Message-ID: <1391717992.6733.232.camel@snotra.buserror.net>
Subject: Re: [PATCH] Convert powerpc simple spinlocks into ticket locks
From: Scott Wood <scottwood@freescale.com>
To: Torsten Duwe <duwe@lst.de>
CC: Peter Zijlstra <peterz@infradead.org>, <linux-kernel@vger.kernel.org>,
        Paul Mackerras <paulus@samba.org>, Anton Blanchard <anton@samba.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        <linuxppc-dev@lists.ozlabs.org>, Ingo Molnar <mingo@kernel.org>
Date: Thu, 6 Feb 2014 14:19:52 -0600
In-Reply-To: <20140206173727.GA13048@lst.de>
References: <20140206103736.GA18054@lst.de>
	 <20140206163837.GT2936@laptop.programming.kicks-ass.net>
	 <20140206173727.GA13048@lst.de>
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Sender: linux-kernel-owner@vger.kernel.org

On Thu, 2014-02-06 at 18:37 +0100, Torsten Duwe wrote:
> On Thu, Feb 06, 2014 at 05:38:37PM +0100, Peter Zijlstra wrote:
> > On Thu, Feb 06, 2014 at 11:37:37AM +0100, Torsten Duwe wrote:
> > > x86 has them, MIPS has them, ARM has them, even ia64 has them:
> > > ticket locks. They reduce memory bus and cache pressure especially
> > > for contended spinlocks, increasing performance.
> > > 
> > > This patch is a port of the x86 spin locks, mostly written in C,
> > > to the powerpc, introducing inline asm where needed. The pSeries
> > > directed yield for vCPUs is taken care of by an additional "holder"
> > > field in the lock.
> > > 
> > 
> > A few questions; what's with the ppc64 holder thing? Not having a 32bit
> > spinlock_t is sad.
> 
> I must admit that I haven't tested the patch on non-pseries ppc64 nor on
> ppc32. Only ppc64 has the ldarx and I tried to atomically replace the 
> holder along with the locks. That might prove unneccessary.

Why is the functionality of holder only required on 64-bit?  We have too
many 32/64 differences as is.  Perhaps on 32-bit a lower max number of
CPUs could be assumed, to make it fit in one word.

> > Can you pair lwarx with sthcx ? I couldn't immediately find the answer
> > in the PowerISA doc. If so I think you can do better by being able to
> > atomically load both tickets but only storing the head without affecting
> > the tail.
> 
> V2.06b, Book II, Chapter 3, "sthcx" says:
> | If a reservation exists and the length associated [...] is not 2 bytes,
> | it is undefined whether (RS)_48:63 are stored [...]
> 
> That doesn't make me feel comfortable :(

Plus, sthcx doesn't exist on all PPC chips.

-Scott


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