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

[Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>]

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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