linux-kernel - Re: [PATCH 3/3] virtio_blk: implement blk_mq

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Message-ID: <7cc7f19b-34b3-1501-898d-3f41e047d766@redhat.com>
Date:   Tue, 25 May 2021 09:22:48 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Christoph Hellwig <hch@....de>,
        Stefan Hajnoczi <stefanha@...hat.com>
Cc:     virtualization@...ts.linux-foundation.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Jason Wang <jasowang@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        slp@...hat.com, sgarzare@...hat.com,
        "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH 3/3] virtio_blk: implement blk_mq_ops->poll()

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