linux-kernel - Re: [RFC PATCH 0/7] Improve swiotlb performance by using physical addresses

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Date:	Sat, 6 Oct 2012 19:57:51 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Alexander Duyck <alexander.h.duyck@...el.com>
Cc:	Andi Kleen <andi@...stfloor.org>, konrad.wilk@...cle.com,
	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	rob@...dley.net, akpm@...ux-foundation.org, joerg.roedel@....com,
	bhelgaas@...gle.com, shuahkhan@...il.com,
	linux-kernel@...r.kernel.org, devel@...uxdriverproject.org,
	x86@...nel.org, torvalds@...ux-foundation.org
Subject: Re: [RFC PATCH 0/7] Improve swiotlb performance by using physical addresses

> Inlining everything did speed things up a bit, but I still didn't reach
> the same speed I achieved using the patch set.  However I did notice the
> resulting swiotlb code was considerably larger.

Thanks. So your patch makes sense, but imho should pursue the inlining
in parallel for other call sites.

> assembly, is replaced with 8 lines of assembly and becomes inline.  In
> addition we drop the number of calls to __phys_addr from 9 to 2 by
> dropping them all from swiotlb.  By my math I am probably saving about
> 120 instructions per packet.  I suspect all of that would probably be
> cutting the number of instructions per packet enough to probably account
> for a 5% difference when you consider I am running at about 1.5Mpps per
> core on a 2.7Ghz processor.

Maybe it's just me, but that's somehow sad for one if() and a su
btraction

BTW __pa used to be a simple subtraction, the if () was just added to
handle the few call sites for x86-64 that do __pa(&text_symbol).
Maybe we should just go back to the old __pa_symbol() for those cases,
then __pa could be the simple subtraction it used to was again
and it could be inlined and everyone would be happy.

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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