linux-kernel - Re: [PATCH v2 1/2] sched: make each sched class handle its rq assignment in their own class

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Message-ID: <20151014092630.GX6455@byungchulpark-X58A-UD3R>
Date:	Wed, 14 Oct 2015 18:26:30 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	yuyang.du@...el.com, pjt@...gle.com, efault@....de,
	tglx@...utronix.de
Subject: Re: [PATCH v2 1/2] sched: make each sched class handle its rq
 assignment in their own class

On Wed, Oct 14, 2015 at 11:00:16AM +0200, Peter Zijlstra wrote:
> On Wed, Oct 14, 2015 at 05:42:09PM +0900, byungchul.park@....com wrote:
> > +static inline void set_task_rq(struct task_struct *p, unsigned int cpu)
> > +{
> > +	const struct sched_class *class;
> > +
> > +	for_each_class(class) {
> > +		if (class->set_task_rq)
> > +			class->set_task_rq(p, cpu);
> > +	}
> > +}
> 
> So I worry about this, because the class structures are not all in the
> same translation unit, GCC cannot (without -fwhole-program) optimize
> that all away.

i wondered if it was so, and it was.

> 
> This means we'll do 5 cacheline loads and 2 indirect calls, on _every_
> cpu migration.

i agree with your this concern. to avoid this concern, it can be done by
hard coding as current code.. but we will lose code flexability. i thought
migration overhead was not so important since it hardly happens.

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