linux-kernel - Re: Jump Label initialization

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Date:	Thu, 29 Sep 2011 09:37:59 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Jason Baron <jbaron@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Michael Ellerman <michael@...erman.id.au>,
	Jan Glauber <jang@...ux.vnet.ibm.com>,
	David Daney <ddaney@...iumnetworks.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: Jump Label initialization

On Wed, 2011-09-28 at 19:14 -0700, Jeremy Fitzhardinge wrote:
> Hi all,
> 
> I'm trying to use the jump label machinery as part of the pv ticketlock
> work I'm doing on x86.
> 
> The problem I'm having at the moment is that I do my spinlock setup in
> smp_prepare_boot_cpu(), which happens before jump_label_init() gets
> called, and so the latter goes and nops out all my enabled jump label key.
> 
> I'm experimenting at the moment with a patch to allow
> jump_label_enable() to be called fairly early, and have that be
> respected by jump_label_init().  I'm doing this by replacing
> arch_jump_label_poke_text_early() with
> arch_jump_label_transform_early(), which shares most of its code with
> its non-early variant, except that it expects to run in a pre-SMP
> environment.
> 
> Does this seem plausible? (I haven't tested it yet.)
> 
> The x86, mips and sparc patches are fairly simple; I forgot to look at
> powerpc, and I didn't fully investigate s390.
> 
> While my current use-case is x86-specific, it seems generally useful to
> make the jump_label machinery available as early as possible. I wonder
> if you have any suggestions about how to handle this?

I also remember talking to Jason about a way to initialize a jump label
enabled, instead of the default disabled.

So yeah, I think your stuff would be useful..
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