Re: [PATCH v3 1/2] io_uring: avoid whole io_wq_work copy for requests completed inline

[Jens Axboe <axboe@kernel.dk>]

On 5/28/20 3:15 AM, Xiaoguang Wang wrote:
> If requests can be submitted and completed inline, we don't need to
> initialize whole io_wq_work in io_init_req(), which is an expensive
> operation, add a new 'REQ_F_WORK_INITIALIZED' to control whether
> io_wq_work is initialized.
> 
> I use /dev/nullb0 to evaluate performance improvement in my physical
> machine:
>   modprobe null_blk nr_devices=1 completion_nsec=0
>   sudo taskset -c 60 fio  -name=fiotest -filename=/dev/nullb0 -iodepth=128
>   -thread -rw=read -ioengine=io_uring -direct=1 -bs=4k -size=100G -numjobs=1
>   -time_based -runtime=120
> 
> before this patch:
> Run status group 0 (all jobs):
>    READ: bw=724MiB/s (759MB/s), 724MiB/s-724MiB/s (759MB/s-759MB/s),
>    io=84.8GiB (91.1GB), run=120001-120001msec
> 
> With this patch:
> Run status group 0 (all jobs):
>    READ: bw=761MiB/s (798MB/s), 761MiB/s-761MiB/s (798MB/s-798MB/s),
>    io=89.2GiB (95.8GB), run=120001-120001msec
> 
> About 5% improvement.

I think this is a big enough of a win to warrant looking closer
at this. Just a quick comment from me so far:

> @@ -2923,7 +2943,10 @@ static int io_fsync(struct io_kiocb *req, bool force_nonblock)
>  {
>  	/* fsync always requires a blocking context */
>  	if (force_nonblock) {
> -		req->work.func = io_fsync_finish;
> +		if (!(req->flags & REQ_F_WORK_INITIALIZED))
> +			init_io_work(req, io_fsync_finish);
> +		else
> +			req->work.func = io_fsync_finish;

This pattern is repeated enough to warrant a helper, ala:

static void io_req_init_async(req, func)
{
	if (req->flags & REQ_F_WORK_INITIALIZED)
		req->work.func = func;
	else
		init_io_work(req, func);
}

also swapped the conditions, I tend to find it easier to read without
the negation.

-- 
Jens Axboe