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: <20151014090016.GR17308@twins.programming.kicks-ass.net>
Date:	Wed, 14 Oct 2015 11:00:16 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	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 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.

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