rcu-tasks: Stop RCU Tasks Trace from scanning idle tasks

Now that RCU scans both running tasks and tasks that have blocked within
their current RCU Tasks Trace read-side critical section, there is no
need for it to scan the idle tasks.  After all, an idle loop should not
be remain within an RCU Tasks Trace read-side critical section across
exit from idle, and from a BPF viewpoint, functions invoked from the
idle loop should not sleep.  So only running idle tasks can be within
RCU Tasks Trace read-side critical sections.

This commit therefore removes the scan of the idle tasks from the
rcu_tasks_trace_postscan() function.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: KP Singh <kpsingh@kernel.org>

kernel/rcu/tasks.h

@@ -1533,16 +1533,10 @@ static void rcu_tasks_trace_pregp_step(struct list_head *hop)
 }
 
 /*
- * Do intermediate processing between task and holdout scans and
+ * Do intermediate processing between task and holdout scans.
- * pick up the idle tasks.
  */
 static void rcu_tasks_trace_postscan(struct list_head *hop)
 {
-	int cpu;
-
-	for_each_online_cpu(cpu)
-		rcu_tasks_trace_pertask(idle_task(cpu), hop);
-
 	// Re-enable CPU hotplug now that the tasklist scan has completed.
 	cpus_read_unlock();