Re: [PATCH v4 6/7] sched/deadline: Deferrable dl server

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Sep 06, 2023 at 04:58:11PM +0200, Daniel Bristot de Oliveira wrote:

> > Yeah, it's a wee hack to move it to the zero-laxity point. I was
> > considering if it makes sense to push that down and make it available
> > for all DL tasks, but I'm not sure..
> 
> It might be useful in the future, like when DL dominates all other schedulers, so
> we can have a way to schedule a deferred work, like kworkers... :-) But it might be
> too early for that..

So... that scheme I was pushing where we unconditionally decrement
fair_server.dl_runtime from update_curr_fair(), that relies on it being
a proper zero-laxity scheduler, and doesn't work with the proposed defer
hack.

That is, it relies on dl_runtime > 0 during throttle, and you explicitly
set it 0.

Now, I've not looked at all this code in detail in a minute, but would
not something like the below work?

AFAICT the regular dl_task_timer() callback works to make it go, because
replenish will see positive runtime (or not, when already consumed) and
DTRT.


Index: linux-2.6/include/linux/sched.h
===================================================================
--- linux-2.6.orig/include/linux/sched.h
+++ linux-2.6/include/linux/sched.h
@@ -657,6 +657,7 @@ struct sched_dl_entity {
 	unsigned int			dl_non_contending : 1;
 	unsigned int			dl_overrun	  : 1;
 	unsigned int			dl_server         : 1;
+	unsigned int			dl_zerolax        : 1;
 
 	/*
 	 * Bandwidth enforcement timer. Each -deadline task has its
Index: linux-2.6/kernel/sched/deadline.c
===================================================================
--- linux-2.6.orig/kernel/sched/deadline.c
+++ linux-2.6/kernel/sched/deadline.c
@@ -895,6 +895,16 @@ static void replenish_dl_entity(struct s
 		dl_se->dl_yielded = 0;
 	if (dl_se->dl_throttled)
 		dl_se->dl_throttled = 0;
+
+	/*
+	 * If this is a zero-laxity task, and we're before the zero-laxity
+	 * point, throttle it.
+	 */
+	if (dl_se->dl_zerolax &&
+	    dl_time_before(dl_se->deadline - dl_se->runtime, rq_clock(rq))) {
+		if (!is_dl_boosted(dl_se) && start_dl_timer(dl_se))
+			dl_se->dl_throttled = 1;
+	}
 }
 
 /*
@@ -1078,7 +1088,12 @@ static int start_dl_timer(struct sched_d
 	 * that it is actually coming from rq->clock and not from
 	 * hrtimer's time base reading.
 	 */
-	act = ns_to_ktime(dl_next_period(dl_se));
+	if (dl_se->dl_zerolax && !dl_se->dl_throttled) {
+		act = ns_to_ktime(dl_se->deadline - dl_se->runtime);
+	} else {
+		act = ns_to_ktime(dl_next_period(dl_se));
+	}
+
 	now = hrtimer_cb_get_time(timer);
 	delta = ktime_to_ns(now) - rq_clock(rq);
 	act = ktime_add_ns(act, delta);
@@ -1794,6 +1809,13 @@ enqueue_dl_entity(struct sched_dl_entity
 		setup_new_dl_entity(dl_se);
 	}
 
+	/*
+	 * If we are still throttled, eg. we got replenished but are a
+	 * zero-laxity task and still got to wait, don't enqueue.
+	 */
+	if (dl_se->dl_throttled)
+		return;
+
 	__enqueue_dl_entity(dl_se);
 }