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

[Nitesh Shetty <nj.shetty@samsung.com>]

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

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);
 
 	return 0;
 }

base-commit: 834a4a689699090a406d1662b03affa8b155d025
-- 
2.43.0