linux-kernel - Re: [PATCH] sched,kvm: Fix KVM preempt

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Date:	Fri, 3 Jul 2015 17:26:15 +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 17:16, Peter Zijlstra wrote:
> On Fri, Jul 03, 2015 at 03:17:13PM +0200, Peter Zijlstra wrote:
>> But, could we rework the code so that you register the preempt notifier
>> when creating the vcpu thread and leave it installed forevermore?
> 
> OK, it looks like there is no fixed relation between a thread and a vcpu
> :/
> 
> Would something like the below (on top of all the others) work for you?

This looks fine, but one would do the same thing without the previous
patch, i.e. directly on top of Linus's master---right?  The patch in tip
is a red herring, and the hunks below are reverting large parts of it.

I'm going to send a pull request to Linus anyway, either tonight or
tomorrow morning.  Send it with SoB, so that it applies to Linus's
master, and I can include it.  This way bisection is also better preserved.

Paolo

> ---
>  include/linux/preempt.h |  2 ++
>  kernel/sched/core.c     | 18 +++++++++++++++---
>  virt/kvm/kvm_main.c     |  7 +++++--
>  3 files changed, 22 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/preempt.h b/include/linux/preempt.h
> index 0f1534acaf60..84991f185173 100644
> --- a/include/linux/preempt.h
> +++ b/include/linux/preempt.h
> @@ -293,6 +293,8 @@ struct preempt_notifier {
>  	struct preempt_ops *ops;
>  };
>  
> +void preempt_notifier_inc(void);
> +void preempt_notifier_dec(void);
>  void preempt_notifier_register(struct preempt_notifier *notifier);
>  void preempt_notifier_unregister(struct preempt_notifier *notifier);
>  
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 6169c167ac98..5ddbcb720fc6 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2320,13 +2320,27 @@ void wake_up_new_task(struct task_struct *p)
>  
>  static struct static_key preempt_notifier_key = STATIC_KEY_INIT_FALSE;
>  
> +void preempt_notifier_inc(void)
> +{
> +	static_key_slow_inc(&preempt_notifier_key);
> +}
> +EXPORT_SYMBOL_GPL(preempt_notifier_inc);
> +
> +void preempt_notifier_dec(void)
> +{
> +	static_key_slow_dec(&preempt_notifier_key);
> +}
> +EXPORT_SYMBOL_GPL(preempt_notifier_dec);
> +
>  /**
>   * preempt_notifier_register - tell me when current is being preempted & rescheduled
>   * @notifier: notifier struct to register
>   */
>  void preempt_notifier_register(struct preempt_notifier *notifier)
>  {
> -	static_key_slow_inc(&preempt_notifier_key);
> +	if (!static_key_false(&preempt_notifier_key))
> +		WARN(1, "registering preempt_notifier while notifiers disabled\n");
> +
>  	/*
>  	 * Avoid preemption while changing the preempt notifier list.
>  	 */
> @@ -2350,8 +2364,6 @@ void preempt_notifier_unregister(struct preempt_notifier *notifier)
>  	preempt_disable();
>  	hlist_del(&notifier->link);
>  	preempt_enable();
> -
> -	static_key_slow_dec(&preempt_notifier_key);
>  }
>  EXPORT_SYMBOL_GPL(preempt_notifier_unregister);
>  
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index d7aafa0458a0..8136be28d76c 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -128,9 +128,9 @@ int vcpu_load(struct kvm_vcpu *vcpu)
>  
>  	if (mutex_lock_killable(&vcpu->mutex))
>  		return -EINTR;
> -	preempt_notifier_register(&vcpu->preempt_notifier);
>  
>  	cpu = get_cpu();
> +	preempt_notifier_register(&vcpu->preempt_notifier);
>  	kvm_arch_vcpu_load(vcpu, cpu);
>  	put_cpu();
>  	return 0;
> @@ -140,8 +140,8 @@ void vcpu_put(struct kvm_vcpu *vcpu)
>  {
>  	preempt_disable();
>  	kvm_arch_vcpu_put(vcpu);
> -	preempt_enable();
>  	preempt_notifier_unregister(&vcpu->preempt_notifier);
> +	preempt_enable();
>  	mutex_unlock(&vcpu->mutex);
>  }
>  
> @@ -513,6 +513,8 @@ static struct kvm *kvm_create_vm(unsigned long type)
>  	list_add(&kvm->vm_list, &vm_list);
>  	spin_unlock(&kvm_lock);
>  
> +	preempt_notifier_inc();
> +
>  	return kvm;
>  
>  out_err:
> @@ -612,6 +614,7 @@ static void kvm_destroy_vm(struct kvm *kvm)
>  	cleanup_srcu_struct(&kvm->irq_srcu);
>  	cleanup_srcu_struct(&kvm->srcu);
>  	kvm_arch_free_vm(kvm);
> +	preempt_notifier_dec();
>  	hardware_disable_all();
>  	mmdrop(mm);
>  }
> 
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