linux-kernel - Re: [PATCH v4] kvm: make vcpu life cycle separated from kvm instance

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Date:	Thu, 15 Dec 2011 11:10:37 +0200
From:	Gleb Natapov <gleb@...hat.com>
To:	Liu Ping Fan <kernelfans@...il.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, avi@...hat.com,
	aliguori@...ibm.com, mtosatti@...hat.com, jan.kiszka@....de
Subject: Re: [PATCH v4] kvm: make vcpu life cycle separated from kvm instance

On Thu, Dec 15, 2011 at 12:28:48PM +0800, Liu Ping Fan wrote:
> From: Liu Ping Fan <pingfank@...ux.vnet.ibm.com>
> 
> Currently, vcpu can be destructed only when kvm instance destroyed.
> Change this to vcpu's destruction before kvm instance, so vcpu MUST
> and CAN be destroyed before kvm's destroy.
> 
I see reference counting is back.

> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index d9cfb78..71dda47 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -141,6 +141,7 @@ void vcpu_load(struct kvm_vcpu *vcpu)
>  {
>  	int cpu;
>  
> +	kvm_vcpu_get(vcpu);
>  	mutex_lock(&vcpu->mutex);
>  	if (unlikely(vcpu->pid != current->pids[PIDTYPE_PID].pid)) {
>  		/* The thread running this VCPU changed. */
> @@ -163,6 +164,7 @@ void vcpu_put(struct kvm_vcpu *vcpu)
>  	preempt_notifier_unregister(&vcpu->preempt_notifier);
>  	preempt_enable();
>  	mutex_unlock(&vcpu->mutex);
> +	kvm_vcpu_put(vcpu);
>  }
>  
Why is kvm_vcpu_get/kvm_vcpu_put is needed in vcpu_load/vcpu_put? 
As far as I see load/put are only called in vcpu ioctl,
kvm_arch_vcpu_setup(), kvm_arch_vcpu_destroy() and kvm_arch_destroy_vm().

kvm_arch_vcpu_setup() and kvm_arch_vcpu_destroy() are called before vcpu is
added to vcpus list, so it can't be accessed by other thread at this
point. kvm_arch_destroy_vm() is  called on KVM destruction path when all
vcpus should be destroyed already. So the only interesting place is vcpu
ioctl and I think we are protected by fd refcount there. vcpu fd can't
be closed while ioctl is executing for that vcpu. Otherwise we would
have problem now too.

> @@ -1539,12 +1547,10 @@ EXPORT_SYMBOL_GPL(kvm_resched);
>  void kvm_vcpu_on_spin(struct kvm_vcpu *me)
>  {
>  	struct kvm *kvm = me->kvm;
> -	struct kvm_vcpu *vcpu;
> -	int last_boosted_vcpu = me->kvm->last_boosted_vcpu;
> -	int yielded = 0;
> -	int pass;
> -	int i;
> -
> +	struct kvm_vcpu *vcpu, *v;
> +	struct task_struct *task = NULL;
> +	struct pid *pid;
> +	int pass, firststart, lastone, yielded;
>  	/*
>  	 * We boost the priority of a VCPU that is runnable but not
>  	 * currently running, because it got preempted by something
> @@ -1552,15 +1558,22 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
>  	 * VCPU is holding the lock that we need and will release it.
>  	 * We approximate round-robin by starting at the last boosted VCPU.
>  	 */
> -	for (pass = 0; pass < 2 && !yielded; pass++) {
> -		kvm_for_each_vcpu(i, vcpu, kvm) {
> -			struct task_struct *task = NULL;
> -			struct pid *pid;
> -			if (!pass && i < last_boosted_vcpu) {
> -				i = last_boosted_vcpu;
> +	for (pass = 0, firststart = 0; pass < 2 && !yielded; pass++) {
> +
> +		rcu_read_lock();
> +		kvm_for_each_vcpu(vcpu, kvm) {
> +			if (!pass && !firststart &&
> +			    vcpu != kvm->last_boosted_vcpu &&
> +			    kvm->last_boosted_vcpu != NULL) {
> +				vcpu = kvm->last_boosted_vcpu;
> +				firststart = 1;
>  				continue;
> -			} else if (pass && i > last_boosted_vcpu)
> +			} else if (pass && !lastone) {
> +				if (vcpu == kvm->last_boosted_vcpu)
> +					lastone = 1;
> +			} else if (pass && lastone)
>  				break;
> +
>  			if (vcpu == me)
>  				continue;
>  			if (waitqueue_active(&vcpu->wq))
> @@ -1576,15 +1589,29 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me)
>  				put_task_struct(task);
>  				continue;
>  			}
> +			v = kvm_vcpu_get(vcpu);
> +			if (v == NULL)
> +				continue;
> +
> +			rcu_read_unlock();
>  			if (yield_to(task, 1)) {
>  				put_task_struct(task);
> -				kvm->last_boosted_vcpu = i;
> +				mutex_lock(&kvm->lock);
> +				/*Remeber to release it.*/
> +				if (kvm->last_boosted_vcpu != NULL)
> +					kvm_vcpu_put(kvm->last_boosted_vcpu);
> +				kvm->last_boosted_vcpu = vcpu;
> +				mutex_unlock(&kvm->lock);
>  				yielded = 1;
I think we can be smart and protect kvm->last_boosted_vcpu with the same
rcu as vcpus, but yeild_to() can sleep anyway. Hmm may be we should use
srcu in the first place :( Or rewrite the logic of the functions
somehow to call yield_to() outside of the loop. This is heuristics anyway.

--
			Gleb.
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