linux-kernel - Re: [PATCH] x86 rwsem optimization extreme

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Message-ID: <4B7C7BE4.9050908@zytor.com>
Date:	Wed, 17 Feb 2010 15:29:40 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Zachary Amsden <zamsden@...hat.com>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	Avi Kivity <avi@...hat.com>
Subject: Re: [PATCH] x86 rwsem optimization extreme

On 02/17/2010 02:10 PM, Linus Torvalds wrote:
> 
> The cost of 'adc' may happen to be identical in this case, but I suspect 
> you didn't test on UP, where the 'lock' prefix goes away. An unlocked 
> 'add' tends to be faster than an unlocked 'adc'.
> 
> (It's possible that some micro-architectures don't care, since it's a 
> memory op, and they can see that 'C' is set. But it's a fragile assumption 
> that it would always be ok).
> 

FWIW, I don't know of any microarchitecture where adc is slower than
add, *as long as* the setup time for the CF flag is already used up.
However, as I already commented, I don't think this is worth it.  This
inline appears to only be instantiated once, and as such, it takes a
whopping six bytes across the entire kernel.

	-hpa

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