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

[Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com>]

On 07/17/2013 07:43 PM, 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 ?
>
>> How often this will happens anyway.
>>
>
> I know NMIs occur frequently with watchdogs. or used by sysrq-trigger
> etc.. But I am not an expert how frequent it is otherwise.

Forgot to ask if Peter has any points on NMI frequency.

But even
> then if they do not use spinlock, we have no problem as already pointed.
>
> I can measure with debugfs counter how often it happens.