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

[Peter Zijlstra <peterz@infradead.org>]

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?

---
 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);
 }