From: Paolo Bonzini <pbonzini@redhat.com>
To: Christoph Hellwig <hch@lst.de>, Stefan Hajnoczi <stefanha@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>,
"Michael S. Tsirkin" <mst@redhat.com>,
linux-kernel@vger.kernel.org,
virtualization@lists.linux-foundation.org,
linux-block@vger.kernel.org
Subject: Re: [PATCH 3/3] virtio_blk: implement blk_mq_ops->poll()
Date: Tue, 25 May 2021 09:22:48 +0200 [thread overview]
Message-ID: <7cc7f19b-34b3-1501-898d-3f41e047d766@redhat.com> (raw)
In-Reply-To: <20210524145928.GA3873@lst.de>
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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next prev parent reply other threads:[~2021-05-25 7:22 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-20 14:13 [PATCH 0/3] virtio_blk: blk-mq io_poll support Stefan Hajnoczi
2021-05-20 14:13 ` [PATCH 1/3] virtio: add virtioqueue_more_used() Stefan Hajnoczi
2021-05-25 2:23 ` Jason Wang
2021-05-25 8:48 ` Stefan Hajnoczi
2021-05-20 14:13 ` [PATCH 2/3] virtio_blk: avoid repeating vblk->vqs[qid] Stefan Hajnoczi
2021-05-25 2:25 ` Jason Wang
2021-05-20 14:13 ` [PATCH 3/3] virtio_blk: implement blk_mq_ops->poll() Stefan Hajnoczi
2021-05-24 14:59 ` Christoph Hellwig
2021-05-25 7:22 ` Paolo Bonzini [this message]
2021-05-25 7:38 ` Ming Lei
2021-05-25 8:06 ` Paolo Bonzini
2021-05-25 13:20 ` Stefan Hajnoczi
2021-05-25 13:19 ` Stefan Hajnoczi
2021-05-25 3:21 ` Jason Wang
2021-05-25 8:59 ` Stefan Hajnoczi
2021-05-27 5:48 ` Jason Wang
2021-06-03 15:24 ` Stefan Hajnoczi
2021-05-27 2:44 ` Ming Lei
2021-06-03 15:12 ` Stefan Hajnoczi
2021-06-03 15:30 ` [PATCH 0/3] virtio_blk: blk-mq io_poll support Stefan Hajnoczi
2021-06-16 7:43 ` Christoph Hellwig
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