linux-kernel - Re: [RFC][PATCH 0/5] preempt

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Message-ID: <20130814165227.GA21191@laptop.programming.kicks-ass.net>
Date:	Wed, 14 Aug 2013 18:52:27 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Mike Galbraith <bitbucket@...ine.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [RFC][PATCH 0/5] preempt_count rework

On Wed, Aug 14, 2013 at 09:14:34AM -0700, H. Peter Anvin wrote:
> Even so, this_rq() uses __get_cpu_var() and takes its address, which
> turns into a sequence like:
> 
> 	leaq __percpu_runqueues(%rip),%rax
> 	addq %gs:this_cpu_off,%rax
> 
> ... which is better than the above but still more heavyweight than it
> would be if the pointer was itself a percpu variable.

Oh curses, this is because lea can't do segment offsets? So there's no
sane way to get addresses of per-cpu variables.

Because ideally we'd have something like:

  lea %gs:__percpu_runqueues,%rax

So in this case it makes sense to also store the actual pointer; how
unfortunate.
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