linux-kernel - Re: Preemptable Ticket Spinlock

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Message-ID: <51A73E5C.4010608@linux.vnet.ibm.com>
Date:	Thu, 30 May 2013 17:26:12 +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>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: Preemptable Ticket Spinlock

On 04/23/2013 07:12 AM, Raghavendra K T wrote:
> 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.

Sorry for late reply.

Here is the branch with pvpspinlock V9 version in github reabsed to  3.10-rc

https://github.com/ktraghavendra/linux/tree/pvspinlock_v9

planning post a formal email in a separate thread with link a to this
branch (instead of spamming with 19 patches)

Main changes w.r.t v8 are
- Changed spin_threshold to 32k to avoid excess halt exits that are 
causing undercommit degradation (after PLE handler improvement).
- Added  kvm_irq_delivery_to_apic (suggested by Gleb)
- optimized halt exit path to use PLE handler

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