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 08:25:19PM +0530, 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?
> 
Why would we disable it if its purpose is to improve handling of
contended locks? NMI is only special because it is impossible to handle
and should not happen anyway.

--
                        Gleb.

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