[v8,10/10] sched: make scale_rt invariant with frequency

Message ID 1414745252-4895-11-git-send-email-vincent.guittot@linaro.org
State New
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Commit Message

Vincent Guittot Oct. 31, 2014, 8:47 a.m.
The average running time of RT tasks is used to estimate the remaining compute
capacity for CFS tasks. This remaining capacity is the original capacity scaled
down by a factor (aka scale_rt_capacity). This estimation of available capacity
must also be invariant with frequency scaling.

A frequency scaling factor is applied on the running time of the RT tasks for
computing scale_rt_capacity.

In sched_rt_avg_update, we scale the RT execution time like below:
rq->rt_avg += rt_delta * arch_scale_freq_capacity() >> SCHED_CAPACITY_SHIFT

Then, scale_rt_capacity can be summarized by:
scale_rt_capacity = SCHED_CAPACITY_SCALE -
		((rq->rt_avg << SCHED_CAPACITY_SHIFT) / period)

We can optimize by removing right and left shift in the computation of rq->rt_avg
and scale_rt_capacity

The call to arch_scale_frequency_capacity in the rt scheduling path might be
a concern for RT folks because I'm not sure whether we can rely on
arch_scale_freq_capacity to be short and efficient ?

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
 kernel/sched/fair.c  | 17 +++++------------
 kernel/sched/sched.h |  4 +++-
 2 files changed, 8 insertions(+), 13 deletions(-)

Comments

Peter Zijlstra Nov. 3, 2014, 3:51 p.m. | #1
On Fri, Oct 31, 2014 at 09:47:32AM +0100, Vincent Guittot wrote:
> The call to arch_scale_frequency_capacity in the rt scheduling path might be
> a concern for RT folks because I'm not sure whether we can rely on
> arch_scale_freq_capacity to be short and efficient ?

Well, you put it in quite a hot path on the CFS side too, so it had
better be blazing fast anyhow.


That said, can you move all the running + invariance bits at the
beginning of the patch set, these appear to be the least controversial
bits and 'desired' by quite a few people, so we'll try and get those
agreed upon and merged first.
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Vincent Guittot Nov. 3, 2014, 4:14 p.m. | #2
On 3 November 2014 16:51, Peter Zijlstra <peterz@infradead.org> wrote:
> On Fri, Oct 31, 2014 at 09:47:32AM +0100, Vincent Guittot wrote:
>> The call to arch_scale_frequency_capacity in the rt scheduling path might be
>> a concern for RT folks because I'm not sure whether we can rely on
>> arch_scale_freq_capacity to be short and efficient ?
>
> Well, you put it in quite a hot path on the CFS side too, so it had
> better be blazing fast anyhow.
>
>
> That said, can you move all the running + invariance bits at the
> beginning of the patch set, these appear to be the least controversial
> bits and 'desired' by quite a few people, so we'll try and get those
> agreed upon and merged first.

ok

I'm going to resend with the following order

sched: add utilization_avg_contrib
sched: Track group sched_entity usage contributions
sched: remove frequency scaling from cpu_capacity
sched: Make sched entity usage tracking scale-invariant
sched: make scale_rt invariant with frequency
sched: add per rq cpu_capacity_orig
sched: get CPU's usage statistic
sched: replace capacity_factor by usage
sched: add SD_PREFER_SIBLING for SMT level
sched: move cfs task on a CPU with higher capacity
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Patch

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 31c3ec2..02e8f7f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5817,7 +5817,7 @@  unsigned long __weak arch_scale_cpu_capacity(struct sched_domain *sd, int cpu)
 static unsigned long scale_rt_capacity(int cpu)
 {
 	struct rq *rq = cpu_rq(cpu);
-	u64 total, available, age_stamp, avg;
+	u64 total, used, age_stamp, avg;
 	s64 delta;
 
 	/*
@@ -5833,19 +5833,12 @@  static unsigned long scale_rt_capacity(int cpu)
 
 	total = sched_avg_period() + delta;
 
-	if (unlikely(total < avg)) {
-		/* Ensures that capacity won't end up being negative */
-		available = 0;
-	} else {
-		available = total - avg;
-	}
+	used = div_u64(avg, total);
 
-	if (unlikely((s64)total < SCHED_CAPACITY_SCALE))
-		total = SCHED_CAPACITY_SCALE;
+	if (likely(used < SCHED_CAPACITY_SCALE))
+		return SCHED_CAPACITY_SCALE - used;
 
-	total >>= SCHED_CAPACITY_SHIFT;
-
-	return div_u64(available, total);
+	return 1;
 }
 
 static void update_cpu_capacity(struct sched_domain *sd, int cpu)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 7963981..8fd30c1 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1313,9 +1313,11 @@  static inline int hrtick_enabled(struct rq *rq)
 
 #ifdef CONFIG_SMP
 extern void sched_avg_update(struct rq *rq);
+extern unsigned long arch_scale_freq_capacity(struct sched_domain *sd, int cpu);
+
 static inline void sched_rt_avg_update(struct rq *rq, u64 rt_delta)
 {
-	rq->rt_avg += rt_delta;
+	rq->rt_avg += rt_delta * arch_scale_freq_capacity(NULL, cpu_of(rq));
 	sched_avg_update(rq);
 }
 #else