From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andi Kleen <andi@firstfloor.org>
Subject: Re: Preemptable Ticket Spinlock
Date: Mon, 22 Apr 2013 14:56:44 -0700
Message-ID: <m2k3nuw7oj.fsf@firstfloor.org>
References: <CAJocwccu5QQyuKRvgNyPSFz2K_rzCW419W9-XdSUYOL7+KqQKg@mail.gmail.com>
	<51745650.9050204@redhat.com>
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Cc: Jiannan Ouyang <ouyang@cs.pitt.edu>,
	LKML <linux-kernel@vger.kernel.org>,
	Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Avi Kivity <avi.kivity@gmail.com>,
	Gleb Natapov <gleb@redhat.com>, Ingo Molnar <mingo@redhat.com>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	Srikar <srikar@linux.vnet.ibm.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"Nikunj A. Dadhania" <nikunj@linux.vnet.ibm.com>,
	KVM <kvm@vger.kernel.org>, Thomas Gleixner <tglx@linutronix.de>,
	Chegu Vinod <chegu_vinod@hp.com>,
	"Andrew M. Theurer" <habanero@linux.vnet.ibm.com>,
	Srivatsa Vaddagiri <srivatsa.vaddagiri@gmail.com>,
	Andrew Jones <drjones@redhat.com>,
	Karen Noel <knoel@redhat.com>
To: Rik van Riel <riel@redhat.com>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <51745650.9050204@redhat.com> (Rik van Riel's message of "Sun, 21
	Apr 2013 17:12:48 -0400")
Sender: linux-kernel-owner@vger.kernel.org
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Rik van Riel <riel@redhat.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.

-Andi
        
-- 
ak@linux.intel.com -- Speaking for myself only