sched/fair: Tweak pick_next_entity()

Currently, pick_next_entity(...) has the following structure
(simplified):

  [...]
  if (last_buddy_ok())
    result = last_buddy;
  if (next_buddy_ok())
    result = next_buddy;
  [...]

The intended behavior is to prefer next buddy over last buddy;
the current code somewhat obfuscates this, and also wastes
cycles checking the last buddy when eventually the next buddy is
picked up.

So this patch refactors two 'ifs' above into

  [...]
  if (next_buddy_ok())
      result = next_buddy;
  else if (last_buddy_ok())
      result = last_buddy;
  [...]

Signed-off-by: Peter Oskolkov <posk@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guitttot@linaro.org>
Link: https://lkml.kernel.org/r/20200930173532.1069092-1-posk@google.com

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index fc3410b..cec6cf9 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4465,17 +4465,17 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
                        se = second;
        }
 
-       /*
-        * Prefer last buddy, try to return the CPU to a preempted task.
-        */
-       if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1)
-               se = cfs_rq->last;
-
-       /*
-        * Someone really wants this to run. If it's not unfair, run it.
-        */
-       if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1)
+       if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1) {
+               /*
+                * Someone really wants this to run. If it's not unfair, run it.
+                */
                se = cfs_rq->next;
+       } else if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1) {
+               /*
+                * Prefer last buddy, try to return the CPU to a preempted task.
+                */
+               se = cfs_rq->last;
+       }
 
        clear_buddies(cfs_rq, se);