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Date: Fri, 3 Jul 2015 14:31:25 +0200 From: Paolo Bonzini <pbonzini@...hat.com> To: Peter Zijlstra <peterz@...radead.org> Cc: Pontus Fuchs <pontus.fuchs@...il.com>, Linus Torvalds <torvalds@...ux-foundation.org>, mingo@...hat.com, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, gleb@...nel.org Subject: Re: [PATCH] sched,kvm: Fix KVM preempt_notifier usage 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. Paolo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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