linux-kernel - Re: [PATCH v3 1/2] qspinlock: Introducing a 4-byte queue spinlock implementation

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Date:	Fri, 31 Jan 2014 20:51:33 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@...com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.de>,
	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michel Lespinasse <walken@...gle.com>,
	Andi Kleen <andi@...stfloor.org>,
	Rik van Riel <riel@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	George Spelvin <linux@...izon.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Daniel J Blueman <daniel@...ascale.com>,
	Alexander Fyodorov <halcy@...dex.ru>,
	Aswin Chandramouleeswaran <aswin@...com>,
	Scott J Norton <scott.norton@...com>,
	Thavatchai Makphaibulchoke <thavatchai.makpahibulchoke@...com>
Subject: Re: [PATCH v3 1/2] qspinlock: Introducing a 4-byte queue spinlock
 implementation

On Fri, Jan 31, 2014 at 02:24:33PM -0500, Waiman Long wrote:
> On 01/31/2014 10:08 AM, Peter Zijlstra wrote:
> >Whereas for the straight cmpxchg() you'd get something relatively simple
> >like:
> >
> >   mov %edx,%eax
> >   lock cmpxchg %ecx,(%rbx)
> >   cmp %edx,%eax
> >   jne ...
> 
> I believe the speeds of the lock functions are about the same. However,
> qspinlock has a much simpler unlock function which probably account of most
> of the speed gain.

The unlock path for ticket locks is a single "add[bw] $0x1,()", that should be
as fast as the single "movb 0,()" you have.

> >I suppose we could from the ticket code more and optimize the
> >uncontended path, but that'll make the contended path more expensive
> >again, although probably not as bad as hitting a new cacheline.
> 
> I don't get what you are trying to say.

I said we could probably make the ticket lock function faster for the
uncontended case by making the contended case slightly more expensive.
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