Re: [PATCH 0/6] sched/fair: Compute capacity invariant load/utilization tracking

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 07/09/15 13:42, Peter Zijlstra wrote:
> On Mon, Aug 31, 2015 at 11:24:49AM +0200, Peter Zijlstra wrote:
> 
>> A quick run here gives:
>>
>> IVB-EP (2*20*2):
> 
> As noted by someone; that should be 2*10*2, for a total of 40 cpus in
> this machine.
> 
>>
>> perf stat --null --repeat 10 -- perf bench sched messaging -g 50 -l 5000
>>
>> Before:				After:
>> 5.484170711 ( +-  0.74% )	5.590001145 ( +-  0.45% )
>>
>> Which is an almost 2% slowdown :/
>>
>> I've yet to look at what happens.
> 
> OK, so it appears this is link order nonsense. When I compared profiles
> between the series, the one function that had significant change was
> skb_release_data(), which doesn't make much sense.
> 
> If I do a 'make clean' in front of each build, I get a repeatable
> improvement with this patch set (although how much of that is due to the
> patches itself or just because of code movement is as yet undetermined).
> 
> I'm of a mind to apply these patches; with two patches on top, which
> I'll post shortly.
> 

-- >8 --

From: Dietmar Eggemann <dietmar.eggemann@arm.com>
Date: Mon, 7 Sep 2015 14:57:22 +0100
Subject: [PATCH] sched/fair: Defer calling scaling functions

Do not call the scaling functions in case time goes backwards or the
last update of the sched_avg structure has happened less than 1024ns
ago.

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
---
 kernel/sched/fair.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d6ca8d987a63..3445d2fb38f4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2552,8 +2552,7 @@ __update_load_avg(u64 now, int cpu, struct sched_avg *sa,
 	u64 delta, scaled_delta, periods;
 	u32 contrib;
 	unsigned int delta_w, scaled_delta_w, decayed = 0;
-	unsigned long scale_freq = arch_scale_freq_capacity(NULL, cpu);
-	unsigned long scale_cpu = arch_scale_cpu_capacity(NULL, cpu);
+	unsigned long scale_freq, scale_cpu;
 
 	delta = now - sa->last_update_time;
 	/*
@@ -2574,6 +2573,9 @@ __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);
+
 	/* delta_w is the amount already accumulated against our next period */
 	delta_w = sa->period_contrib;
 	if (delta + delta_w >= 1024) {
-- 
1.9.1