From: "H. Peter Anvin" <hpa@zytor.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Zachary Amsden <zamsden@redhat.com>,
linux-kernel@vger.kernel.org,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>,
x86@kernel.org, Avi Kivity <avi@redhat.com>
Subject: Re: [PATCH] x86 rwsem optimization extreme
Date: Wed, 17 Feb 2010 15:29:40 -0800 [thread overview]
Message-ID: <4B7C7BE4.9050908@zytor.com> (raw)
In-Reply-To: <alpine.LFD.2.00.1002171403160.4141@localhost.localdomain>
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
next prev parent reply other threads:[~2010-02-17 23:30 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-02-17 21:58 [PATCH] x86 rwsem optimization extreme Zachary Amsden
2010-02-17 22:10 ` Linus Torvalds
2010-02-17 22:29 ` H. Peter Anvin
2010-02-17 23:29 ` H. Peter Anvin [this message]
2010-02-18 1:03 ` Zachary Amsden
2010-02-18 1:53 ` Linus Torvalds
2010-02-18 1:59 ` H. Peter Anvin
2010-02-18 4:25 ` Zachary Amsden
2010-02-18 8:12 ` Andi Kleen
2010-02-18 8:24 ` Zachary Amsden
2010-02-18 9:29 ` Andi Kleen
2010-02-18 10:55 ` Ingo Molnar
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