[PATCH v4] sched/core: Minor optimize ttwu_runnable()

From: Chengming Zhou
Date: Fri Dec 02 2022 - 03:07:07 EST


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.

Reorganize ttwu_do_wakeup() and ttwu_do_activate() to make
ttwu_do_wakeup() only mark the task runnable, so it can be used
in ttwu_runnable() and try_to_wake_up() fast paths.

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>
---
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 | 73 ++++++++++++++++++++++++---------------------
1 file changed, 39 insertions(+), 34 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 314c2c0219d9..d8216485b0ad 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3623,14 +3623,39 @@ ttwu_stat(struct task_struct *p, int cpu, int wake_flags)
}

/*
- * Mark the task runnable and perform wakeup-preemption.
+ * Mark the task runnable.
*/
-static void ttwu_do_wakeup(struct rq *rq, struct task_struct *p, int wake_flags,
- struct rq_flags *rf)
+static inline void ttwu_do_wakeup(struct task_struct *p)
{
- check_preempt_curr(rq, p, wake_flags);
WRITE_ONCE(p->__state, TASK_RUNNING);
trace_sched_wakeup(p);
+}
+
+static void
+ttwu_do_activate(struct rq *rq, struct task_struct *p, int wake_flags,
+ struct rq_flags *rf)
+{
+ int en_flags = ENQUEUE_WAKEUP | ENQUEUE_NOCLOCK;
+
+ lockdep_assert_rq_held(rq);
+
+ if (p->sched_contributes_to_load)
+ rq->nr_uninterruptible--;
+
+#ifdef CONFIG_SMP
+ if (wake_flags & WF_MIGRATED)
+ en_flags |= ENQUEUE_MIGRATED;
+ else
+#endif
+ if (p->in_iowait) {
+ delayacct_blkio_end(p);
+ atomic_dec(&task_rq(p)->nr_iowait);
+ }
+
+ activate_task(rq, p, en_flags);
+ check_preempt_curr(rq, p, wake_flags);
+
+ ttwu_do_wakeup(p);

#ifdef CONFIG_SMP
if (p->sched_class->task_woken) {
@@ -3660,31 +3685,6 @@ static void ttwu_do_wakeup(struct rq *rq, struct task_struct *p, int wake_flags,
#endif
}

-static void
-ttwu_do_activate(struct rq *rq, struct task_struct *p, int wake_flags,
- struct rq_flags *rf)
-{
- int en_flags = ENQUEUE_WAKEUP | ENQUEUE_NOCLOCK;
-
- lockdep_assert_rq_held(rq);
-
- if (p->sched_contributes_to_load)
- rq->nr_uninterruptible--;
-
-#ifdef CONFIG_SMP
- if (wake_flags & WF_MIGRATED)
- en_flags |= ENQUEUE_MIGRATED;
- else
-#endif
- if (p->in_iowait) {
- delayacct_blkio_end(p);
- atomic_dec(&task_rq(p)->nr_iowait);
- }
-
- activate_task(rq, p, en_flags);
- ttwu_do_wakeup(rq, p, wake_flags, rf);
-}
-
/*
* Consider @p being inside a wait loop:
*
@@ -3718,9 +3718,15 @@ 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);
+ }
+ ttwu_do_wakeup(p);
ret = 1;
}
__task_rq_unlock(rq, &rf);
@@ -4086,8 +4092,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
goto out;

trace_sched_waking(p);
- WRITE_ONCE(p->__state, TASK_RUNNING);
- trace_sched_wakeup(p);
+ ttwu_do_wakeup(p);
goto out;
}

--
2.37.2