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SubjectRe: [PATCH 3/3] virtio_blk: implement blk_mq_ops->poll()
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On 24/05/21 16:59, Christoph Hellwig wrote:
> On Thu, May 20, 2021 at 03:13:05PM +0100, Stefan Hajnoczi wrote:
>> Possible drawbacks of this approach:
>>
>> - Hardware virtio_blk implementations may find virtqueue_disable_cb()
>> expensive since it requires DMA. If such devices become popular then
>> the virtio_blk driver could use a similar approach to NVMe when
>> VIRTIO_F_ACCESS_PLATFORM is detected in the future.
>>
>> - If a blk_poll() thread is descheduled it not only hurts polling
>> performance but also delays completion of non-REQ_HIPRI requests on
>> that virtqueue since vq notifications are disabled.
>
> Yes, I think this is a dangerous configuration. What argument exists
> again just using dedicated poll queues?

There isn't an equivalent of the admin queue in virtio-blk, which would
allow the guest to configure the desired number of poll queues. The
number of queues is fixed.

Could the blk_poll() thread use preempt notifiers to enable/disable
callbacks, for example using two new .poll_start and .end_poll callbacks
to struct blk_mq_ops?

Paolo

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