linux-kernel - Re: [PATCH] x86/asm/entry/64: pack interrupt dispatch table tighter

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Message-ID: <551EDD7E.3050505@zytor.com>
Date:	Fri, 03 Apr 2015 11:35:42 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Denys Vlasenko <dvlasenk@...hat.com>
CC:	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/asm/entry/64: pack interrupt dispatch table tighter

On 04/03/2015 11:08 AM, Linus Torvalds wrote:
> On Fri, Apr 3, 2015 at 9:54 AM, Denys Vlasenko <dvlasenk@...hat.com> wrote:
>>
>> How about this version?
>> It's still isn't a star of readability,
>> but the structure of the 32-byte code block is more visible now...
> 
> Do we really even want to be this clever in the first place?
> 
> The thing is, when we take an interrupt:
> 
>  (a) the L1 I$ is always cold
> 
>  (b) the instruction decoder has never had time to run ahead
> 
>  (c) there are usually not that many different interrupts anyway, even
> under load (ie you'd have maybe disk and networking)
> 
>  (d) we intentionally spread out the different interrupt vector numbers
> 
>  (e) the 32-byte block thing is questionable, most older
> micro-architectures fetch in 16-byte blocks iirc.
> 

For the record, I actually measured the impact of the jump-to-jump when
I wrote it.  It has a small, *but measurable*, positive impact.

	-hpa

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