linux-kernel - Re: Preemptable Ticket Spinlock

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Message-ID: <5175E71F.8070307@linux.vnet.ibm.com>
Date:	Tue, 23 Apr 2013 07:12:55 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Rik van Riel <riel@...hat.com>,
	Jiannan Ouyang <ouyang@...pitt.edu>,
	LKML <linux-kernel@...r.kernel.org>,
	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 04/23/2013 01:19 AM, Peter Zijlstra wrote:
> On Mon, 2013-04-22 at 08:52 -0400, Rik van Riel wrote:
>> On 04/22/2013 07:51 AM, Peter Zijlstra wrote:
>>> On Sun, 2013-04-21 at 17:12 -0400, Rik van Riel wrote:
>>>>
>>>> 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.
>>>
>>> ISTR that paravirt ticket locks already do that and use the lsb to
>>> indicate the unlock needs to perform wakeups.
>>>
>>> Also, since all of this is virt nonsense, shouldn't it live in the
>>> paravirt ticket lock code and leave the native code as is?
>>
>> Sure, but that is still no reason not to have the virt
>> implementation be as fast as possible, and share the same
>> data type as the non-virt implementation.
>
> It has to share the same data-type..
>
>> Also, is it guaranteed that the native spin_lock code has
>> not been called yet before we switch over to the paravirt
>> functions?
>>
>> 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..
>

pv-ticket spinlock went on hold state, after Avi acked because of:

though on non-PLE, we get a huge advantage, on PLE machine the benefit 
was not as impressive (~10% as you stated in email chain) compared to 
the complexity of the patches.
So Avi suggested to try PLE improvements first, so they are going upstream.

https://lkml.org/lkml/2012/7/18/247
https://lkml.org/lkml/2013/1/22/104
https://lkml.org/lkml/2013/2/6/345 (on the way in kvm tree)

Current status of PV spinlock:
I have the rebased patches of pv spinlocks and experimenting with latest 
kernel.I have
Gleb's irq delivery incorporated into the patch series. But I am 
thinknig whether I can
improve some guest side logic in unlock.
I will probably setup a githup and post the link soon.

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