linux-kernel - Re: [patch 5/8] x86: use this_cpu_xxx to replace percpu

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Message-ID: <20120306074601.GD23669@elte.hu>
Date:	Tue, 6 Mar 2012 08:46:01 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	akpm@...ux-foundation.org
Cc:	tglx@...utronix.de, hpa@...or.com, alex.shi@...el.com,
	cl@...two.org, sfr@...b.auug.org.au, tj@...nel.org,
	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [patch 5/8] x86: use this_cpu_xxx to replace percpu_xxx funcs


* akpm@...ux-foundation.org <akpm@...ux-foundation.org> wrote:

> From: Alex Shi <alex.shi@...el.com>
> Subject: x86: use this_cpu_xxx to replace percpu_xxx funcs
> 
> Since percpu_xxx() serial functions are duplicate with this_cpu_xxx(). 
> Removing percpu_xxx() definition and replacing them by this_cpu_xxx() in
> code.
> 
> And further more, as Christoph Lameter's requirement, I try to use
> __this_cpu_xx to replace this_cpu_xxx if it is in preempt safe scenario.
> The preempt safe scenarios include:
> 1, in irq/softirq/nmi handler
> 2, protected by preempt_disable
> 3, protected by spin_lock
> 4, if the code context imply that it is preempt safe, like the code is
> follows or be followed a preempt safe code.
> 
> BTW, In fact, this_cpu_xxx are same as __this_cpu_xxx since all funcs
> implement in a single instruction for x86 machine.  But it maybe other
> platforms' performance.

but this is an x86 only patch.

> -	percpu_write(irq_regs, new_regs);
> +	__this_cpu_write(irq_regs, new_regs);

So what's the point of all this ugly churn?

Will percpu_write() be removed altogether? If so then the 
changelog should say *that*, and explain that on x86 this is a 
simple renaming of the API, not a long explanation about 
scenarios that don't actually matter.

If percpu_write() is not being removed then I don't see how this 
patch is an improvement: it's supposed to result in the same 
instructions being emitted, and __this_cpu_write() et al are 
distinctly longer to write ...

So what's the plan here?

Thanks,

	Ingo
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