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

[Gleb Natapov <gleb@xxxxxxxxxx>]

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.

> >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. 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.
> 
When you run perf you will see a lot of NMIs, but those should not take
any locks.

--
                        Gleb.

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