linux-kernel - Re: [PATCH RFC/RFB] x86_64, i386: interrupt dispatch changes

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Message-ID: <49178E89.2000307@zytor.com>
Date:	Sun, 09 Nov 2008 17:29:45 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Alexander van Heukelum <heukelum@...tmail.fm>
CC:	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Alexander van Heukelum <heukelum@...lshack.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>, lguest@...abs.org,
	jeremy@...source.com, Steven Rostedt <srostedt@...hat.com>,
	Mike Travis <travis@....com>
Subject: Re: [PATCH RFC/RFB] x86_64, i386: interrupt dispatch changes

Alexander van Heukelum wrote:
> 
> In general: after applying the patch, latencies are more
> often seen by the rdtsctest. It also seems to cause a
> small percentage decrease in speed of hackbench. 
> Looking at the latency histograms I believe this is
> a real effect, but I could not do enough boots/runs to
> make this a certainty from the runtimes alone.
> 
> At least for this PC, doing hpa's suggested cleanup of
> the stub table is the right way to go for now... A
> second option would be to get rid of the stub table by
> assigning each important vector a unique handler and
> to make sure those handlers do not rely on the vector
> number at all.
> 

Hi Alexander,

First of all, great job on the timing analysis.  I believe this confirms
the concerns that I had about this technique.

Here is a prototype patch of the compressed IRQ stubs -- this patch
compresses them down to 7 stubs per 32-byte cache line (or part of cache
line) at the expense of a back-to-back jmp which has the potential of
being ugly on some pipelines (we can only get 4 stubs into 32 bytes
without that).

Would you be willing to run your timing test on this patch?  This isn't
submission-quality since it commingles multiple changes, and it needs
some cleanup, but it should be useful for testing.

As a side benefit it eliminates some gratuitous differences between the
32- and 64-bit code.

	-hpa

View attachment "irqstubs.patch" of type "text/x-patch" (7318 bytes)