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From: Paolo Bonzini <pbonzini@redhat.com>
To: Christoph Hellwig <hch@lst.de>, Stefan Hajnoczi <stefanha@redhat.com>
Cc: virtualization@lists.linux-foundation.org,
	linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
	Jason Wang <jasowang@redhat.com>, Jens Axboe <axboe@kernel.dk>,
	slp@redhat.com, sgarzare@redhat.com,
	"Michael S. Tsirkin" <mst@redhat.com>
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


  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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