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

Peter Zijlstra
2015-07-03T15:17:18Z

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);
}