[PATCH] sched/schedutil : optimize utilization scaling for guest kernel

[Vincent Guittot <vincent.guittot@linaro.org>]

Scaling the utilization of CPUs with irq util_avg in schedutil doesn't give
any benefit and just waste CPU cycles when irq time is not accounted but
only steal time.
Add an internal _scale_irq_capacity() for scale_rt_capacity but scale
cpu utilization in schedutil only if we are accounting irq time.

Suggested-by: Wanpeng Li <kernellwp@gmail.com>
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
 kernel/sched/fair.c  |  2 +-
 kernel/sched/sched.h | 22 ++++++++++++++++++++--
 2 files changed, 21 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 309c93f..c1334be 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7501,7 +7501,7 @@ static unsigned long scale_rt_capacity(int cpu)
 
 	free = max - used;
 
-	return scale_irq_capacity(free, irq, max);
+	return _scale_irq_capacity(free, irq, max);
 }
 
 static void update_cpu_capacity(struct sched_domain *sd, int cpu)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 4a2e8ca..1003d69 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2221,14 +2221,14 @@ static inline unsigned long cpu_util_irq(struct rq *rq)
 }
 
 static inline
-unsigned long scale_irq_capacity(unsigned long util, unsigned long irq, unsigned long max)
+unsigned long _scale_irq_capacity(unsigned long util, unsigned long irq, unsigned long max)
 {
 	util *= (max - irq);
 	util /= max;
 
 	return util;
-
 }
+
 #else
 static inline unsigned long cpu_util_irq(struct rq *rq)
 {
@@ -2236,8 +2236,26 @@ static inline unsigned long cpu_util_irq(struct rq *rq)
 }
 
 static inline
+unsigned long _scale_irq_capacity(unsigned long util, unsigned long irq, unsigned long max)
+{
+	return util;
+}
+#endif
+
+/*
+ * scale_irq_capacity is used by schedutil to scale utilization only when
+ * irq time is accounted. This scaling is not necessary when only virtual time
+ * is accounted as guest doesn't have access to frequency scaling.
+ */
+#ifdef CONFIG_IRQ_TIME_ACCOUNTING
+
+#define scale_irq_capacity _scale_irq_capacity
+
+#else
+static inline
 unsigned long scale_irq_capacity(unsigned long util, unsigned long irq, unsigned long max)
 {
 	return util;
 }
 #endif
+
-- 
2.7.4