Re: [RFC][PATCH 5/5] sched: Add ttwu_queue support for delayed tasks

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jun 06, 2025 at 05:03:36PM +0200, Vincent Guittot wrote:
> On Tue, 20 May 2025 at 12:18, Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > One of the things lost with introduction of DELAY_DEQUEUE is the
> > ability of TTWU to move those tasks around on wakeup, since they're
> > on_rq, and as such, need to be woken in-place.
> 
> I was thinking that you would call select_task_rq() somewhere in the
> wake up path of delayed entity to get a chance to migrate it which was
> one reason for the perf regression (and which would have also been
> useful for EAS case) but IIUC, 

FWIW, the trivial form of all this is something like the below. The
problem is that performance sucks :/ For me it is worse than not doing
it. But perhaps it is the right thing for the more complicated cases ?

On my SPR:

schbench-6.9.0-1.txt:average rps: 2975450.75
schbench-6.9.0-2.txt:average rps: 2975464.38
schbench-6.9.0-3.txt:average rps: 2974881.02

(these patches)
schbench-6.15.0-dirty-1.txt:average rps: 3029984.58
schbench-6.15.0-dirty-2.txt:average rps: 3034723.10
schbench-6.15.0-dirty-3.txt:average rps: 3033893.33

TTWU_QUEUE_DELAYED
schbench-6.15.0-dirty-delayed-1.txt:average rps: 3048778.58
schbench-6.15.0-dirty-delayed-2.txt:average rps: 3049587.90
schbench-6.15.0-dirty-delayed-3.txt:average rps: 3045826.95

NO_DELAY_DEQUEUE
schbench-6.15.0-dirty-no_delay-1.txt:average rps: 3043629.03
schbench-6.15.0-dirty-no_delay-2.txt:average rps: 3046054.47
schbench-6.15.0-dirty-no_delay-3.txt:average rps: 3044736.37

TTWU_DEQUEUE
schbench-6.15.0-dirty-dequeue-1.txt:average rps: 3008790.80
schbench-6.15.0-dirty-dequeue-2.txt:average rps: 3017497.33
schbench-6.15.0-dirty-dequeue-3.txt:average rps: 3005858.57



Index: linux-2.6/kernel/sched/core.c
===================================================================
--- linux-2.6.orig/kernel/sched/core.c
+++ linux-2.6/kernel/sched/core.c
@@ -3770,8 +3770,13 @@ static int __ttwu_runnable(struct rq *rq
 		return 0;
 
 	update_rq_clock(rq);
-	if (p->se.sched_delayed)
+	if (p->se.sched_delayed) {
+		if (sched_feat(TTWU_DEQUEUE)) {
+			dequeue_task(rq, p, DEQUEUE_NOCLOCK | DEQUEUE_DELAYED | DEQUEUE_SLEEP);
+			return 0;
+		}
 		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
Index: linux-2.6/kernel/sched/features.h
===================================================================
--- linux-2.6.orig/kernel/sched/features.h
+++ linux-2.6/kernel/sched/features.h
@@ -84,6 +84,7 @@ SCHED_FEAT(TTWU_QUEUE, true)
 SCHED_FEAT(TTWU_QUEUE_ON_CPU, true)
 SCHED_FEAT(TTWU_QUEUE_DELAYED, false)
 SCHED_FEAT(TTWU_QUEUE_DEFAULT, false)
+SCHED_FEAT(TTWU_DEQUEUE, false)
 
 /*
  * When doing wakeups, attempt to limit superfluous scans of the LLC domain.