linux-kernel - Re: [PATCH RFC V10 15/18] kvm : Paravirtual ticketlocks support for linux guests running on KVM hypervisor

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Date:	Wed, 17 Jul 2013 20:50:13 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	mingo@...hat.com, jeremy@...p.org, x86@...nel.org,
	konrad.wilk@...cle.com, hpa@...or.com, pbonzini@...hat.com,
	linux-doc@...r.kernel.org, habanero@...ux.vnet.ibm.com,
	xen-devel@...ts.xensource.com, peterz@...radead.org,
	mtosatti@...hat.com, stefano.stabellini@...citrix.com,
	andi@...stfloor.org, ouyang@...pitt.edu, agraf@...e.de,
	chegu_vinod@...com, torvalds@...ux-foundation.org,
	avi.kivity@...il.com, tglx@...utronix.de, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, riel@...hat.com, drjones@...hat.com,
	virtualization@...ts.linux-foundation.org,
	srivatsa.vaddagiri@...il.com
Subject: Re: [PATCH RFC V10 15/18] kvm : Paravirtual ticketlocks support for
 linux guests running on KVM hypervisor

On 07/17/2013 08:25 PM, Raghavendra K T wrote:
> On 07/17/2013 08:14 PM, Gleb Natapov wrote:
>> On Wed, Jul 17, 2013 at 07:43:01PM +0530, Raghavendra K T wrote:
>>> On 07/17/2013 06:55 PM, Gleb Natapov wrote:
>>>> On Wed, Jul 17, 2013 at 06:25:05PM +0530, Raghavendra K T wrote:
>>>>> On 07/17/2013 06:15 PM, Gleb Natapov wrote:
>>>>>> On Wed, Jul 17, 2013 at 03:35:37PM +0530, Raghavendra K T wrote:
>>>>>>>>> Instead of halt we started with a sleep hypercall in those
>>>>>>>>>   versions. Changed to halt() once Avi suggested to reuse
>>>>>>>>> existing sleep.
>>>>>>>>>
>>>>>>>>> If we use older hypercall with few changes like below:
>>>>>>>>>
>>>>>>>>> kvm_pv_wait_for_kick_op(flags, vcpu, w->lock )
>>>>>>>>> {
>>>>>>>>>   // a0 reserved for flags
>>>>>>>>> if (!w->lock)
>>>>>>>>> return;
>>>>>>>>> DEFINE_WAIT
>>>>>>>>> ...
>>>>>>>>> end_wait
>>>>>>>>> }
>>>>>>>>>
>>>>>>>> How would this help if NMI takes lock in critical section. The
>>>>>>>> thing
>>>>>>>> that may happen is that lock_waiting->want may have NMI lock
>>>>>>>> value, but
>>>>>>>> lock_waiting->lock will point to non NMI lock. Setting of want
>>>>>>>> and lock
>>>>>>>> have to be atomic.
>>>>>>>
>>>>>>> True. so we are here
>>>>>>>
>>>>>>>          non NMI lock(a)
>>>>>>>          w->lock = NULL;
>>>>>>>          smp_wmb();
>>>>>>>          w->want = want;
>>>>>>>                                 NMI
>>>>>>>                           <---------------------
>>>>>>>                            NMI lock(b)
>>>>>>>                            w->lock = NULL;
>>>>>>>                            smp_wmb();
>>>>>>>                            w->want = want;
>>>>>>>                            smp_wmb();
>>>>>>>                            w->lock = lock;
>>>>>>>                           ---------------------->
>>>>>>>          smp_wmb();
>>>>>>>          w->lock = lock;
>>>>>>>
>>>>>>> so how about fixing like this?
>>>>>>>
>>>>>>> again:
>>>>>>>          w->lock = NULL;
>>>>>>>          smp_wmb();
>>>>>>>          w->want = want;
>>>>>>>          smp_wmb();
>>>>>>>          w->lock = lock;
>>>>>>>
>>>>>>> if (!lock || w->want != want) goto again;
>>>>>>>
>>>>>> NMI can happen after the if() but before halt and the same situation
>>>>>> we are trying to prevent with IRQs will occur.
>>>>>
>>>>> True, we can not fix that. I thought to fix the inconsistency of
>>>>> lock,want pair.
>>>>> But NMI could happen after the first OR condition also.
>>>>> /me thinks again
>>>>>
>>>> lock_spinning() can check that it is called in nmi context and bail
>>>> out.
>>>
>>> Good point.
>>> I think we can check for even irq context and bailout so that in irq
>>> context we continue spinning instead of slowpath. no ?
>>>
>> That will happen much more often and irq context is no a problem anyway.
>>
>
> Yes. It is not a problem. But my idea was to not to enter slowpath lock
> during irq processing. Do you think that is a good idea?
>
> I 'll now experiment how often we enter slowpath in irq context.
>

With dbench 1.5x run, on my 32cpu / 16core sandybridge, I saw
around 10  spinlock slowpath  entered from the irq context.

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