linux-kernel - Re: Preemptable Ticket Spinlock

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Message-ID: <51759E43.2080003@redhat.com>
Date:	Mon, 22 Apr 2013 16:32:03 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Jiannan Ouyang <ouyang@...pitt.edu>,
	LKML <linux-kernel@...r.kernel.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	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 04:08 PM, Peter Zijlstra wrote:
> On Mon, 2013-04-22 at 15:56 -0400, Rik van Riel wrote:
>> On 04/22/2013 03:49 PM, Peter Zijlstra wrote:
>>> On Mon, 2013-04-22 at 08:52 -0400, Rik van Riel wrote:
>>
>>>> If the native spin_lock code has been called already at
>>>> that time, the native code would still need to be modified
>>>> to increment the ticket number by 2, so we end up with a
>>>> compatible value in each spin lock's .tickets field, and
>>>> prevent a deadlock after we switch over to the paravirt
>>>> variant.
>>>
>>> I thought the stuff already made it upstream, but apparently not; the
>>> lastest posting I'm aware of is here:
>>>
>>>     https://lkml.org/lkml/2012/5/2/105
>>>
>>> That stuff changes the normal ticket increment as well..
>>
>> Jiannan,
>>
>> It looks like the patch above could make a good patch
>> 1 (or 2) in your patch series :)
>
> I much prefer the entire series from Jeremy since it maintains the
> ticket semantics and doesn't degrade the lock to unfair under
> contention.
>
> Now I suppose there's a reason its not been merged yet and I suspect
> its !paravirt hotpath impact which wasn't rightly justified or somesuch
> so maybe someone can work on that or so.. dunno.

IIRC one of the reasons was that the performance improvement wasn't
as obvious.  Rescheduling VCPUs takes a fair amount of time, quite
probably more than the typical hold time of a spinlock.

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