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

[Chengming Zhou]

ttwu_runnable() is used as a fast wakeup path when the wakee task
is between set_current_state() and schedule(), in which case all
we need to do is change p->state back to TASK_RUNNING. So we don't
need to update_rq_clock() and check_preempt_curr() in this case.

Some performance numbers using mmtests/perfpipe on Intel Xeon server:

                           linux-next                patched
Min       Time        8.67 (   0.00%)        8.66 (   0.13%)
1st-qrtle Time        8.83 (   0.00%)        8.72 (   1.19%)
2nd-qrtle Time        8.90 (   0.00%)        8.76 (   1.57%)
3rd-qrtle Time        8.98 (   0.00%)        8.82 (   1.82%)
Max-1     Time        8.67 (   0.00%)        8.66 (   0.13%)
Max-5     Time        8.67 (   0.00%)        8.66 (   0.13%)
Max-10    Time        8.79 (   0.00%)        8.69 (   1.09%)
Max-90    Time        9.01 (   0.00%)        8.84 (   1.94%)
Max-95    Time        9.02 (   0.00%)        8.85 (   1.86%)
Max-99    Time        9.02 (   0.00%)        8.88 (   1.56%)
Max       Time        9.59 (   0.00%)        8.89 (   7.29%)
Amean     Time        8.92 (   0.00%)        8.77 *   1.65%*

Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
---
 kernel/sched/core.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 87c9cdf37a26..3785418de127 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3718,9 +3718,8 @@ 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);
+		WRITE_ONCE(p->__state, TASK_RUNNING);
+		trace_sched_wakeup(p);
 		ret = 1;
 	}
 	__task_rq_unlock(rq, &rf);
-- 
2.37.2