Re: [PATCH] io_uring/rsrc: send exact nr_segs for fixed buffer

[Jens Axboe <axboe@kernel.dk>]

On 4/15/25 11:44 PM, Nitesh Shetty wrote:
> Sending exact nr_segs, avoids bio split check and processing in
> block layer, which takes around 5%[1] of overall CPU utilization.
> 
> In our setup, we see overall improvement of IOPS from 7.15M to 7.65M [2]
> and 5% less CPU utilization.
> 
> [1]
>      3.52%  io_uring         [kernel.kallsyms]     [k] bio_split_rw_at
>      1.42%  io_uring         [kernel.kallsyms]     [k] bio_split_rw
>      0.62%  io_uring         [kernel.kallsyms]     [k] bio_submit_split
> 
> [2]
> sudo taskset -c 0,1 ./t/io_uring -b512 -d128 -c32 -s32 -p1 -F1 -B1 -n2
> -r4 /dev/nvme0n1 /dev/nvme1n1

This must be a regression, do you know which block/io_uring side commit
caused the splits to be done for fixed buffers?

> Signed-off-by: Nitesh Shetty <nj.shetty@samsung.com>
> ---
>  io_uring/rsrc.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/io_uring/rsrc.c b/io_uring/rsrc.c
> index b36c8825550e..6fd3a4a85a9c 100644
> --- a/io_uring/rsrc.c
> +++ b/io_uring/rsrc.c
> @@ -1096,6 +1096,9 @@ static int io_import_fixed(int ddir, struct iov_iter *iter,
>  			iter->iov_offset = offset & ((1UL << imu->folio_shift) - 1);
>  		}
>  	}
> +	iter->nr_segs = (iter->bvec->bv_offset + iter->iov_offset +
> +		iter->count + ((1UL << imu->folio_shift) - 1)) /
> +		(1UL << imu->folio_shift);

	iter->nr_segs = (iter->bvec->bv_offset + iter->iov_offset +
		iter->count + ((1UL << imu->folio_shift) - 1)) >> imu->folio_shift;

to avoid a division, seems worthwhile?

-- 
Jens Axboe