[PATCH] block: virtio_blk: don't hold spin lock during world switch

Jens Axboe
Fri, 30 May 2014 03:19:14 -0000

On 2014-05-29 20:49, Ming Lei wrote:
> Firstly, it isn't necessary to hold lock of vblk->vq_lock
> when notifying hypervisor about queued I/O.
>
> Secondly, virtqueue_notify() will cause world switch and
> it may take long time on some hypervisors(such as, qemu-arm),
> so it isn't good to hold the lock and block other vCPUs.
>
> On arm64 quad core VM(qemu-kvm), the patch can increase I/O
> performance a lot with VIRTIO_RING_F_EVENT_IDX enabled:
> 	- without the patch: 14K IOPS
> 	- with the patch: 34K IOPS

Patch looks good to me. I don't see a hit on my qemu-kvm testing, but it
definitely makes sense and I can see it hurting in other places.

--
Jens Axboe