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From: "Nakajima, Jun" <jun.nakajima@intel.com>
To: "Calin A. Culianu" <calin@ajvar.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: RE: SMP Pentium4 -- PAUSE Instruction
Date: Tue, 3 Dec 2002 07:57:09 -0800 
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That one is for tight loops. Spin locks are inlined, and PAUSE is used like:

#define spin_lock_string \
        "\n1:\t" \
        "lock ; decb %0\n\t" \
        "js 2f\n" \
        LOCK_SECTION_START("") \
        "2:\t" \
        "cmpb $0,%0\n\t" \
        "rep;nop\n\t" \ <---
        "jle 2b\n\t" \
        "jmp 1b\n" \
        LOCK_SECTION_END

Also take a look at arch/i386/kernel/semaphore.c for read/write_locks.

Jun

> -----Original Message-----
> From: Calin A. Culianu [mailto:calin@ajvar.org]
> Sent: Tuesday, December 03, 2002 7:42 AM
> To: Linux Kernel Mailing List
> Subject: SMP Pentium4 -- PAUSE Instruction
> 
> 
> I as wondering -- according to Intel's docs they recommend that on a P4
> processor to use the PAUSE instruction (aka rep followed by a nop) inside
> any spin loop (such as one used in SMP spinlock code) in order to both
> improve processor performance and reduce power consumption.
> 
> Is this instruction being used in spin-wait loops?  For some reason, I am
> having a hard time figuring out whether or not it is being used.  There is
> a rep_nop() in processor.h.. but I can't determine if that is being called
> for spin lock lock/unlock code.
> 
> 
> -Calin
> 
> 
> -
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