Re: [PATCH] vhost: configure all host notifiers in a single MR transaction

[longpeng2--- via <qemu-devel@nongnu.org>]

在 2022/11/21 12:01, Jason Wang 写道:
> On Fri, Nov 18, 2022 at 10:49 PM Longpeng(Mike) <longpeng2@huawei.com> wrote:
>>
>> From: Longpeng <longpeng2@huawei.com>
>>
>> This allows the vhost device to batch the setup of all its host notifiers.
>> This significantly reduces the device starting time, e.g. the vhost-vDPA
>> generic device [1] start time reduce from 376ms to 9.1ms for a VM with
>> 64 vCPUs and 3 vDPA device(64vq per device).
> 
> Great, I think we need to do this for host_notifiers_mr as well. This
> helps for the case when the notification area could be mapped directly
> to guests.
> 
Batch and commit once for host_notifiers_mrs can reduce the cost from 
423ms to 32ms, testing on vdpasim_blk (3 devices and 64 vqs per device) 
with doorbell passthrough support.
I'll append a patch in the next version.

>>
>> [1] https://www.mail-archive.com/qemu-devel@nongnu.org/msg921541.html
>>
>> Signed-off-by: Longpeng <longpeng2@huawei.com>
>> ---
>>   hw/virtio/vhost.c | 39 ++++++++++++++++++++++++++++++++++++++-
>>   1 file changed, 38 insertions(+), 1 deletion(-)
>>
>> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
>> index d1c4c20b8c..bf82d9b176 100644
>> --- a/hw/virtio/vhost.c
>> +++ b/hw/virtio/vhost.c
>> @@ -1507,6 +1507,7 @@ void vhost_dev_cleanup(struct vhost_dev *hdev)
>>   int vhost_dev_enable_notifiers(struct vhost_dev *hdev, VirtIODevice *vdev)
>>   {
>>       BusState *qbus = BUS(qdev_get_parent_bus(DEVICE(vdev)));
>> +    int vq_init_count = 0;
>>       int i, r, e;
>>
>>       /* We will pass the notifiers to the kernel, make sure that QEMU
>> @@ -1518,6 +1519,12 @@ int vhost_dev_enable_notifiers(struct vhost_dev *hdev, VirtIODevice *vdev)
>>           goto fail;
>>       }
>>
>> +    /*
>> +     * Batch all the host notifiers in a single transaction to avoid
>> +     * quadratic time complexity in address_space_update_ioeventfds().
>> +     */
>> +    memory_region_transaction_begin();
>> +
>>       for (i = 0; i < hdev->nvqs; ++i) {
>>           r = virtio_bus_set_host_notifier(VIRTIO_BUS(qbus), hdev->vq_index + i,
>>                                            true);
>> @@ -1525,19 +1532,33 @@ int vhost_dev_enable_notifiers(struct vhost_dev *hdev, VirtIODevice *vdev)
>>               error_report("vhost VQ %d notifier binding failed: %d", i, -r);
>>               goto fail_vq;
>>           }
>> +
>> +        vq_init_count++;
>>       }
> 
> Nit, the name needs some tweak, it's actually the number of the host
> notifiers that is initialized. And we can count it out of the loop.
> 
Ok, I will refer to virtio_device_start_ioeventfd_impl().

>>
>> +    memory_region_transaction_commit();
>> +
>>       return 0;
>>   fail_vq:
>> -    while (--i >= 0) {
>> +    for (i = 0; i < vq_init_count; i++) {
> 
> It looks to me there's no need for this change.
> 
> Others look good.
> 
> Thanks
> 
>>           e = virtio_bus_set_host_notifier(VIRTIO_BUS(qbus), hdev->vq_index + i,
>>                                            false);
>>           if (e < 0) {
>>               error_report("vhost VQ %d notifier cleanup error: %d", i, -r);
>>           }
>>           assert (e >= 0);
>> +    }
>> +
>> +    /*
>> +     * The transaction expects the ioeventfds to be open when it
>> +     * commits. Do it now, before the cleanup loop.
>> +     */
>> +    memory_region_transaction_commit();
>> +
>> +    for (i = 0; i < vq_init_count; i++) {
>>           virtio_bus_cleanup_host_notifier(VIRTIO_BUS(qbus), hdev->vq_index + i);
>>       }
>> +
>>       virtio_device_release_ioeventfd(vdev);
>>   fail:
>>       return r;
>> @@ -1553,6 +1574,12 @@ void vhost_dev_disable_notifiers(struct vhost_dev *hdev, VirtIODevice *vdev)
>>       BusState *qbus = BUS(qdev_get_parent_bus(DEVICE(vdev)));
>>       int i, r;
>>
>> +    /*
>> +     * Batch all the host notifiers in a single transaction to avoid
>> +     * quadratic time complexity in address_space_update_ioeventfds().
>> +     */
>> +    memory_region_transaction_begin();
>> +
>>       for (i = 0; i < hdev->nvqs; ++i) {
>>           r = virtio_bus_set_host_notifier(VIRTIO_BUS(qbus), hdev->vq_index + i,
>>                                            false);
>> @@ -1560,8 +1587,18 @@ void vhost_dev_disable_notifiers(struct vhost_dev *hdev, VirtIODevice *vdev)
>>               error_report("vhost VQ %d notifier cleanup failed: %d", i, -r);
>>           }
>>           assert (r >= 0);
>> +    }
>> +
>> +    /*
>> +     * The transaction expects the ioeventfds to be open when it
>> +     * commits. Do it now, before the cleanup loop.
>> +     */
>> +    memory_region_transaction_commit();
>> +
>> +    for (i = 0; i < hdev->nvqs; ++i) {
>>           virtio_bus_cleanup_host_notifier(VIRTIO_BUS(qbus), hdev->vq_index + i);
>>       }
>> +
>>       virtio_device_release_ioeventfd(vdev);
>>   }
>>
>> --
>> 2.23.0
>>
> 
> .