Message-ID: <551EDD7E.3050505@zytor.com>
Date: Fri, 03 Apr 2015 11:35:42 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Linus Torvalds <torvalds@linux-foundation.org>,
        Denys Vlasenko <dvlasenk@redhat.com>
CC: Ingo Molnar <mingo@kernel.org>, Steven Rostedt <rostedt@goodmis.org>,
        Borislav Petkov <bp@alien8.de>, Andy Lutomirski <luto@amacapital.net>,
        Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Alexei Starovoitov <ast@plumgrid.com>, Will Drewry <wad@chromium.org>,
        Kees Cook <keescook@chromium.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] x86/asm/entry/64: pack interrupt dispatch table tighter
References: <1428059634-11782-1-git-send-email-dvlasenk@redhat.com>	<20150403140336.GA16422@gmail.com>	<551EC5CD.3050901@redhat.com> <CA+55aFxHSCUmsWZR4ey_NfZep-h5oyEoUL15GamQt9v17znyvQ@mail.gmail.com>
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On 04/03/2015 11:08 AM, Linus Torvalds wrote:
> On Fri, Apr 3, 2015 at 9:54 AM, Denys Vlasenko <dvlasenk@redhat.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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