linux-kernel - Re: [PATCH v4 1/3] qspinlock: Introducing a 4-byte queue spinlock implementation

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Date:	Tue, 18 Feb 2014 19:50:13 -0500
From:	Waiman Long <waiman.long@...com>
To:	Peter Zijlstra <peterz@...radead.org>
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 v4 1/3] qspinlock: Introducing a 4-byte queue spinlock
 implementation

On 02/18/2014 04:34 PM, Peter Zijlstra wrote:
> On Tue, Feb 18, 2014 at 02:39:31PM -0500, Waiman Long wrote:
>> The #ifdef is harder to take away here. The point is that doing a 32-bit
>> exchange may accidentally steal the lock with the additional code to handle
>> that. Doing a 16-bit exchange, on the other hand, will never steal the lock
>> and so don't need the extra handling code. I could construct a function with
>> different return values to handle the different cases if you think it will
>> make the code easier to read.
> Does it really pay to use xchg() with all those fixup cases? Why not
> have a single cmpxchg() loop that does just the exact atomic op you
> want?

The main reason for using xchg instead of cmpxchg is its performance 
impact when the lock is heavily contended. Under those circumstances, a 
task may need to do several tries of read+atomic-RMV before getting it 
right. This may cause a lot of cacheline contention. With xchg, we need 
at most 2 atomic ops. Using cmpxchg() does simplify the code a bit at 
the expense of performance with heavy contention.

-Longman
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