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Date: Thu, 17 Sep 2015 11:51:38 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Morten Rasmussen <morten.rasmussen@....com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steve Muckle <steve.muckle@...aro.org>,
"mingo@...hat.com" <mingo@...hat.com>,
"daniel.lezcano@...aro.org" <daniel.lezcano@...aro.org>,
"yuyang.du@...el.com" <yuyang.du@...el.com>,
"mturquette@...libre.com" <mturquette@...libre.com>,
"rjw@...ysocki.net" <rjw@...ysocki.net>,
Juri Lelli <Juri.Lelli@....com>,
"sgurrappadi@...dia.com" <sgurrappadi@...dia.com>,
"pang.xunlei@....com.cn" <pang.xunlei@....com.cn>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/6] sched/fair: Get rid of scaling utilization by
capacity_orig
On Fri, Sep 11, 2015 at 06:22:47PM +0100, Morten Rasmussen wrote:
> I have done some runs with the proposed fixes added:
>
> 1. PeterZ's util_sum shift fix (change util_sum).
> 2. Morten's scaling of weight instead of time (reduce bit loss).
> 3. PeterZ's unconditional calls to arch*() functions (compiler opt).
>
> To be clear: 2 includes 1, and 3 includes 1 and 2.
>
> Runs where done with the default (#define) implementation of the
> arch-functions and with arch specific implementation for ARM.
> Results:
>
> perf numbers are average of three (x10) runs. Raw data is available
> further down.
>
> ARM TC2 #mul #mul_all perf bench
> arch*() default arm default arm default arm
>
> 1 shift_fix 10 16 22 36 13.401 13.288
> 2 scaled_weight 12 14 30 32 13.282 13.238
> 3 unconditional 12 14 26 32 13.296 13.427
>
> Intel E5-2690 #mul #mul_all perf bench
> arch*() default default default
>
> 1 shift_fix 13 14.786
> 2 scaled_weight 18 15.078
> 3 unconditional 14 15.195
>
>
> Overall it appears that fewer 'mul' instructions doesn't necessarily
> mean better perf bench score. For ARM, 2 seems the best choice overall.
I suspect you're paying for having to do an actual load which can miss
there. So that makes sense.
> While 1 is better for Intel.
Right, because GCC shits itself with those conditionals. Weirdly though;
the below version does not seem so affected.
> I suggest that I spin a v2 of this series and go with scaled_weight to
> reduce bit loss. Any objections?
Just playing devils advocate to myself; how about cgroups? Will not a
per-cpu share of the cgroup weight often be very small?
So I had a little play, and I'm not at all convinced we want to do this
(I've not actually ran any numbers on it, but I can well imagine the
extra condition to hurt on branch miss predict) but it does show GCC
need not always get confused.
---
kernel/sched/fair.c | 58 +++++++++++++++++++++++++++++++++++------------------
1 file changed, 38 insertions(+), 20 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 9176f7c588a8..1b60fbe3b86c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2519,7 +2519,25 @@ static u32 __compute_runnable_contrib(u64 n)
#error "load tracking assumes 2^10 as unit"
#endif
-#define cap_scale(v, s) ((v)*(s) >> SCHED_CAPACITY_SHIFT)
+static __always_inline unsigned long fp_mult2(unsigned long x, unsigned long y)
+{
+ y *= x;
+ y >>= 10;
+
+ return y;
+}
+
+static __always_inline unsigned long fp_mult3(unsigned long x, unsigned long y, unsigned long z)
+{
+ if (x > y)
+ swap(x,y);
+
+ z *= y;
+ z >>= 10;
+ z *= x;
+
+ return z;
+}
/*
* We can represent the historical contribution to runnable average as the
@@ -2553,9 +2571,9 @@ 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)
{
- u64 delta, scaled_delta, periods;
+ u64 delta, periods;
u32 contrib;
- unsigned int delta_w, scaled_delta_w, decayed = 0;
+ unsigned int delta_w, decayed = 0;
unsigned long scale_freq, scale_cpu;
delta = now - sa->last_update_time;
@@ -2577,8 +2595,10 @@ __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)
+ scale_freq = arch_scale_freq_capacity(NULL, cpu);
+ if (running)
+ scale_cpu = arch_scale_cpu_capacity(NULL, cpu);
/* delta_w is the amount already accumulated against our next period */
delta_w = sa->period_contrib;
@@ -2594,16 +2614,14 @@ __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;
- if (cfs_rq) {
- cfs_rq->runnable_load_sum +=
- weight * scaled_delta_w;
- }
+ unsigned long t = fp_mult3(delta_w, weight, scale_freq);
+ sa->load_sum += t;
+ if (cfs_rq)
+ cfs_rq->runnable_load_sum += t;
}
if (running)
- sa->util_sum += scaled_delta_w * scale_cpu;
+ sa->util_sum += delta_w * fp_mult2(scale_cpu, scale_freq);
delta -= delta_w;
@@ -2620,25 +2638,25 @@ __update_load_avg(u64 now, int cpu, struct sched_avg *sa,
/* Efficiently calculate \sum (1..n_period) 1024*y^i */
contrib = __compute_runnable_contrib(periods);
- contrib = cap_scale(contrib, scale_freq);
if (weight) {
- sa->load_sum += weight * contrib;
+ unsigned long t = fp_mult3(contrib, weight, scale_freq);
+ sa->load_sum += t;
if (cfs_rq)
- cfs_rq->runnable_load_sum += weight * contrib;
+ cfs_rq->runnable_load_sum += t;
}
if (running)
- sa->util_sum += contrib * scale_cpu;
+ sa->util_sum += contrib * fp_mult2(scale_cpu, scale_freq);
}
/* Remainder of delta accrued against u_0` */
- scaled_delta = cap_scale(delta, scale_freq);
if (weight) {
- sa->load_sum += weight * scaled_delta;
+ unsigned long t = fp_mult3(delta, weight, scale_freq);
+ sa->load_sum += t;
if (cfs_rq)
- cfs_rq->runnable_load_sum += weight * scaled_delta;
+ cfs_rq->runnable_load_sum += t;
}
if (running)
- sa->util_sum += scaled_delta * scale_cpu;
+ sa->util_sum += delta * fp_mult2(scale_cpu, scale_freq);
sa->period_contrib += delta;
--
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