KVM: eventfd: Use synchronize_srcu_expedited() on shutdown

When hot-unplug a device which has many queues, and guest CPU will has
huge jitter, and unplugging is very slow.

It turns out synchronize_srcu() in irqfd_shutdown() caused the guest
jitter and unplugging latency, so replace synchronize_srcu() with
synchronize_srcu_expedited(), to accelerate the unplugging, and reduce
the guest OS jitter, this accelerates the VM reboot too.

Signed-off-by: Li RongQing <lirongqing@baidu.com>
Message-ID: <20240711121130.38917-1-lirongqing@baidu.com>
[Call it just once in irqfd_resampler_shutdown. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
index 229570059a1bb5fc44be96499b17090d13150d4e..992f9beb3e7d0403d7f3a4d780025a5660fdbaa5 100644 (file)
--- a/virt/kvm/eventfd.c
+++ b/virt/kvm/eventfd.c
@@ -97,18 +97,19 @@ irqfd_resampler_shutdown(struct kvm_kernel_irqfd *irqfd)
        mutex_lock(&kvm->irqfds.resampler_lock);
 
        list_del_rcu(&irqfd->resampler_link);
-       synchronize_srcu(&kvm->irq_srcu);
 
        if (list_empty(&resampler->list)) {
                list_del_rcu(&resampler->link);
                kvm_unregister_irq_ack_notifier(kvm, &resampler->notifier);
                /*
-                * synchronize_srcu(&kvm->irq_srcu) already called
+                * synchronize_srcu_expedited(&kvm->irq_srcu) already called
                 * in kvm_unregister_irq_ack_notifier().
                 */
                kvm_set_irq(kvm, KVM_IRQFD_RESAMPLE_IRQ_SOURCE_ID,
                            resampler->notifier.gsi, 0, false);
                kfree(resampler);
+       } else {
+               synchronize_srcu_expedited(&kvm->irq_srcu);
        }
 
        mutex_unlock(&kvm->irqfds.resampler_lock);
@@ -126,7 +127,7 @@ irqfd_shutdown(struct work_struct *work)
        u64 cnt;
 
        /* Make sure irqfd has been initialized in assign path. */
-       synchronize_srcu(&kvm->irq_srcu);
+       synchronize_srcu_expedited(&kvm->irq_srcu);
 
        /*
         * Synchronize with the wait-queue and unhook ourselves to prevent
@@ -384,7 +385,7 @@ kvm_irqfd_assign(struct kvm *kvm, struct kvm_irqfd *args)
                }
 
                list_add_rcu(&irqfd->resampler_link, &irqfd->resampler->list);
-               synchronize_srcu(&kvm->irq_srcu);
+               synchronize_srcu_expedited(&kvm->irq_srcu);
 
                mutex_unlock(&kvm->irqfds.resampler_lock);
        }
@@ -523,7 +524,7 @@ void kvm_unregister_irq_ack_notifier(struct kvm *kvm,
        mutex_lock(&kvm->irq_lock);
        hlist_del_init_rcu(&kian->link);
        mutex_unlock(&kvm->irq_lock);
-       synchronize_srcu(&kvm->irq_srcu);
+       synchronize_srcu_expedited(&kvm->irq_srcu);
        kvm_arch_post_irq_ack_notifier_list_update(kvm);
 }
 
@@ -608,7 +609,7 @@ kvm_irqfd_release(struct kvm *kvm)
 
 /*
  * Take note of a change in irq routing.
- * Caller must invoke synchronize_srcu(&kvm->irq_srcu) afterwards.
+ * Caller must invoke synchronize_srcu_expedited(&kvm->irq_srcu) afterwards.
  */
 void kvm_irq_routing_update(struct kvm *kvm)
 {