Re: [PATCH 5/6] sched/fair: Get rid of scaling utilization by capacity_orig

[Dietmar Eggemann]

On 04/09/15 00:51, Steve Muckle wrote:
> Hi Morten, Dietmar,
> 
> On 08/14/2015 09:23 AM, Morten Rasmussen wrote:
> ...
>> + * 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
> 
> I see util_sum is scaled by SCHED_LOAD_SHIFT at the end of
> __update_load_avg(). If there is now an assumption that util_avg may be
> used directly as a capacity value, should it be changed to
> SCHED_CAPACITY_SHIFT? These are equal right now, not sure if they will
> always be or if they can be combined.

You're referring to the code line

2647   sa->util_avg = (sa->util_sum << SCHED_LOAD_SHIFT) / LOAD_AVG_MAX;

in __update_load_avg()?

Here we actually scale by 'SCHED_LOAD_SCALE/LOAD_AVG_MAX' so both values are
load related.

LOAD (UTIL) and CAPACITY have the same SCALE and SHIFT values because
SCHED_LOAD_RESOLUTION is always defined to 0. scale_load() and
scale_load_down() are also NOPs so this area is probably 
worth a separate clean-up.
Beyond that, I'm not sure if the current functionality is
broken if we use different SCALE and SHIFT values for LOAD and CAPACITY?

> 
>> + * capacity_orig is the cpu_capacity available at * the highest frequency
> 
> spurious *
> 
> thanks,
> Steve
> 

Fixed.

Thanks, 

-- Dietmar

-- >8 --

From: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Date: Fri, 14 Aug 2015 17:23:13 +0100
Subject: [PATCH] sched/fair: Get rid of scaling utilization by capacity_orig

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@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Signed-off-by: Morten Rasmussen <morten.rasmussen@xxxxxxx>
---
 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 2074d45a67c2..a73ece2372f5 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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