sched: Micro optimization in pick_next_task() and in check_preempt_curr()

This introduces an optimization based on xxx_sched_class addresses
in two hot scheduler functions: pick_next_task() and check_preempt_curr().

It is possible to compare pointers to sched classes to check, which
of them has a higher priority, instead of current iterations using
for_each_class().

One more result of the patch is that size of object file becomes a little
less (excluding added BUG_ON(), which goes in __init section):

$size kernel/sched/core.o
         text     data      bss	    dec	    hex	filename
before:  66446    18957	    676	  86079	  1503f	kernel/sched/core.o
after:   66398    18957	    676	  86031	  1500f	kernel/sched/core.o

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/711a9c4b-ff32-1136-b848-17c622d548f3@yandex.ru

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 81640fe0eae8fd47c5f87128852603362671f58a..0a08d0c235d4538a5a12bf7e5404a719c3a12467 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1412,20 +1412,10 @@ static inline void check_class_changed(struct rq *rq, struct task_struct *p,
 
 void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags)
 {
-       const struct sched_class *class;
-
-       if (p->sched_class == rq->curr->sched_class) {
+       if (p->sched_class == rq->curr->sched_class)
                rq->curr->sched_class->check_preempt_curr(rq, p, flags);
-       } else {
-               for_each_class(class) {
-                       if (class == rq->curr->sched_class)
-                               break;
-                       if (class == p->sched_class) {
-                               resched_curr(rq);
-                               break;
-                       }
-               }
-       }
+       else if (p->sched_class > rq->curr->sched_class)
+               resched_curr(rq);
 
        /*
         * A queue event has occurred, and we're going to schedule.  In
@@ -4003,8 +3993,7 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
         * higher scheduling class, because otherwise those loose the
         * opportunity to pull in more work from other CPUs.
         */
-       if (likely((prev->sched_class == &idle_sched_class ||
-                   prev->sched_class == &fair_sched_class) &&
+       if (likely(prev->sched_class <= &fair_sched_class &&
                   rq->nr_running == rq->cfs.h_nr_running)) {
 
                p = pick_next_task_fair(rq, prev, rf);