Date: Wed, 19 Feb 2014 09:52:25 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <waiman.long@hp.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Arnd Bergmann <arnd@arndb.de>,
        linux-arch@vger.kernel.org, x86@kernel.org,
        linux-kernel@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michel Lespinasse <walken@google.com>,
        Andi Kleen <andi@firstfloor.org>, Rik van Riel <riel@redhat.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>,
        George Spelvin <linux@horizon.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Daniel J Blueman <daniel@numascale.com>,
        Alexander Fyodorov <halcy@yandex.ru>,
        Aswin Chandramouleeswaran <aswin@hp.com>,
        Scott J Norton <scott.norton@hp.com>,
        Thavatchai Makphaibulchoke <thavatchai.makpahibulchoke@hp.com>
Subject: Re: [PATCH v4 1/3] qspinlock: Introducing a 4-byte queue spinlock
 implementation
Message-ID: <20140219085225.GH27965@twins.programming.kicks-ass.net>
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On Tue, Feb 18, 2014 at 07:50:13PM -0500, Waiman Long wrote:
> 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.

Have you actually measured this?
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