linux-kernel - Re: [patch 1/4] x86: FIFO ticket spinlocks

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Message-ID: <20071102000051.GB20144@wotan.suse.de>
Date:	Fri, 2 Nov 2007 01:00:51 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Chuck Ebbert <cebbert@...hat.com>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <ak@...e.de>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [patch 1/4] x86: FIFO ticket spinlocks

On Thu, Nov 01, 2007 at 04:01:45PM -0400, Chuck Ebbert wrote:
> On 11/01/2007 10:03 AM, Nick Piggin wrote:
> 
> [edited to show the resulting code]
> 
> > +	__asm__ __volatile__ (
> > +		LOCK_PREFIX "xaddw %w0, %1\n"
> > +		"1:\t"
> > +		"cmpb %h0, %b0\n\t"
> > +		"je 2f\n\t"
> > +		"rep ; nop\n\t"
> > +		"movb %1, %b0\n\t"
> > +		/* don't need lfence here, because loads are in-order */
> >  		"jmp 1b\n"
> > +		"2:"
> > +		:"+Q" (inc), "+m" (lock->slock)
> > +		:
> > +		:"memory", "cc");
> >  }
> 
> If you really thought you might get long queues, you could figure out
> how far back you are and use that to determine how long to wait before
> testing the lock again. That cmpb could become a subb without adding
> overhead to the fast path -- that would give you the queue length (or
> its complement anyway.)

Indeed. You can use this as a really nice input into a backoff
algorithm (eg. if you're next in line, don't back off, or at least
don't go into exponential backoff; if you've got people in front
of you, start throttling harder).

I think I'll leave that to SGI if they come up with a big x86 SSI ;)
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