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SubjectRe: [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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