linux-kernel - Re: [RFC PATCH] x86/64: Optimize the effective instruction cache footprint of kernel functions

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Message-ID: <555DC39C.8060502@redhat.com>
Date:	Thu, 21 May 2015 13:38:04 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>
CC:	Andy Lutomirski <luto@...capital.net>,
	Davidlohr Bueso <dave@...olabs.net>,
	Peter Anvin <hpa@...or.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Borislav Petkov <bp@...en8.de>,
	Peter Zijlstra <peterz@...radead.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Brian Gerst <brgerst@...il.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jason Low <jason.low2@...com>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH] x86/64: Optimize the effective instruction cache
 footprint of kernel functions

On 05/20/2015 02:21 PM, Denys Vlasenko wrote:
> So what we need is to put something like ".p2align 64,,7"
> before every function.
> 
> (
> 	Why 7?
> 
> 	defconfig vmlinux (w/o FRAME_POINTER) has 42141 functions.
> 	6923 of them have 1st insn 5 or more bytes long,
> 	5841 of them have 1st insn 6 or more bytes long,
> 	5095 of them have 1st insn 7 or more bytes long,
> 	786 of them have 1st insn 8 or more bytes long,
> 	548 of them have 1st insn 9 or more bytes long,
> 	375 of them have 1st insn 10 or more bytes long,
> 	73 of them have 1st insn 11 or more bytes long,
> 	one of them has 1st insn 12 bytes long:
> 	    this "heroic" instruction is in local_touch_nmi()
> 		65 48 c7 05 44 3c 00 7f 00 00 00 00
> 		movq $0x0,%gs:0x7f003c44(%rip)
> 
> 	Thus ensuring that at least seven first bytes do not cross
> 	64-byte boundary would cover >98% of all functions.
> )	
> 
> gcc can't do that right now. With -falign-functions=N,
> it emits ".p2align next_power_of_2(N),,N-1"
> 
> We need to make it just a tiny bit smarter.
> 
>> We'd need toolchain help to do saner alignment.
> 
> Yep.
> I'm going to create a gcc BZ with a feature request,
> unless you disagree with my musings above.

The BZ is here:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66240

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