Date: Wed, 20 May 2015 05:09:39 +0000 (GMT)
From: Sarbojit Ganguly <ganguly.s@samsung.com>
Subject: Re: Re: [RFC] arm: Add for atomic half word exchange
To: Peter Zijlstra <peterz@infradead.org>,
        Sarbojit Ganguly <ganguly.s@samsung.com>
Cc: Arnd Bergmann <arnd@arndb.de>,
        "linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "mingo@redhat.com" <mingo@redhat.com>, "hpa@zytor.com" <hpa@zytor.com>,
        "Waiman.Long@hp.com" <Waiman.Long@hp.com>,
        "raghavendra.kt@linux.vnet.ibm.com" 
	<raghavendra.kt@linux.vnet.ibm.com>,
        "oleg@redhat.com" <oleg@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        SHARAN ALLUR <sharan.allur@samsung.com>,
        "torvalds@linux-foundation.org" <torvalds@linux-foundation.org>,
        VIKRAM MUPPARTHI <vikram.m@samsung.com>,
        SUNEEL KUMAR SURIMANI <suneel@samsung.com>
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Yes, the main advantage of Qspinlock code can be observed in NUMA but when I tested in an embedded system, a slight advantage was observed.


------- Original Message -------
Sender : Peter Zijlstra<peterz@infradead.org>
Date : May 19, 2015 21:43 (GMT+09:00)
Title : Re: [RFC] arm: Add for atomic half word exchange

On Tue, May 19, 2015 at 11:20:13AM +0000, Sarbojit Ganguly wrote:
> On Tuesday 19 May 2015 09:39:33 Sarbojit Ganguly wrote:
> > Since 16 bit half word exchange was not there and MCS based
> > qspinlock by Waiman's xchg_tail() requires an atomic exchange on a
> > half word, here is a small modification to __xchg() code.

Can you actually see a performance improvement with the qspinlock code
on ARM ?

The real improvements on x86 were on NUMA systems; although there were
real improvements on light loads as well.


Note that ARM (or any load-store arch) could get rid of all the cmpxchg
loops in that code. Although I suppose we replaced the most common ones
with these unconditional atomics already -- like that xchg16 -- so
implementing those with ll/sc, as you did, should be near optimal.


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