Messages in this thread | | | Date | Wed, 17 Jul 2013 20:25:19 +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 08:14 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.
>>> 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.
Yes. I just verified that with benchmark runs, and with perf running, there was not even a single nmi hitting the lock_spinning.
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