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SubjectRe: [PATCH 5/6] sched/fair: Get rid of scaling utilization by capacity_orig
On Tue, Sep 08, 2015 at 05:53:31PM +0100, Morten Rasmussen wrote:
> On Tue, Sep 08, 2015 at 03:31:58PM +0100, Morten Rasmussen wrote:

> > On Tue, Sep 08, 2015 at 02:52:05PM +0200, Peter Zijlstra wrote:
> > But if we apply the scaling to the weight instead of time, we would only
> > have to apply it once and not three times like it is now? So maybe we
> > can end up with almost the same number of multiplications.
> >
> > We might be loosing bits for low priority task running on cpus at a low
> > frequency though.
>
> Something like the below. We should be saving one multiplication.

> @@ -2577,8 +2575,13 @@ __update_load_avg(u64 now, int cpu, struct sched_avg *sa,
> return 0;
> sa->last_update_time = now;
>
> - scale_freq = arch_scale_freq_capacity(NULL, cpu);
> - scale_cpu = arch_scale_cpu_capacity(NULL, cpu);
> + if (weight || running)
> + scale_freq = arch_scale_freq_capacity(NULL, cpu);
> + if (weight)
> + scaled_weight = weight * scale_freq >> SCHED_CAPACITY_SHIFT;
> + if (running)
> + scale_freq_cpu = scale_freq * arch_scale_cpu_capacity(NULL, cpu)
> + >> SCHED_CAPACITY_SHIFT;
>
> /* delta_w is the amount already accumulated against our next period */
> delta_w = sa->period_contrib;
> @@ -2594,16 +2597,15 @@ __update_load_avg(u64 now, int cpu, struct sched_avg *sa,
> * period and accrue it.
> */
> delta_w = 1024 - delta_w;
> - scaled_delta_w = cap_scale(delta_w, scale_freq);
> if (weight) {
> - sa->load_sum += weight * scaled_delta_w;
> + sa->load_sum += scaled_weight * delta_w;
> if (cfs_rq) {
> cfs_rq->runnable_load_sum +=
> - weight * scaled_delta_w;
> + scaled_weight * delta_w;
> }
> }
> if (running)
> - sa->util_sum += scaled_delta_w * scale_cpu;
> + sa->util_sum += delta_w * scale_freq_cpu;
>
> delta -= delta_w;
>

Sadly that makes the code worse; I get 14 mul instructions where
previously I had 11.

What happens is that GCC gets confused and cannot constant propagate the
new variables, so what used to be shifts now end up being actual
multiplications.

With this, I get back to 11. Can you see what happens on ARM where you
have both functions defined to non constants?

---
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2551,10 +2551,10 @@ static __always_inline int
__update_load_avg(u64 now, int cpu, struct sched_avg *sa,
unsigned long weight, int running, struct cfs_rq *cfs_rq)
{
+ unsigned long scaled_weight, scale_freq, scale_freq_cpu;
+ unsigned int delta_w, decayed = 0;
u64 delta, periods;
u32 contrib;
- unsigned int delta_w, decayed = 0;
- unsigned long scaled_weight = 0, scale_freq, scale_freq_cpu = 0;

delta = now - sa->last_update_time;
/*
@@ -2575,13 +2575,10 @@ __update_load_avg(u64 now, int cpu, stru
return 0;
sa->last_update_time = now;

- if (weight || running)
- scale_freq = arch_scale_freq_capacity(NULL, cpu);
- if (weight)
- scaled_weight = weight * scale_freq >> SCHED_CAPACITY_SHIFT;
- if (running)
- scale_freq_cpu = scale_freq * arch_scale_cpu_capacity(NULL, cpu)
- >> SCHED_CAPACITY_SHIFT;
+ scale_freq = arch_scale_freq_capacity(NULL, cpu);
+
+ scaled_weight = weight * scale_freq >> SCHED_CAPACITY_SHIFT;
+ scale_freq_cpu = scale_freq * arch_scale_cpu_capacity(NULL, cpu) >> SCHED_CAPACITY_SHIFT;

/* delta_w is the amount already accumulated against our next period */
delta_w = sa->period_contrib;

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