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: <20151014093024.GB3816@twins.programming.kicks-ass.net>
Date:	Wed, 14 Oct 2015 11:30:24 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Byungchul Park <byungchul.park@....com>
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 06:26:30PM +0900, Byungchul Park wrote:
> 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.

--combine might be enough, and might make sense for all the
kernel/sched/ file, but that too is not something we currently do.

> > 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.

It happens quite a lot, just not _as_ often as regular context
switching.
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