linux-kernel - [PATCH 4/7] sched: Optimize pick_next

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Message-Id: <20191108131909.603037345@infradead.org>
Date:   Fri, 08 Nov 2019 14:15:57 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     mingo@...nel.org, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, juri.lelli@...hat.com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de
Cc:     linux-kernel@...r.kernel.org, qperret@...gle.com,
        valentin.schneider@....com, qais.yousef@....com,
        ktkhai@...tuozzo.com, peterz@...radead.org
Subject: [PATCH 4/7] sched: Optimize pick_next_task()

Ever since we moved the sched_class defenitions into their own files,
the constant expression {fair,idle}_sched_class.pick_next_task() is
not in fact a compile time constant anymore and results in an indirect
call (barring LTO).

Fix that by exposing pick_next_task_{fair,idle}() directly, this gets
rid of the indirect call (and RETPOLINE) on the fast path.

Also remove the unlikely() from the idle case, it is in fact /the/ way
we select idle -- and that is a very common thing to do.

Performance for will-it-scale/sched_yield improves by 2% (as reported
by 0-day).

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 kernel/sched/core.c  |    6 +++---
 kernel/sched/fair.c  |    2 +-
 kernel/sched/idle.c  |    2 +-
 kernel/sched/sched.h |    3 +++
 4 files changed, 8 insertions(+), 5 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3917,14 +3917,14 @@ pick_next_task(struct rq *rq, struct tas
 		    prev->sched_class == &fair_sched_class) &&
 		   rq->nr_running == rq->cfs.h_nr_running)) {
 
-		p = fair_sched_class.pick_next_task(rq, prev, rf);
+		p = pick_next_task_fair(rq, prev, rf);
 		if (unlikely(p == RETRY_TASK))
 			goto restart;
 
 		/* Assumes fair_sched_class->next == idle_sched_class */
-		if (unlikely(!p)) {
+		if (!p) {
 			put_prev_task(rq, prev);
-			p = idle_sched_class.pick_next_task(rq, NULL, NULL);
+			p = pick_next_task_idle(rq, NULL, NULL);
 		}
 
 		return p;
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6599,7 +6599,7 @@ static void check_preempt_wakeup(struct
 		set_last_buddy(se);
 }
 
-static struct task_struct *
+struct task_struct *
 pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 {
 	struct cfs_rq *cfs_rq = &rq->cfs;
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -391,7 +391,7 @@ static void set_next_task_idle(struct rq
 	schedstat_inc(rq->sched_goidle);
 }
 
-static struct task_struct *
+struct task_struct *
 pick_next_task_idle(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 {
 	struct task_struct *next = rq->idle;
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1821,6 +1821,9 @@ static inline bool sched_fair_runnable(s
 	return rq->cfs.nr_running > 0;
 }
 
+extern struct task_struct *pick_next_task_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf);
+extern struct task_struct *pick_next_task_idle(struct rq *rq, struct task_struct *prev, struct rq_flags *rf);
+
 #ifdef CONFIG_SMP
 
 extern void update_group_capacity(struct sched_domain *sd, int cpu);