[PATCH V2] sched/fair: Dequeue sched_delayed tasks when waking to a busy CPU

[Mike Galbraith]

On Thu, 2024-11-21 at 06:56 -0500, Phil Auld wrote:
> On Wed, Nov 20, 2024 at 07:37:39PM +0100 Mike Galbraith wrote:
> > On Tue, 2024-11-19 at 12:51 +0100, Mike Galbraith wrote:
> > > On Tue, 2024-11-19 at 06:30 -0500, Phil Auld wrote:
> > > >
> > > > This, below, by itself, did not do help and caused a small slowdown on some
> > > > other tests.  Did this need to be on top of the wakeup change?
> > >
> > > No, that made a mess.
> >
> > Rashly speculating that turning mobile kthread component loose is what
> > helped your write regression...
> >
> > You could try adding (p->flags & PF_KTHREAD) to the wakeup patch to
> > only turn hard working kthreads loose to try to dodge service latency.
> > Seems unlikely wakeup frequency * instances would combine to shred fio
> > the way turning tbench loose did.
> >
>
> Thanks, I'll try that.

You may still want to try that, but my box says probably not.  Playing
with your write command line, the players I see are pinned kworkers and
mobile fio instances.

Maybe try the below instead. The changelog is obsolete BS unless you
say otherwise, but while twiddled V2 will still migrate tbench a bit,
and per trace_printk() does still let all kinds of stuff wander off to
roll the SIS dice, it does not even scratch the paint of the formerly
obliterated tbench progression.

Question: did wiping off the evil leave any meaningful goodness behind?

---

sched/fair: Dequeue sched_delayed tasks when waking to a busy CPU

Phil Auld (Redhat) reported an fio benchmark regression having been found
to have been caused by addition of the DELAY_DEQUEUE feature, suggested it
may be related to wakees losing the ability to migrate, and confirmed that
restoration of same indeed did restore previous performance.

V2: do not rip buddies apart, convenient on/off switch

Fixes: 152e11f6df29 ("sched/fair: Implement delayed dequeue")
Signed-off-by: Mike Galbraith <efault@xxxxxx>
---
 kernel/sched/core.c     |   51 ++++++++++++++++++++++++++++++------------------
 kernel/sched/features.h |    5 ++++
 kernel/sched/sched.h    |    5 ++++
 3 files changed, 42 insertions(+), 19 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3783,28 +3783,41 @@ ttwu_do_activate(struct rq *rq, struct t
  */
 static int ttwu_runnable(struct task_struct *p, int wake_flags)
 {
-	struct rq_flags rf;
-	struct rq *rq;
-	int ret = 0;
-
-	rq = __task_rq_lock(p, &rf);
-	if (task_on_rq_queued(p)) {
-		update_rq_clock(rq);
-		if (p->se.sched_delayed)
-			enqueue_task(rq, p, ENQUEUE_NOCLOCK | ENQUEUE_DELAYED);
-		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.
-			 */
-			wakeup_preempt(rq, p, wake_flags);
+	CLASS(__task_rq_lock, rq_guard)(p);
+	struct rq *rq = rq_guard.rq;
+
+	if (!task_on_rq_queued(p))
+		return 0;
+
+	update_rq_clock(rq);
+	if (p->se.sched_delayed) {
+		int queue_flags = ENQUEUE_DELAYED | ENQUEUE_NOCLOCK;
+		int dequeue = sched_feat(DEQUEUE_DELAYED);
+
+		/*
+		 * Since sched_delayed means we cannot be current anywhere,
+		 * dequeue it here and have it fall through to the
+		 * select_task_rq() case further along in ttwu() path.
+		 * Note: Do not rip buddies apart else chaos follows.
+		 */
+		if (dequeue && rq->nr_running > 1 && p->nr_cpus_allowed > 1 &&
+		    !(rq->curr->last_wakee == p || p->last_wakee == rq->curr)) {
+			dequeue_task(rq, p, DEQUEUE_SLEEP | queue_flags);
+			return 0;
 		}
-		ttwu_do_wakeup(p);
-		ret = 1;
+
+		enqueue_task(rq, p, queue_flags);
+	}
+	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.
+		 */
+		wakeup_preempt(rq, p, wake_flags);
 	}
-	__task_rq_unlock(rq, &rf);
+	ttwu_do_wakeup(p);

-	return ret;
+	return 1;
 }

 #ifdef CONFIG_SMP
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -47,6 +47,11 @@ SCHED_FEAT(CACHE_HOT_BUDDY, true)
  * DELAY_ZERO clips the lag on dequeue (or wakeup) to 0.
  */
 SCHED_FEAT(DELAY_DEQUEUE, true)
+/*
+ * Free ONLY non-buddy delayed tasks to wakeup-migrate to avoid taking.
+ * an unnecessary latency hit.  Rending buddies asunder inflicts harm.
+ */
+SCHED_FEAT(DEQUEUE_DELAYED, true)
 SCHED_FEAT(DELAY_ZERO, true)

 /*
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1783,6 +1783,11 @@ task_rq_unlock(struct rq *rq, struct tas
 	raw_spin_unlock_irqrestore(&p->pi_lock, rf->flags);
 }

+DEFINE_LOCK_GUARD_1(__task_rq_lock, struct task_struct,
+		    _T->rq = __task_rq_lock(_T->lock, &_T->rf),
+		    __task_rq_unlock(_T->rq, &_T->rf),
+		    struct rq *rq; struct rq_flags rf)
+
 DEFINE_LOCK_GUARD_1(task_rq_lock, struct task_struct,
 		    _T->rq = task_rq_lock(_T->lock, &_T->rf),
 		    task_rq_unlock(_T->rq, _T->lock, &_T->rf),