perf machine: Factor out machine__idle_thread()

Factor out machine__idle_thread() so it can be re-used for guest machines.

A thread is needed to find executable code, even for the guest kernel. To
avoid possible future pid number conflicts, the idle thread can be used.
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Andi Kleen <ak@linux.intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: https://lore.kernel.org/r/20210218095801.19576-7-adrian.hunter@intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/machine.c

@@ -598,6 +598,24 @@ struct thread *machine__find_thread(struct machine *machine, pid_t pid,
 	return th;
 }
 
+/*
+ * Threads are identified by pid and tid, and the idle task has pid == tid == 0.
+ * So here a single thread is created for that, but actually there is a separate
+ * idle task per cpu, so there should be one 'struct thread' per cpu, but there
+ * is only 1. That causes problems for some tools, requiring workarounds. For
+ * example get_idle_thread() in builtin-sched.c, or thread_stack__per_cpu().
+ */
+struct thread *machine__idle_thread(struct machine *machine)
+{
+	struct thread *thread = machine__findnew_thread(machine, 0, 0);
+
+	if (!thread || thread__set_comm(thread, "swapper", 0) ||
+	    thread__set_namespaces(thread, 0, NULL))
+		pr_err("problem inserting idle task for machine pid %d\n", machine->pid);
+
+	return thread;
+}
+
 struct comm *machine__thread_exec_comm(struct machine *machine,
 				       struct thread *thread)
 {

tools/perf/util/machine.h

@@ -106,6 +106,7 @@ u8 machine__addr_cpumode(struct machine *machine, u8 cpumode, u64 addr);
 
 struct thread *machine__find_thread(struct machine *machine, pid_t pid,
 				    pid_t tid);
+struct thread *machine__idle_thread(struct machine *machine);
 struct comm *machine__thread_exec_comm(struct machine *machine,
 				       struct thread *thread);
 

tools/perf/util/session.c

@@ -1789,32 +1789,13 @@ struct thread *perf_session__findnew(struct perf_session *session, pid_t pid)
 	return machine__findnew_thread(&session->machines.host, -1, pid);
 }
 
-/*
- * Threads are identified by pid and tid, and the idle task has pid == tid == 0.
- * So here a single thread is created for that, but actually there is a separate
- * idle task per cpu, so there should be one 'struct thread' per cpu, but there
- * is only 1. That causes problems for some tools, requiring workarounds. For
- * example get_idle_thread() in builtin-sched.c, or thread_stack__per_cpu().
- */
 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");
-		err = -1;
-	}
+	struct thread *thread = machine__idle_thread(&session->machines.host);
 
-	if (thread == NULL || thread__set_namespaces(thread, 0, NULL)) {
-		pr_err("problem inserting idle task.\n");
-		err = -1;
-	}
-
-	/* machine__findnew_thread() got the thread, so put it */
+	/* machine__idle_thread() got the thread, so put it */
 	thread__put(thread);
-	return err;
+	return thread ? 0 : -1;
 }
 
 static void