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, 1 insertions, 28 deletions

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index dbdabd76f192..858c4cc6f99b 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1553,38 +1553,11 @@ pick_next_task_rt(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
         struct task_struct *p;
         struct rt_rq *rt_rq = &rq->rt;
 
-        if (need_pull_rt_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_rt_task(rq);
-                rq_repin_lock(rq, rf);
-                /*
-                 * pull_rt_task() can drop (and re-acquire) rq->lock; this
-                 * means a dl or stop task can slip in, in which case we need
-                 * to re-start task selection.
-                 */
-                if (unlikely((rq->stop && task_on_rq_queued(rq->stop)) ||
-                             rq->dl.dl_nr_running))
-                        return RETRY_TASK;
-        }
-
-        /*
-         * We may dequeue prev's rt_rq in put_prev_task().
-         * So, we update time before rt_queued check.
-         */
-        if (prev->sched_class == &rt_sched_class)
-                update_curr_rt(rq);
+        WARN_ON_ONCE(prev || rf);
 
         if (!rt_rq->rt_queued)
                 return NULL;
 
-        put_prev_task(rq, prev);
-
         p = _pick_next_task_rt(rq);
 
         set_next_task_rt(rq, p);