[PATCH] sched: Skip update_rq_clock() in enqueue_task() following right after dequeue_task()

[Kirill Tkhai]

Add new flag ENQUEUE_NO_CLK to skip microscopic rq->clock update.

This update is less even than in places where skip_clock_update is used.

Signed-off-by: Kirill Tkhai <tkhai@xxxxxxxxx>
CC: Ingo Molnar <mingo@xxxxxxxxxx>
CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CC: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 kernel/sched/core.c  |   12 +++++++-----
 kernel/sched/sched.h |    1 +
 2 files changed, 8 insertions(+), 5 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9d06ad6..0b99057 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -764,7 +764,9 @@ static void set_load_weight(struct task_struct *p)
 
 static void enqueue_task(struct rq *rq, struct task_struct *p, int flags)
 {
-	update_rq_clock(rq);
+	if (!(flags & ENQUEUE_NO_CLK))
+		update_rq_clock(rq);
+
 	sched_info_queued(p);
 	p->sched_class->enqueue_task(rq, p, flags);
 }
@@ -3107,7 +3109,7 @@ void set_user_nice(struct task_struct *p, long nice)
 	delta = p->prio - old_prio;
 
 	if (on_rq) {
-		enqueue_task(rq, p, 0);
+		enqueue_task(rq, p, ENQUEUE_NO_CLK);
 		/*
 		 * If the task increased its priority or is running and
 		 * lowered its priority, then reschedule its CPU:
@@ -3414,7 +3416,7 @@ recheck:
 	if (running)
 		p->sched_class->set_curr_task(rq);
 	if (on_rq)
-		enqueue_task(rq, p, 0);
+		enqueue_task(rq, p, ENQUEUE_NO_CLK);
 
 	check_class_changed(rq, p, prev_class, oldprio);
 	task_rq_unlock(rq, p, &flags);
@@ -6562,7 +6564,7 @@ static void normalize_task(struct rq *rq, struct task_struct *p)
 		dequeue_task(rq, p, 0);
 	__setscheduler(rq, p, SCHED_NORMAL, 0);
 	if (on_rq) {
-		enqueue_task(rq, p, 0);
+		enqueue_task(rq, p, ENQUEUE_NO_CLK);
 		resched_task(rq->curr);
 	}
 
@@ -6777,7 +6779,7 @@ void sched_move_task(struct task_struct *tsk)
 	if (unlikely(running))
 		tsk->sched_class->set_curr_task(rq);
 	if (on_rq)
-		enqueue_task(rq, tsk, 0);
+		enqueue_task(rq, tsk, ENQUEUE_NO_CLK);
 
 	task_rq_unlock(rq, tsk, &flags);
 }
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index ef0a7b2..4b8dfd7 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -957,6 +957,7 @@ static const u32 prio_to_wmult[40] = {
 #else
 #define ENQUEUE_WAKING		0
 #endif
+#define ENQUEUE_NO_CLK		8	/* skip update_rq_clock() */
 
 #define DEQUEUE_SLEEP		1
 
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