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 03:35 PM, Raghavendra K T wrote:
> On 07/17/2013 03:04 PM, Gleb Natapov wrote:
>> On Wed, Jul 17, 2013 at 12:12:35AM +0530, Raghavendra K T wrote:
>>>> I do not think it is very rare to get interrupt between
>>>> local_irq_restore() and halt() under load since any interrupt that
>>>> occurs between local_irq_save() and local_irq_restore() will be
>>>> delivered
>>>> immediately after local_irq_restore(). Of course the chance of no
>>>> other
>>>> random interrupt waking lock waiter is very low, but waiter can sleep
>>>> for much longer then needed and this will be noticeable in
>>>> performance.
>>>
>>> Yes, I meant the entire thing. I did infact turned WARN on
>>> w->lock==null before halt() [ though we can potentially have irq right
>>> after that ], but did not hit so far.
>> Depends on your workload of course. To hit that you not only need to get
>> interrupt in there, but the interrupt handler needs to take contended
>> spinlock.
>>
>
> Yes. Agree.
>
>>>
>>>> BTW can NMI handler take spinlocks? If it can what happens if NMI is
>>>> delivered in a section protected by
>>>> local_irq_save()/local_irq_restore()?
>>>>
>>>
>>> Had another idea if NMI, halts are causing problem until I saw
>>> PeterZ's reply similar to V2 of pvspinlock posted  here:
>>>
>>>   https://lkml.org/lkml/2011/10/23/211
>>>
>>> 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;
>
Sorry,
I meant if (!w->lock || w->want !=want) here


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