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Date: Fri, 12 Nov 2010 14:20:37 -0800
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Jeremy Fitzhardinge <jeremy@goop.org>
CC: Peter Zijlstra <peterz@infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Nick Piggin <npiggin@suse.de>, Jan Beulich <JBeulich@novell.com>,
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        Linux Virtualization <virtualization@lists.linux-foundation.org>,
        Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Subject: Re: [PATCH 00/20] x86: ticket lock rewrite and paravirtualization
References: <cover.1288794124.git.jeremy.fitzhardinge@citrix.com> <4CDDBBD3.5050903@zytor.com> <4CDDBCE4.80906@goop.org>
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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