sched: Provide a wake up API without sending freq notifications

Each time a task wakes up, scheduler evaluates its load and notifies
governor if the resulting frequency of destination CPU is larger than
a threshold. However, some governor wakes up a separate task that
handles frequency change, which again calls wake_up_process().

This is dangerous because if the task being woken up meets the
threshold and ends up being moved around, there is a potential for
endless recursive notifications.

Introduce a new API for waking up a task without triggering
frequency notification.

Change-Id: I24261af81b7dc410c7fb01eaa90920b8d66fbd2a
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>

include/linux/sched.h

@@ -2596,6 +2596,7 @@ extern void xtime_update(unsigned long ticks);
 
 extern int wake_up_state(struct task_struct *tsk, unsigned int state);
 extern int wake_up_process(struct task_struct *tsk);
+extern int wake_up_process_no_notif(struct task_struct *tsk);
 extern void wake_up_new_task(struct task_struct *tsk);
 #ifdef CONFIG_SMP
  extern void kick_process(struct task_struct *tsk);

kernel/sched/core.c

@@ -4360,6 +4360,9 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
 	u64 wallclock;
 	struct related_thread_group *grp = NULL;
 #endif
+	bool freq_notif_allowed = !(wake_flags & WF_NO_NOTIFIER);
+
+	wake_flags &= ~WF_NO_NOTIFIER;
 
 	/*
 	 * If we are going to wake up a thread waiting for CONDITION we
@@ -4477,11 +4480,14 @@ out:
 		atomic_notifier_call_chain(&migration_notifier_head,
 					   0, (void *)&mnd);
 
-	if (!same_freq_domain(src_cpu, cpu)) {
-		check_for_freq_change(cpu_rq(cpu));
-		check_for_freq_change(cpu_rq(src_cpu));
-	} else if (heavy_task)
-		check_for_freq_change(cpu_rq(cpu));
+	if (freq_notif_allowed) {
+		if (!same_freq_domain(src_cpu, cpu)) {
+			check_for_freq_change(cpu_rq(cpu));
+			check_for_freq_change(cpu_rq(src_cpu));
+		} else if (heavy_task) {
+			check_for_freq_change(cpu_rq(cpu));
+		}
+	}
 
 	return success;
 }
@@ -4561,6 +4567,26 @@ int wake_up_process(struct task_struct *p)
 }
 EXPORT_SYMBOL(wake_up_process);
 
+/**
+ * wake_up_process_no_notif - Wake up a specific process without notifying
+ * governor
+ * @p: The process to be woken up.
+ *
+ * Attempt to wake up the nominated process and move it to the set of runnable
+ * processes.
+ *
+ * Return: 1 if the process was woken up, 0 if it was already running.
+ *
+ * It may be assumed that this function implies a write memory barrier before
+ * changing the task state if and only if any tasks are woken up.
+ */
+int wake_up_process_no_notif(struct task_struct *p)
+{
+	WARN_ON(task_is_stopped_or_traced(p));
+	return try_to_wake_up(p, TASK_NORMAL, WF_NO_NOTIFIER);
+}
+EXPORT_SYMBOL(wake_up_process_no_notif);
+
 int wake_up_state(struct task_struct *p, unsigned int state)
 {
 	return try_to_wake_up(p, state, 0);

kernel/sched/sched.h

@@ -1596,6 +1596,7 @@ static inline void finish_lock_switch(struct rq *rq, struct task_struct *prev)
 #define WF_SYNC		0x01		/* waker goes to sleep after wakeup */
 #define WF_FORK		0x02		/* child wakeup after fork */
 #define WF_MIGRATED	0x4		/* internal use, task got migrated */
+#define WF_NO_NOTIFIER	0x08		/* do not notify governor */
 
 /*
  * To aid in avoiding the subversion of "niceness" due to uneven distribution