Hello Ziyang,

On Mon, Oct 24, 2022 at 05:48:51PM +0800, Ziyang Zhang wrote:

On 2022/10/23 17:38, Ming Lei wrote:

quoted

task_work_add() is used for waking ubq daemon task with one batch
of io requests/commands queued. However, task_work_add() isn't
exported for module code, and it is still debatable if the symbol
should be exported.

Fortunately we still have io_uring_cmd_complete_in_task() which just
can't handle batched wakeup for us.

Add one one llist into ublk_queue and call io_uring_cmd_complete_in_task()
via current command for running them via task work.

This way cleans up current code a lot, meantime allow us to wakeup
ubq daemon task after queueing batched requests/io commands.

Hi, Ming

This patch works and I have run some tests to compare current version(ucmd)
with your patch(ucmd-batch).

iodepth=128 numjobs=1 direct=1 bs=4k

--------------------------------------------
ublk loop target, the backend is a file.
IOPS(k)

type		ucmd		ucmd-batch
seq-read	54.7		54.2	
rand-read	52.8		52.0

--------------------------------------------
ublk null target
IOPS(k)

type		ucmd		ucmd-batch
seq-read	257		257
rand-read	252		253


I find that io_req_task_work_add() puts task_work node into a llist
first, then it may call task_work_add() to run batched task_works. So do we really
need such llist in ublk_drv? I think io_uring has already considered task_work batch
optimization.

BTW, task_work_add() in ublk_drv achieves
higher IOPS(about 5-10% on my machine) than io_uring_cmd_complete_in_task()
in ublk_drv.

Yeah, that is same with my observation, and motivation of this patch is
to get same performance with task_work_add by building ublk_drv as
module. One win of task_work_add() is that we get exact batching info
meantime only send TWA_SIGNAL_NO_IPI for whole batch, that is basically
what the patch is doing, but needs help of the following ublksrv patch:

https://github.com/ming1/ubdsrv/commit/dce6d1d222023c1641292713b311ced01e6dc548

which sets IORING_SETUP_COOP_TASKRUN for ublksrv's uring, then
io_uring_cmd_complete_in_task will notify via TWA_SIGNAL_NO_IPI, and 5+%
IOPS boost is observed on loop/001 by putting image on SSD in my test
VM.

Thanks, 
Ming