Re: [RFC][PATCH 12/13] sched/deadline: Introduce deadline servers

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Aug 08, 2019 at 11:45:46AM +0200, Juri Lelli wrote:
> I'd like to take this last sentence back, I was able to run a few boot +
> hackbench + shutdown cycles with the following applied (guess too much
> debug printks around before).

I've changed that slightly; the merged delta looks like:


--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3913,6 +3913,13 @@ pick_next_task(struct rq *rq, struct tas
 		if (unlikely(!p))
 			p = idle_sched_class.pick_next_task(rq, prev, rf);
 
+		/*
+		 * This is the fast path; it cannot be a DL server pick;
+		 * therefore even if @p == @prev, ->server must be NULL.
+		 */
+		if (prev->server)
+			p->server = NULL;
+
 		return p;
 	}
 
@@ -3925,13 +3932,18 @@ pick_next_task(struct rq *rq, struct tas
 	if (!rq->nr_running)
 		newidle_balance(rq, rf);
 
+	/*
+	 * We've updated @prev and no longer need the server link, clear it.
+	 * Must be done before ->pick_next_task() because that can (re)set
+	 * ->server.
+	 */
+	if (prev->server)
+		prev->server = NULL;
+
 	for_each_class(class) {
 		p = class->pick_next_task(rq, NULL, NULL);
-		if (p) {
-			if (p->sched_class == class && p->server)
-				p->server = NULL;
+		if (p)
 			return p;
-		}
 	}
 
 	/* The idle class should always have a runnable task: */
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1312,6 +1312,10 @@ void dl_server_update(struct sched_dl_en
 
 void dl_server_start(struct sched_dl_entity *dl_se)
 {
+	if (!dl_server(dl_se)) {
+		dl_se->dl_server = 1;
+		setup_new_dl_entity(dl_se);
+	}
 	enqueue_dl_entity(dl_se, dl_se, ENQUEUE_WAKEUP);
 }
 
@@ -1324,12 +1328,9 @@ void dl_server_init(struct sched_dl_enti
 		    dl_server_has_tasks_f has_tasks,
 		    dl_server_pick_f pick)
 {
-	dl_se->dl_server = 1;
 	dl_se->rq = rq;
 	dl_se->server_has_tasks = has_tasks;
 	dl_se->server_pick = pick;
-
-	setup_new_dl_entity(dl_se);
 }
 
 /*
@@ -2855,6 +2856,7 @@ static void __dl_clear_params(struct sch
 	dl_se->dl_yielded		= 0;
 	dl_se->dl_non_contending	= 0;
 	dl_se->dl_overrun		= 0;
+	dl_se->dl_server		= 0;
 }
 
 void init_dl_entity(struct sched_dl_entity *dl_se)