sched: Add rq::ttwu_pending

In preparation of removing rq->wake_list, replace the
!list_empty(rq->wake_list) with rq->ttwu_pending. This is not fully
equivalent as this new variable is racy.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200526161908.070399698@infradead.org

kernel/sched/core.c

@@ -2275,13 +2275,21 @@ static int ttwu_remote(struct task_struct *p, int wake_flags)
 void sched_ttwu_pending(void)
 {
 	struct rq *rq = this_rq();
-	struct llist_node *llist = llist_del_all(&rq->wake_list);
+	struct llist_node *llist;
 	struct task_struct *p, *t;
 	struct rq_flags rf;
 
+	llist = llist_del_all(&rq->wake_list);
 	if (!llist)
 		return;
 
+	/*
+	 * rq::ttwu_pending racy indication of out-standing wakeups.
+	 * Races such that false-negatives are possible, since they
+	 * are shorter lived that false-positives would be.
+	 */
+	WRITE_ONCE(rq->ttwu_pending, 0);
+
 	rq_lock_irqsave(rq, &rf);
 	update_rq_clock(rq);
 
@@ -2318,6 +2326,7 @@ static void __ttwu_queue_wakelist(struct task_struct *p, int cpu, int wake_flags
 
 	p->sched_remote_wakeup = !!(wake_flags & WF_MIGRATED);
 
+	WRITE_ONCE(rq->ttwu_pending, 1);
 	if (llist_add(&p->wake_entry, &rq->wake_list)) {
 		if (!set_nr_if_polling(rq->idle))
 			smp_call_function_single_async(cpu, &rq->wake_csd);
@@ -4705,7 +4714,7 @@ int idle_cpu(int cpu)
 		return 0;
 
 #ifdef CONFIG_SMP
-	if (!llist_empty(&rq->wake_list))
+	if (rq->ttwu_pending)
 		return 0;
 #endif
 

kernel/sched/debug.c

@@ -638,7 +638,6 @@ do {									\
 
 	P(nr_running);
 	P(nr_switches);
-	P(nr_load_updates);
 	P(nr_uninterruptible);
 	PN(next_balance);
 	SEQ_printf(m, "  .%-30s: %ld\n", "curr->pid", (long)(task_pid_nr(rq->curr)));

kernel/sched/fair.c

@@ -8590,7 +8590,7 @@ static int idle_cpu_without(int cpu, struct task_struct *p)
 	 */
 
 #ifdef CONFIG_SMP
-	if (!llist_empty(&rq->wake_list))
+	if (rq->ttwu_pending)
 		return 0;
 #endif
 

kernel/sched/sched.h

@@ -895,7 +895,9 @@ struct rq {
 	atomic_t		nohz_flags;
 #endif /* CONFIG_NO_HZ_COMMON */
 
-	unsigned long		nr_load_updates;
+#ifdef CONFIG_SMP
+	unsigned int		ttwu_pending;
+#endif
 	u64			nr_switches;
 
 #ifdef CONFIG_UCLAMP_TASK