linux-kernel - [PATCH 5/6] sched/fair: Get rid of scaling utilization by capacity

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Message-Id: <1439569394-11974-6-git-send-email-morten.rasmussen@arm.com>
Date:	Fri, 14 Aug 2015 17:23:13 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	peterz@...radead.org, mingo@...hat.com
Cc:	vincent.guittot@...aro.org, daniel.lezcano@...aro.org,
	Dietmar Eggemann <Dietmar.Eggemann@....com>,
	yuyang.du@...el.com, mturquette@...libre.com, rjw@...ysocki.net,
	Juri Lelli <Juri.Lelli@....com>, sgurrappadi@...dia.com,
	pang.xunlei@....com.cn, linux-kernel@...r.kernel.org,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Morten Rasmussen <morten.rasmussen@....com>
Subject: [PATCH 5/6] sched/fair: Get rid of scaling utilization by capacity_orig

From: Dietmar Eggemann <dietmar.eggemann@....com>

Utilization is currently scaled by capacity_orig, but since we now have
frequency and cpu invariant cfs_rq.avg.util_avg, frequency and cpu scaling
now happens as part of the utilization tracking itself.
So cfs_rq.avg.util_avg should no longer be scaled in cpu_util().

Cc: Ingo Molnar <mingo@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@....com>
Signed-off-by: Morten Rasmussen <morten.rasmussen@....com>
---
 kernel/sched/fair.c | 38 ++++++++++++++++++++++----------------
 1 file changed, 22 insertions(+), 16 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 4cc3050..34e24181 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4824,33 +4824,39 @@ static int select_idle_sibling(struct task_struct *p, int target)
 done:
 	return target;
 }
+
 /*
  * cpu_util returns the amount of capacity of a CPU that is used by CFS
  * tasks. The unit of the return value must be the one of capacity so we can
  * compare the utilization with the capacity of the CPU that is available for
  * CFS task (ie cpu_capacity).
- * cfs.avg.util_avg is the sum of running time of runnable tasks on a
- * CPU. It represents the amount of utilization of a CPU in the range
- * [0..SCHED_LOAD_SCALE]. The utilization of a CPU can't be higher than the
- * full capacity of the CPU because it's about the running time on this CPU.
- * Nevertheless, cfs.avg.util_avg can be higher than SCHED_LOAD_SCALE
- * because of unfortunate rounding in util_avg or just
- * after migrating tasks until the average stabilizes with the new running
- * time. So we need to check that the utilization stays into the range
- * [0..cpu_capacity_orig] and cap if necessary.
- * Without capping the utilization, a group could be seen as overloaded (CPU0
- * utilization at 121% + CPU1 utilization at 80%) whereas CPU1 has 20% of
- * available capacity.
+ *
+ * cfs_rq.avg.util_avg is the sum of running time of runnable tasks plus the
+ * recent utilization of currently non-runnable tasks on a CPU. It represents
+ * the amount of utilization of a CPU in the range [0..capacity_orig] where
+ * capacity_orig is the cpu_capacity available at * the highest frequency
+ * (arch_scale_freq_capacity()).
+ * The utilization of a CPU converges towards a sum equal to or less than the
+ * current capacity (capacity_curr <= capacity_orig) of the CPU because it is
+ * the running time on this CPU scaled by capacity_curr.
+ *
+ * Nevertheless, cfs_rq.avg.util_avg can be higher than capacity_curr or even
+ * higher than capacity_orig because of unfortunate rounding in
+ * cfs.avg.util_avg or just after migrating tasks and new task wakeups until
+ * the average stabilizes with the new running time. We need to check that the
+ * utilization stays within the range of [0..capacity_orig] and cap it if
+ * necessary. Without utilization capping, a group could be seen as overloaded
+ * (CPU0 utilization at 121% + CPU1 utilization at 80%) whereas CPU1 has 20% of
+ * available capacity. We allow utilization to overshoot capacity_curr (but not
+ * capacity_orig) as it useful for predicting the capacity required after task
+ * migrations (scheduler-driven DVFS).
  */
 static int cpu_util(int cpu)
 {
 	unsigned long util = cpu_rq(cpu)->cfs.avg.util_avg;
 	unsigned long capacity = capacity_orig_of(cpu);
 
-	if (util >= SCHED_LOAD_SCALE)
-		return capacity;
-
-	return (util * capacity) >> SCHED_LOAD_SHIFT;
+	return (util >= capacity) ? capacity : util;
 }
 
 /*
-- 
1.9.1

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