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From: Denys Vlasenko <vda.linux@googlemail.com>
Date: Sat, 16 Apr 2016 23:08:34 +0200
Message-ID: <CAK1hOcMPXi99Gv2EMm9s60urAHCSKsMG8bhw3RBjcFJtO0MUcg@mail.gmail.com>
Subject: Re: [RFC PATCH] x86/64: Optimize the effective instruction cache
 footprint of kernel functions
To: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Ingo Molnar <mingo@kernel.org>, Andy Lutomirski <luto@amacapital.net>,
        Davidlohr Bueso <dave@stgolabs.net>, Peter Anvin <hpa@zytor.com>,
        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>,
        Brian Gerst <brgerst@gmail.com>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>, Jason Low <jason.low2@hp.com>,
        "linux-tip-commits@vger.kernel.org" 
	<linux-tip-commits@vger.kernel.org>,
        Arjan van de Ven <arjan@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>
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On Thu, May 21, 2015 at 1:38 PM, Denys Vlasenko <dvlasenk@redhat.com> wrote:
> 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

...and now this BZ has a working patch, which implements e.g.
-falign-functions=64,7