[PATCH v5 1/2] sched/core: Minor optimize ttwu_runnable()

[Chengming Zhou]

ttwu_runnable() is used as a fast wakeup path when the wakee task
is running on CPU or runnable on RQ, in both cases we can just
set its state to TASK_RUNNING to prevent a sleep.

If the wakee task is on_cpu running, we don't need to update_rq_clock()
or check_preempt_curr().

But if the wakee task is on_rq && !on_cpu (e.g. an IRQ hit before
the task got to schedule() and the task been preempted), we should
check_preempt_curr() to see if it can preempt the current running.

This also removes the class->task_woken() callback from ttwu_runnable(),
which wasn't required per the RT/DL implementations: any required push
operation would have been queued during class->set_next_task() when p
got preempted.

ttwu_runnable() also loses the update to rq->idle_stamp, as by definition
the rq cannot be idle in this scenario.

Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
Suggested-by: Valentin Schneider <vschneid@xxxxxxxxxx>
Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Reviewed-by: Valentin Schneider <vschneid@xxxxxxxxxx>
---
v5:
 - Split the reorganization and optimization into two patches
   per Chen Yu. Thanks!

v4:
 - s/This patch reorg/Reorganize/ per Bagas Sanjaya. Thanks!

v3:
 - Improve the changelog per Valentin Schneider. Thanks!

v2:
 - keep check_preempt_curr() for on_rq && !on_cpu case in ttwu_runnable(),
   per Valentin Schneider.
 - reorg ttwu_do_wakeup() and ttwu_do_activate() code, so ttwu_do_wakeup()
   can be reused in ttwu_runnable(), per Peter Zijlstra.
 - reuse ttwu_do_wakeup() in try_to_wake_up() (p == current) fast path too,
   so ttwu_do_wakeup() become the only place we mark task runnable.
---
 kernel/sched/core.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 314c2c0219d9..60aa7774e5f4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3718,9 +3718,16 @@ static int ttwu_runnable(struct task_struct *p, int wake_flags)
 
 	rq = __task_rq_lock(p, &rf);
 	if (task_on_rq_queued(p)) {
-		/* check_preempt_curr() may use rq clock */
-		update_rq_clock(rq);
-		ttwu_do_wakeup(rq, p, wake_flags, &rf);
+		if (!task_on_cpu(rq, p)) {
+			/*
+			 * When on_rq && !on_cpu the task is preempted, see if
+			 * it should preempt whatever is current there now.
+			 */
+			update_rq_clock(rq);
+			check_preempt_curr(rq, p, wake_flags);
+		}
+		WRITE_ONCE(p->__state, TASK_RUNNING);
+		trace_sched_wakeup(p);
 		ret = 1;
 	}
 	__task_rq_unlock(rq, &rf);
-- 
2.37.2