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    SubjectRe: [PATCH] sched,kvm: Fix KVM preempt_notifier usage
    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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