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Subject[tip: sched/core] sched/core: Micro-optimize ttwu_runnable()
The following commit has been merged into the sched/core branch of tip:

Commit-ID: efe09385864f3441c71711f91e621992f9423c01
Author: Chengming Zhou <>
AuthorDate: Fri, 23 Dec 2022 18:32:56 +08:00
Committer: Ingo Molnar <>
CommitterDate: Sat, 07 Jan 2023 10:48:38 +01:00

sched/core: Micro-optimize ttwu_runnable()

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.

Suggested-by: Valentin Schneider <>
Suggested-by: Peter Zijlstra <>
Signed-off-by: Chengming Zhou <>
Signed-off-by: Ingo Molnar <>
Reviewed-by: Valentin Schneider <>
Reviewed-by: Steven Rostedt (Google) <>
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 f99ee69..255a318 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3720,9 +3720,16 @@ 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 the task that is current now.
+ */
+ update_rq_clock(rq);
+ check_preempt_curr(rq, p, wake_flags);
+ }
+ trace_sched_wakeup(p);
ret = 1;
__task_rq_unlock(rq, &rf);
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