Messages in this thread | | | Date | Wed, 17 Jul 2013 20:52:07 +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 08:41 PM, Gleb Natapov wrote: > 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. >
Yes. agreed. indeed I saw degradation if we allow the slowpath spinlock to loop again.
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