[RFC][PATCH 1/5] sched/deadline: Less agressive dl_server handling

[Peter Zijlstra <peterz@infradead.org>]

Chris reported that commit 5f6bd380c7bd ("sched/rt: Remove default
bandwidth control") caused a significant dip in his favourite
benchmark of the day. Simply disabling dl_server cured things.

His workload hammers the 0->1, 1->0 transitions, and the
dl_server_{start,stop}() overhead kills it -- fairly obviously a bad
idea in hind sight and all that.

Change things around to only disable the dl_server when there has not
been a fair task around for a whole period. Since the default period
is 1 second, this ensures the benchmark never trips this, overhead
gone.

Fixes: 557a6bfc662c ("sched/fair: Add trivial fair server")
Reported-by: Chris Mason <clm@meta.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 include/linux/sched.h   |    1 +
 kernel/sched/deadline.c |   31 +++++++++++++++++++++++++++----
 2 files changed, 28 insertions(+), 4 deletions(-)

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -702,6 +702,7 @@ struct sched_dl_entity {
 	unsigned int			dl_defer	  : 1;
 	unsigned int			dl_defer_armed	  : 1;
 	unsigned int			dl_defer_running  : 1;
+	unsigned int			dl_server_idle    : 1;
 
 	/*
 	 * Bandwidth enforcement timer. Each -deadline task has its
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1215,6 +1215,8 @@ static void __push_dl_task(struct rq *rq
 /* a defer timer will not be reset if the runtime consumed was < dl_server_min_res */
 static const u64 dl_server_min_res = 1 * NSEC_PER_MSEC;
 
+static bool dl_server_stopped(struct sched_dl_entity *dl_se);
+
 static enum hrtimer_restart dl_server_timer(struct hrtimer *timer, struct sched_dl_entity *dl_se)
 {
 	struct rq *rq = rq_of_dl_se(dl_se);
@@ -1234,6 +1236,7 @@ static enum hrtimer_restart dl_server_ti
 
 		if (!dl_se->server_has_tasks(dl_se)) {
 			replenish_dl_entity(dl_se);
+			dl_server_stopped(dl_se);
 			return HRTIMER_NORESTART;
 		}
 
@@ -1639,8 +1642,10 @@ void dl_server_update_idle_time(struct r
 void dl_server_update(struct sched_dl_entity *dl_se, s64 delta_exec)
 {
 	/* 0 runtime = fair server disabled */
-	if (dl_se->dl_runtime)
+	if (dl_se->dl_runtime) {
+		dl_se->dl_server_idle = 0;
 		update_curr_dl_se(dl_se->rq, dl_se, delta_exec);
+	}
 }
 
 void dl_server_start(struct sched_dl_entity *dl_se)
@@ -1663,7 +1668,7 @@ void dl_server_start(struct sched_dl_ent
 		setup_new_dl_entity(dl_se);
 	}
 
-	if (!dl_se->dl_runtime)
+	if (!dl_se->dl_runtime || dl_se->dl_server_active)
 		return;
 
 	dl_se->dl_server_active = 1;
@@ -1672,7 +1677,7 @@ void dl_server_start(struct sched_dl_ent
 		resched_curr(dl_se->rq);
 }
 
-void dl_server_stop(struct sched_dl_entity *dl_se)
+static void __dl_server_stop(struct sched_dl_entity *dl_se)
 {
 	if (!dl_se->dl_runtime)
 		return;
@@ -1684,6 +1689,24 @@ void dl_server_stop(struct sched_dl_enti
 	dl_se->dl_server_active = 0;
 }
 
+static bool dl_server_stopped(struct sched_dl_entity *dl_se)
+{
+	if (!dl_se->dl_server_active)
+		return false;
+
+	if (dl_se->dl_server_idle) {
+		__dl_server_stop(dl_se);
+		return true;
+	}
+
+	dl_se->dl_server_idle = 1;
+	return false;
+}
+
+void dl_server_stop(struct sched_dl_entity *dl_se)
+{
+}
+
 void dl_server_init(struct sched_dl_entity *dl_se, struct rq *rq,
 		    dl_server_has_tasks_f has_tasks,
 		    dl_server_pick_f pick_task)
@@ -2435,7 +2458,7 @@ static struct task_struct *__pick_task_d
 	if (dl_server(dl_se)) {
 		p = dl_se->server_pick_task(dl_se);
 		if (!p) {
-			if (dl_server_active(dl_se)) {
+			if (!dl_server_stopped(dl_se)) {
 				dl_se->dl_yielded = 1;
 				update_curr_dl_se(rq, dl_se, 0);
 			}