linux-kernel - Re: [PATCH 00/20] x86: ticket lock rewrite and paravirtualization

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Message-ID: <4CDDBBD3.5050903@zytor.com>
Date:	Fri, 12 Nov 2010 14:12:35 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...e.de>,
	Jan Beulich <JBeulich@...ell.com>, Avi Kivity <avi@...hat.com>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Linux Virtualization <virtualization@...ts.linux-foundation.org>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
Subject: Re: [PATCH 00/20] x86: ticket lock rewrite and paravirtualization

On 11/03/2010 07:59 AM, Jeremy Fitzhardinge wrote:
> 
>       - with an unmodified struct spinlock, it can check to see if
>         head == tail after unlock; if not, then there's someone else
>         trying to lock, and we can do a kick.  Unfortunately this
>         generates very high level of redundant kicks, because the
>         waiting CPU might not have blocked yet (which is the common
>         case)
> 

How high is "very high" here -- most of the time (so that any mitigation
on the slow patch is useless)?

	-hpa
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