linux-kernel - Re: Preemptable Ticket Spinlock

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Date:	Mon, 22 Apr 2013 19:13:48 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Jiannan Ouyang <ouyang@...pitt.edu>,
	LKML <linux-kernel@...r.kernel.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Avi Kivity <avi.kivity@...il.com>,
	Gleb Natapov <gleb@...hat.com>, Ingo Molnar <mingo@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Srikar <srikar@...ux.vnet.ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	KVM <kvm@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de>,
	Chegu Vinod <chegu_vinod@...com>,
	"Andrew M. Theurer" <habanero@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>,
	Andrew Jones <drjones@...hat.com>,
	Karen Noel <knoel@...hat.com>
Subject: Re: Preemptable Ticket Spinlock

On 04/22/2013 05:56 PM, Andi Kleen wrote:
> Rik van Riel <riel@...hat.com> writes:
>>
>> If we always incremented the ticket number by 2 (instead of 1), then
>> we could use the lower bit of the ticket number as the spinlock.
>
> Spinning on a single bit is very inefficient, as you need to do
> try lock in a loop which is very unfriendly to the MESI state protocol.
> It's much better to have at least three states and allow
> spinning-while-reading-only.
>
> This is typically very visible on systems with >2S.

Absolutely, the spinning should be read-only, until the CPU
sees that the desired bit is clear.  MESI-friendly spinning
is essential.

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