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

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Message-ID: <4CDDBDB5.8000800@zytor.com>
Date:	Fri, 12 Nov 2010 14:20:37 -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>
Subject: Re: [PATCH 00/20] x86: ticket lock rewrite and paravirtualization

On 11/12/2010 02:17 PM, Jeremy Fitzhardinge wrote:
> On 11/12/2010 02:12 PM, H. Peter Anvin wrote:
>> 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)?
> 
> I'll need to remeasure, but I think around 90% of the slowpath entries
> were spurious without this.  In other words, when spinlocks do contend,
> most of the time it isn't very serious and the other cpu doesn't spend
> much time spinning.
> 

90% of the slowpath entries is one thing, my real question is the
fraction of fastpath entries that get diverted to the slowpath.  It
affects where mitigation needs to happen.

	-hpa

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