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Date: Thu, 21 May 2015 13:38:04 +0200
From: Denys Vlasenko <dvlasenk@redhat.com>
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To: Linus Torvalds <torvalds@linux-foundation.org>,
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CC: Andy Lutomirski <luto@amacapital.net>, Davidlohr Bueso <dave@stgolabs.net>,
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        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Tim Chen <tim.c.chen@linux.intel.com>, Borislav Petkov <bp@alien8.de>,
        Peter Zijlstra <peterz@infradead.org>,
        "Chandramouleeswaran, Aswin" <aswin@hp.com>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
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Subject: Re: [RFC PATCH] x86/64: Optimize the effective instruction cache
 footprint of kernel functions
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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