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

[Jens Axboe]

On 2014-05-29 21:34, Ming Lei wrote:
> On Fri, May 30, 2014 at 11:19 AM, Jens Axboe <axboe@xxxxxxxxx> wrote:
> > 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.
>
> 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

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/