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

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Message-ID: <551EE4BE.9090507@zytor.com>
Date:	Fri, 03 Apr 2015 12:06:38 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Denys Vlasenko <dvlasenk@...hat.com>,
	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 11:37 AM, Linus Torvalds wrote:
> On Fri, Apr 3, 2015 at 11:35 AM, H. Peter Anvin <hpa@...or.com> wrote:
>>
>> 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.
> 
> What did you compare against, and how did you measure that? I don't
> see how it could *possibly* be faster than just a simple aligned "push
> + jmp".
> 

I wish I remembered the exact details; it took a fair bit of gathering
numbers as the spread was quite a bit wider than the delta, but in the
end there were two distribution peaks clearly offset.

I seem to remember it involving a loop running RDTSC continuously and
another RDTSC in the interrupt path.

	-hpa


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