Re: [PATCH] virtio-blk: support polling I/O

On Sat, Mar 12, 2022 at 12:28:32AM +0900, Suwan Kim wrote:

This patch supports polling I/O via virtio-blk driver. Polling
feature is enabled based on "VIRTIO_BLK_F_MQ" feature and the number
of polling queues can be set by QEMU virtio-blk-pci property
"num-poll-queues=N". This patch improves the polling I/O throughput
and latency.

The virtio-blk driver doesn't not have a poll function and a poll
queue and it has been operating in interrupt driven method even if
the polling function is called in the upper layer.

virtio-blk polling is implemented upon 'batched completion' of block
layer. virtblk_poll() queues completed request to io_comp_batch->req_list
and later, virtblk_complete_batch() calls unmap function and ends
the requests in batch.

virtio-blk reads the number of queues and poll queues from QEMU
virtio-blk-pci properties ("num-queues=N", "num-poll-queues=M").
It allocates N virtqueues to virtio_blk->vqs[N] and it uses [0..(N-M-1)]
as default queues and [(N-M)..(N-1)] as poll queues. Unlike the default
queues, the poll queues have no callback function.

Regarding HW-SW queue mapping, the default queue mapping uses the
existing method that condsiders MSI irq vector. But the poll queue
doesn't have an irq, so it uses the regular blk-mq cpu mapping.

To enable poll queues, "num-poll-queues=N" property of virtio-blk-pci
needs to be added to QEMU command line. For that, I temporarily
implemented the property on QEMU. Please refer to the git repository below.

	git : https://github.com/asfaca/qemu.git #on master branch commit

For verifying the improvement, I did Fio polling I/O performance test
with io_uring engine with the options below.
(io_uring, hipri, randread, direct=1, bs=512, iodepth=64 numjobs=N)
I set 4 vcpu and 4 virtio-blk queues - 2 default queues and 2 poll
queues for VM.
(-device virtio-blk-pci,num-queues=4,num-poll-queues=2)
As a result, IOPS and average latency improved about 10%.

Test result:

- Fio io_uring poll without virtio-blk poll support
	-- numjobs=1 : IOPS = 297K, avg latency = 214.59us
	-- numjobs=2 : IOPS = 360K, avg latency = 363.88us
	-- numjobs=4 : IOPS = 289K, avg latency = 885.42us

- Fio io_uring poll with virtio-blk poll support
	-- numjobs=1 : IOPS = 332K, avg latency = 192.61us
	-- numjobs=2 : IOPS = 371K, avg latency = 348.31us
	-- numjobs=4 : IOPS = 321K, avg latency = 795.93us

Last year there was a patch series that switched regular queues into
polling queues when HIPRI requests were in flight:
https://lore.kernel.org/linux-block/20210520141305.355961-1-stefanha@redhat.com/T/ (local)

The advantage is that polling is possible without prior device
configuration, making it easier for users.

However, the dynamic approach is a bit more complex and bugs can result
in lost irqs (hung I/O). Christoph Hellwig asked for dedicated polling
queues, which your patch series now delivers.

I think your patch series is worth merging once the comments others have
already made have been addressed. I'll keep an eye out for the VIRTIO
spec change to extend the virtio-blk configuration space, which needs to
be accepted before the Linux can be merged.

A comment would be nice here to explain that regular queues have
interrupts and hence CPU affinity is defined by the core virtio code,
but polling queues have no interrupts so we let the block layer assign
CPU affinity.