[RFCv5 PATCH 08/46] sched: Get rid of scaling usage by cpu_capacity_orig

Morten Rasmussen
2015-07-07T18:23:15Z

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

Since now we have besides frequency invariant also cpu (uarch plus max
system frequency) invariant cfs_rq::utilization_load_avg both, frequency
and cpu scaling happens as part of the load tracking.
So cfs_rq::utilization_load_avg does not have to be scaled by the original
capacity of the cpu again.

Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
---
kernel/sched/fair.c | 34 +++++++++++++++++++++-------------
1 file changed, 21 insertions(+), 13 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 39871a4..0c08cff 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5013,32 +5013,40 @@ static int select_idle_sibling(struct task_struct *p, int target)
done:
return target;
}
+
/*
* get_cpu_usage 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 usage with the capacity of the CPU that is available for CFS
* task (ie cpu_capacity).
+ *
* cfs.utilization_load_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 usage 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.utilization_load_avg can be higher than SCHED_LOAD_SCALE
- * because of unfortunate rounding in avg_period and running_load_avg or just
- * after migrating tasks until the average stabilizes with the new running
- * time. So we need to check that the usage stays into the range
- * [0..cpu_capacity_orig] and cap if necessary.
- * Without capping the usage, a group could be seen as overloaded (CPU0 usage
- * at 121% + CPU1 usage at 80%) whereas CPU1 has 20% of available capacity
+ * [0..capacity_orig] where capacity_orig is the cpu_capacity available at the
+ * highest frequency (arch_scale_freq_capacity()). The usage 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.utilization_load_avg can be higher than
+ * capacity_curr or even higher than capacity_orig because of unfortunate
+ * rounding in avg_period and running_load_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 usage stays into the range
+ * [0..capacity_orig] and cap if necessary. Without capping the usage, a group
+ * could be seen as overloaded (CPU0 usage at 121% + CPU1 usage at 80%) whereas
+ * CPU1 has 20% of available capacity. We allow usage to overshoot
+ * capacity_curr (but not capacity_orig) as it useful for predicting the
+ * capacity required after task migrations (scheduler-driven DVFS).
*/
+
static int get_cpu_usage(int cpu)
{
unsigned long usage = cpu_rq(cpu)->cfs.utilization_load_avg;
- unsigned long capacity = capacity_orig_of(cpu);
+ unsigned long capacity_orig = capacity_orig_of(cpu);

- if (usage >= SCHED_LOAD_SCALE)
- return capacity;
+ if (usage >= capacity_orig)
+ return capacity_orig;

- return (usage * capacity) >> SCHED_LOAD_SHIFT;
+ return usage;
}

/*
--
1.9.1