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
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index fa0d499..b71ed5e 100644
--- a/kernel/sched/core.c
+++ b/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
