linux-kernel - Re: [PATCH RFC V2 3/5] jump_label: if a key has already been initialized, don't nop it out

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Message-ID: <1317925615.4729.14.camel@gandalf.stny.rr.com>
Date:	Thu, 06 Oct 2011 14:26:52 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Jason Baron <jbaron@...hat.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	"David S. Miller" <davem@...emloft.net>,
	David Daney <david.daney@...ium.com>,
	Michael Ellerman <michael@...erman.id.au>,
	Jan Glauber <jang@...ux.vnet.ibm.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Xen Devel <xen-devel@...ts.xensource.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	peterz@...radead.org, rth@...hat.com
Subject: Re: [PATCH RFC V2 3/5] jump_label: if a key has already been
 initialized, don't nop it out

On Thu, 2011-10-06 at 14:10 -0400, Jason Baron wrote:

> > Looks like jmp2 is about 5% faster than jmp5 on Sandybridge with this
> > benchmark.
> > 
> > But insignificant difference on Nehalem.
> > 
> >     J
> 
> It would be cool if we could make the total width 2-bytes, when
> possible.  It might be possible by making the initial 'JUMP_LABEL_INITIAL_NOP'
> as a 'jmp' to the 'l_yes' label. And then patching that with a no-op at boot
> time or link time - letting the compiler pick the width. In that way we could
> get the optimal width...

Why not just do it?

jump_label is encapsulated in arch_static_branch() which on x86 looks
like:

static __always_inline bool arch_static_branch(struct jump_label_key *key)
{
	asm goto("1:"
		JUMP_LABEL_INITIAL_NOP
		".pushsection __jump_table,  \"aw\" \n\t"
		_ASM_ALIGN "\n\t"
		_ASM_PTR "1b, %l[l_yes], %c0 \n\t"
		".popsection \n\t"
		: :  "i" (key) : : l_yes);
	return false;
l_yes:
	return true;
}


That jmp to l_yes should easily be a two byte jump.

If not I'm sure it would be easy to catch it before modifying the code.
And then complain real loudly about it.

-- Steve


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