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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Date:	Thu, 06 Oct 2011 14:39:49 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Jason Baron <jbaron@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	"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 10/06/2011 11:13 AM, H. Peter Anvin wrote:
> On 10/06/2011 11:10 AM, Jason Baron wrote:
>> 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...
>>
> Yes, that would be a win just based on icache footprint alone.

I'm not sure it would be a win, necessarily.  My test with back-to-back
jmp2 was definitely slower than with the nop padding it out to 5 bytes;
I suspect that's a result of having too many jmps within one cacheline. 
Of course, there's no reason why the CPU would optimise for jumps to
jumps, so perhaps its just hitting a "stupid programmer" path.

    J
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