Messages in this thread | | | Date | Fri, 3 Jul 2015 15:17:13 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] sched,kvm: Fix KVM preempt_notifier usage |
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On Fri, Jul 03, 2015 at 02:31:25PM +0200, Paolo Bonzini wrote: > On 03/07/2015 14:19, Peter Zijlstra wrote: > > On Fri, Jul 03, 2015 at 01:12:11PM +0200, Paolo Bonzini wrote: > >> In fact you shouldn't have just tested the patch on a case _without_ > >> preemption notifiers, you should have also benchmarked the impact that > >> static keys have _with_ preemption notifiers. In a > >> not-really-artificial case (one single-processor guest running on the > >> host), the static key patch adds a static_key_slow_inc on a relatively > >> hot path for KVM, which is not acceptable. > > > > Spawning the first vcpu is a hot path? > > This is not *spawning* the first VCPU. Basically any critical section > for vcpu->mutex includes a preempt_notifier_register/unregister pair: > > /* > * Switches to specified vcpu, until a matching vcpu_put() > */ > int vcpu_load(struct kvm_vcpu *vcpu) > { > int cpu; > > if (mutex_lock_killable(&vcpu->mutex)) > return -EINTR; > cpu = get_cpu(); > preempt_notifier_register(&vcpu->preempt_notifier); > kvm_arch_vcpu_load(vcpu, cpu); > put_cpu(); > return 0; > } > > void vcpu_put(struct kvm_vcpu *vcpu) > { > preempt_disable(); > kvm_arch_vcpu_put(vcpu); > preempt_notifier_unregister(&vcpu->preempt_notifier); > preempt_enable(); > mutex_unlock(&vcpu->mutex); > } > > So basically you're adding at least one static_key_slow_inc/dec pair to > every userspace exit.
Ugh, ok that is not what I was expecting to happen. I'll ask Ingo to queue a revert until we can fix this better.
I thought these were vcpu create/destroy functions.
That said, the slow_inc/dec are really only slow on the 0<->!0 transitions.
But, could we rework the code so that you register the preempt notifier when creating the vcpu thread and leave it installed forevermore?
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