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

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Date:	Thu, 2 Apr 2015 17:53:09 +0100
From:	Morten Rasmussen <morten.rasmussen@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Vincent Guittot <vincent.guittot@...aro.org>,
	Ingo Molnar <mingo@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Mike Galbraith <efault@....de>,
	"nicolas.pitre@...aro.org" <nicolas.pitre@...aro.org>,
	Dietmar Eggemann <Dietmar.Eggemann@....com>,
	Linaro Kernel Mailman List <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 Wed, Mar 25, 2015 at 05:33:09PM +0000, Peter Zijlstra wrote:
> On Tue, Mar 24, 2015 at 11:00:57AM +0100, Vincent Guittot wrote:
> > On 23 March 2015 at 14:19, Peter Zijlstra <peterz@...radead.org> wrote:
> > > 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.
> > 
> > yes, it removes useless multiplication when not used by an arch.
> > It also adds a constraint on the arch side which have to define
> > arch_scale_freq_capacity like below:
> > 
> > #define arch_scale_freq_capacity xxx_arch_scale_freq_capacity
> > with xxx_arch_scale_freq_capacity an architecture specific function
> 
> Yeah, but it not being weak should make that a compile time warn/fail,
> which should be pretty easy to deal with.

Could you enlighten me a bit about how to define the arch specific
implementation without getting into trouble? I'm failing miserably :(

I thought the arm arch-specific topology.h file was a good place to put
the define as it get included in sched.h, so I did a:

#define arch_scale_freq_capacity arm_arch_scale_freq_capacity

However, I have to put a function prototype in the same (or some other
included) header file to avoid doing an implicit function definition.
arch_scale_freq_capacity() takes a struct sched_domain pointer, so I
have to include linux/sched.h which leads to circular dependency between
linux/sched.h and topology.h.

The only way out I can think of to create (or find) a new arch-specific
header file that can include linux/sched.h and be included in
kernel/sched/sched.h and have the define and prototype in there.

I must be missing something?

We can drop the sched_domain pointer as we don't use it, but I'm going
to do the same trick for arch_scale_cpu_capacity() as well which does
require the sd pointer.

Finally, is introducing an ARCH_HAS_SCALE_FREQ_CAPACITY or similar a
complete no go?

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 91c6736..5707fb7 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1388,12 +1388,14 @@ static inline int hrtick_enabled(struct rq *rq)
 #ifdef CONFIG_SMP
 extern void sched_avg_update(struct rq *rq);
 
-#ifndef arch_scale_freq_capacity
+#ifndef ARCH_HAS_SCALE_FREQ_CAPACITY
 static __always_inline
 unsigned long arch_scale_freq_capacity(struct sched_domain *sd, int cpu)
 {
 	return SCHED_CAPACITY_SCALE;
 }
+#else
+extern unsigned long arch_scale_freq_capacity(struct sched_domain *sd, int cpu);
 #endif
 
 static inline void sched_rt_avg_update(struct rq *rq, u64 rt_delta)
--
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