linux-kernel - Re: x86 memcpy performance

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Message-ID: <20110815200510.GA10141@liondog.tnic>
Date:	Mon, 15 Aug 2011 22:05:10 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Andrew Lutomirski <luto@....edu>
Cc:	melwyn lobo <linux.melwyn@...il.com>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	borislav.petkov@....com
Subject: Re: x86 memcpy performance

On Mon, Aug 15, 2011 at 03:11:40PM -0400, Andrew Lutomirski wrote:
> > Well, copy_from_user... does a bunch of rep; movsq - if the SSE version
> > shows reasonable speedup there, we might need to make those work too.
> 
> I'm a little surprised that SSE beats fast string operations, but I
> guess benchmarking always wins.

If by fast string operations you mean X86_FEATURE_ERMS, then that's
Intel-only and that actually would need to be benchmarked separately.
Currently, I see speedup for large(r) buffers only vs rep; movsq. But I
dunno about rep; movsb's enhanced rep string tricks Intel does.

> Yes.  But we don't nest that much, and the save/restore isn't all that
> expensive.  And we don't have to save/restore unless kernel entries
> nest and both entries try to use kernel_fpu_begin at the same time.

Yep.

> This whole project may take awhile.  The code in there is a
> poorly-documented mess, even after Hans' cleanups.  (It's a lot worse
> without them, though.)

Oh yeah, this code could use lotsa scrubbing :)

-- 
Regards/Gruss,
    Boris.
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