linux-kernel - Re: [PATCH v10 04/11] sched: Make sched entity usage tracking scale-invariant

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Message-ID: <20150323131905.GF23123@twins.programming.kicks-ass.net>
Date:	Mon, 23 Mar 2015 14:19:05 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	preeti@...ux.vnet.ibm.com, Morten.Rasmussen@....com,
	kamalesh@...ux.vnet.ibm.com, riel@...hat.com, efault@....de,
	nicolas.pitre@...aro.org, dietmar.eggemann@....com,
	linaro-kernel@...ts.linaro.org, Paul Turner <pjt@...gle.com>,
	Ben Segall <bsegall@...gle.com>
Subject: Re: [PATCH v10 04/11] sched: Make sched entity usage tracking
 scale-invariant

On Fri, Feb 27, 2015 at 04:54:07PM +0100, Vincent Guittot wrote:

> +	unsigned long scale_freq = arch_scale_freq_capacity(NULL, cpu);

> +			sa->running_avg_sum += delta_w * scale_freq
> +				>> SCHED_CAPACITY_SHIFT;

so the only thing that could be improved is somehow making this
multiplication go away when the arch doesn't implement the function.

But I'm not sure how to do that without #ifdef.

Maybe a little something like so then... that should make the compiler
get rid of those multiplications unless the arch needs them.


--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2484,8 +2484,6 @@ static u32 __compute_runnable_contrib(u6
 	return contrib + runnable_avg_yN_sum[n];
 }
 
-unsigned long __weak arch_scale_freq_capacity(struct sched_domain *sd, int cpu);
-
 /*
  * We can represent the historical contribution to runnable average as the
  * coefficients of a geometric series.  To do this we sub-divide our runnable
@@ -6015,11 +6013,6 @@ static unsigned long default_scale_capac
 	return SCHED_CAPACITY_SCALE;
 }
 
-unsigned long __weak arch_scale_freq_capacity(struct sched_domain *sd, int cpu)
-{
-	return default_scale_capacity(sd, cpu);
-}
-
 static unsigned long default_scale_cpu_capacity(struct sched_domain *sd, int cpu)
 {
 	if ((sd->flags & SD_SHARE_CPUCAPACITY) && (sd->span_weight > 1))
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1387,7 +1387,14 @@ static inline int hrtick_enabled(struct
 
 #ifdef CONFIG_SMP
 extern void sched_avg_update(struct rq *rq);
-extern unsigned long arch_scale_freq_capacity(struct sched_domain *sd, int cpu);
+
+#ifndef arch_scale_freq_capacity
+static __always_inline
+unsigned long arch_scale_freq_capacity(struct sched_domain *sd, int cpu)
+{
+	return SCHED_CAPACITY_SCALE;
+}
+#endif
 
 static inline void sched_rt_avg_update(struct rq *rq, u64 rt_delta)
 {
--
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