On Thu, Mar 24, 2022 at 10:32:02AM -0400, Michael S. Tsirkin wrote:

On Thu, Mar 24, 2022 at 11:04:49PM +0900, Suwan Kim wrote:

quoted

This patch supports polling I/O via virtio-blk driver. Polling
feature is enabled by module parameter "num_poll_queues" and it
sets dedicated polling queues for virtio-blk. 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 poll queues from module parameter
"num_poll_queues". If VM sets queue parameter as below,
("num-queues=N" [QEMU property], "num_poll_queues=M" [module parameter])
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.

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.

As a result, IOPS and average latency improved about 10%.

Test result:

- Fio io_uring poll without virtio-blk poll support
	-- numjobs=1 : IOPS = 339K, avg latency = 188.33us
	-- numjobs=2 : IOPS = 367K, avg latency = 347.33us
	-- numjobs=4 : IOPS = 383K, avg latency = 682.06us

- Fio io_uring poll with virtio-blk poll support
	-- numjobs=1 : IOPS = 380K, avg latency = 167.87us
	-- numjobs=2 : IOPS = 409K, avg latency = 312.6us
	-- numjobs=4 : IOPS = 413K, avg latency = 619.72us

Reported-by: kernel test robot <redacted>
Signed-off-by: Suwan Kim <redacted>
---
 drivers/block/virtio_blk.c | 101 +++++++++++++++++++++++++++++++++++--
 1 file changed, 97 insertions(+), 4 deletions(-)

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 8c415be86732..3d16f8b753e7 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c

@@ -37,6 +37,10 @@ MODULE_PARM_DESC(num_request_queues,
 		 "0 for no limit. "
 		 "Values > nr_cpu_ids truncated to nr_cpu_ids.");
 
+static unsigned int num_poll_queues;
+module_param(num_poll_queues, uint, 0644);
+MODULE_PARM_DESC(num_poll_queues, "The number of dedicated virtqueues for polling I/O");
+
 static int major;
 static DEFINE_IDA(vd_index_ida);

Is there some way to make it work reasonably without need to set
module parameters? I don't see any other devices with a num_poll_queues
parameter - how do they handle this?

Hi Michael,

NVMe driver uses module parameter.

Please refer to this.
-----
drivers/nvme/host/pci.c

static unsigned int poll_queues;
module_param_cb(poll_queues, &io_queue_count_ops, &poll_queues, 0644);
MODULE_PARM_DESC(poll_queues, "Number of queues to use for polled IO.");
-----

Regards,
Suwan Kim