linux-kernel - Re: [RFC][PATCH 2/2] x86/jump labels: Use etiher 5 byte or 2 byte jumps

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Date:	Wed, 07 Aug 2013 13:57:32 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	linux-kernel@...r.kernel.org
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Jason Baron <jbaron@...mai.com>
Subject: Re: [RFC][PATCH 2/2] x86/jump labels: Use etiher 5 byte or 2 byte
 jumps

On Wed, 2013-08-07 at 13:36 -0400, Steven Rostedt wrote:
> plain text document attachment
> (0002-x86-jump-labels-Use-etiher-5-byte-or-2-byte-jumps.patch)
> From: Steven Rostedt <srostedt@...hat.com>
> 
> Have the jump labels add a "jmp" in the assembly instead
> of a default nop. This will cause the assembler to put in
> either a 2 byte or 5 byte jmp depending on where the target
> lable is.
> 
> Then at compile time, the update_jump_label code will replace
> the jmps with either 2 or 5 byte nops.
> 
> On boot up, the code can be examined to see if the jump label
> uses either a 2 or 5 byte nop and replace it.
> 
> By allowing the jump labels to be 2 bytes, it speeds up the
> nops, not only 2 byte nops are faster than 5 byte nops, but also
> because it saves on cache foot print.
> 
>    text    data     bss     dec     hex filename
> 13403667 3666856 2998272 20068795 13239bb ../nobackup/mxtest/vmlinux-old
> 13398536 3666856 2998272 20063664 13225b0 ../nobackup/mxtest/vmlinux-new
> 
> Converting the current v3.2 trace points saved 5,131 bytes.
> As more places use jump labels, this will have a bigger savings.
> 

FYI, I didn't bother changing the change logs, so these numbers may be
totally bogus now.

-- Steve


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