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

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Date:	Thu, 29 May 2014 21:35:50 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Ming Lei <ming.lei@...onical.com>
CC:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	virtualization@...ts.linux-foundation.org
Subject: 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 <axboe@...nel.dk> 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

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