[tip: sched/core] sched: Micro optimization in pick_next_task() and in check_preempt_curr()

[tip-bot2 for Kirill Tkhai]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     aa93cd53bc1b91b5f99c7b55e3dcc1ac98e99558
Gitweb:        https://git.kernel.org/tip/aa93cd53bc1b91b5f99c7b55e3dcc1ac98e99558
Author:        Kirill Tkhai <tkhai@xxxxxxxxx>
AuthorDate:    Thu, 19 Dec 2019 16:44:55 -05:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Thu, 25 Jun 2020 13:45:44 +02:00

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@xxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: http://lkml.kernel.org/r/711a9c4b-ff32-1136-b848-17c622d548f3@xxxxxxxxx
---
 kernel/sched/core.c | 19 ++++---------------
 1 file changed, 4 insertions(+), 15 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 81640fe..0a08d0c 100644
--- 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);