[PATCH v2 4/4] sched/fair: Abort wakeup when task is no longer in a sleeping state

[Waiman Long]

When a task prepares to sleep and then aborts it somehow, there is
a small chance that a waker may be spinning on the on_cpu flag of
that task waiting for the flag to turn off before doing the wakeup
operation. It may keep on spinning for a long time until that task
actually sleeps leading to spurious wakeup.

This patch adds code to detect the change in task state and abort
the wakeup operation, when appropriate, to free up the waker's cpu
to do other useful works.

Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
---
 kernel/sched/core.c |    9 ++++++++-
 1 files changed, 8 insertions(+), 1 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7e548bd..e4b6e84 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2075,8 +2075,15 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
 	 *
 	 * This ensures that tasks getting woken will be fully ordered against
 	 * their previous state and preserve Program Order.
+	 *
+	 * If the owning cpu decides not to sleep after all by changing back
+	 * its task state, we can return immediately.
 	 */
-	smp_cond_acquire(!p->on_cpu);
+	smp_cond_acquire(!p->on_cpu || !(p->state & state));
+	if (!(p->state & state)) {
+		success = 0;
+		goto out;
+	}
 
 	p->sched_contributes_to_load = !!task_contributes_to_load(p);
 	p->state = TASK_WAKING;
-- 
1.7.1