linux-kernel - Re: [RFC] arm: Add for atomic half word exchange

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Message-ID: <20150519124333.GC3644@twins.programming.kicks-ass.net>
Date:	Tue, 19 May 2015 14:43:33 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Sarbojit Ganguly <ganguly.s@...sung.com>
Cc:	Arnd Bergmann <arnd@...db.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"hpa@...or.com" <hpa@...or.com>,
	"Waiman.Long@...com" <Waiman.Long@...com>,
	"raghavendra.kt@...ux.vnet.ibm.com" 
	<raghavendra.kt@...ux.vnet.ibm.com>,
	"oleg@...hat.com" <oleg@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	SHARAN ALLUR <sharan.allur@...sung.com>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	"vikram.m@...sung.com" <vikram.m@...sung.com>
Subject: 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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