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

On 2014-05-29 21:34, Ming Lei wrote:

On Fri, May 30, 2014 at 11:19 AM, Jens Axboe [off-list ref] wrote:

quoted

On 2014-05-29 20:49, Ming Lei wrote:

quoted

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.

It isn't easy to observe the improvement on x86 VM, especially
with few vCPUs, because qemu-system-x86_64 only takes
several microseconds to handle the notification, but on arm64, it
may take hundreds of microseconds, so the improvement is
obvious on arm VM.

I hope this patch can be merged, at least arm VM can benefit
from it.

If Rusty agrees, I'd like to add it for 3.16 with a stable marker.

-- 
Jens Axboe