sched: Rework pick_next_task() slow-path

Avoid the RETRY_TASK case in the pick_next_task() slow path.

By doing the put_prev_task() early, we get the rt/deadline pull done,
and by testing rq->nr_running we know if we need newidle_balance().

This then gives a stable state to pick a task from.

Since the fast-path is fair only; it means the other classes will
always have pick_next_task(.prev=NULL, .rf=NULL) and we can simplify.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Aaron Lu <aaron.lwe@gmail.com>
Cc: Valentin Schneider <valentin.schneider@arm.com>
Cc: mingo@kernel.org
Cc: Phil Auld <pauld@redhat.com>
Cc: Julien Desfossez <jdesfossez@digitalocean.com>
Cc: Nishanth Aravamudan <naravamudan@digitalocean.com>
Link: https://lkml.kernel.org/r/aa34d24b36547139248f32a30138791ac6c02bd6.1559129225.git.vpillai@digitalocean.com

1 files changed, 2 insertions, 28 deletions

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 2872e15a87cd..0b9cbfb2b1d4 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1761,39 +1761,13 @@ pick_next_task_dl(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 	struct task_struct *p;
 	struct dl_rq *dl_rq;
 
-	dl_rq = &rq->dl;
-
-	if (need_pull_dl_task(rq, prev)) {
-		/*
-		 * This is OK, because current is on_cpu, which avoids it being
-		 * picked for load-balance and preemption/IRQs are still
-		 * disabled avoiding further scheduler activity on it and we're
-		 * being very careful to re-start the picking loop.
-		 */
-		rq_unpin_lock(rq, rf);
-		pull_dl_task(rq);
-		rq_repin_lock(rq, rf);
-		/*
-		 * pull_dl_task() can drop (and re-acquire) rq->lock; this
-		 * means a stop task can slip in, in which case we need to
-		 * re-start task selection.
-		 */
-		if (rq->stop && task_on_rq_queued(rq->stop))
-			return RETRY_TASK;
-	}
+	WARN_ON_ONCE(prev || rf);
 
-	/*
-	 * When prev is DL, we may throttle it in put_prev_task().
-	 * So, we update time before we check for dl_nr_running.
-	 */
-	if (prev->sched_class == &dl_sched_class)
-		update_curr_dl(rq);
+	dl_rq = &rq->dl;
 
 	if (unlikely(!dl_rq->dl_nr_running))
 		return NULL;
 
-	put_prev_task(rq, prev);
-
 	dl_se = pick_next_dl_entity(rq, dl_rq);
 	BUG_ON(!dl_se);