util/session.c
@@ -1311,17 +1311,20 @@ struct thread *perf_session__findnew(struct perf_session *session, pid_t pid)
 	return machine__findnew_thread(&session->machines.host, -1, pid);
 }
 
-struct thread *perf_session__register_idle_thread(struct perf_session *session)
+int perf_session__register_idle_thread(struct perf_session *session)
 {
 	struct thread *thread;
+	int err = 0;
 
 	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;
+		err = -1;
 	}
 
-	return thread;
+	/* machine__findnew_thread() got the thread, so put it */
+	thread__put(thread);
+	return err;
 }
 
 static void perf_session__warn_about_errors(const struct perf_session *session)
@@ -1676,7 +1679,7 @@ int perf_session__process_events(struct perf_session *session)
 	u64 size = perf_data_file__size(session->file);
 	int err;
 
-	if (perf_session__register_idle_thread(session) == NULL)
+	if (perf_session__register_idle_thread(session) < 0)
 		return -ENOMEM;
 
 	if (!perf_data_file__is_pipe(session->file))
diff --git a/tools/perf/util/session.h b/tools/perf/util/session.h
index 3e900c0efc73..5f792e35d4c1 100644
--- a/tools/perf/util/session.h
+++ b/tools/perf/util/session.h
@@ -89,7 +89,7 @@ struct machine *perf_session__findnew_machine(struct perf_session *session, pid_
 }
 
 struct thread *perf_session__findnew(struct perf_session *session, pid_t pid);
-struct thread *perf_session__register_idle_thread(struct perf_session *session);
+int perf_session__register_idle_thread(struct perf_session *session);
 
 size_t perf_session__fprintf(struct perf_session *session, FILE *fp);