Messages in this thread | | | Date | Wed, 17 Jul 2013 20:50:13 +0530 | From | Raghavendra K T <> | Subject | Re: [PATCH RFC V10 15/18] kvm : Paravirtual ticketlocks support for linux guests running on KVM hypervisor |
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On 07/17/2013 08:25 PM, 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? > > I 'll now experiment how often we enter slowpath in irq context. >
With dbench 1.5x run, on my 32cpu / 16core sandybridge, I saw around 10 spinlock slowpath entered from the irq context.
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