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

[Jens Axboe <axboe@kernel.dk>]

On 4/16/25 9:07 AM, Jens Axboe wrote:
> On 4/16/25 9:03 AM, Pavel Begunkov wrote:
>> On 4/16/25 06:44, 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
>>>
>>> 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);
>>
>> That's not going to work with ->is_kbuf as the segments are not uniform in
>> size.
> 
> Oops yes good point.

How about something like this? Trims superflous end segments, if they
exist. The 'offset' section already trimmed the front parts. For
!is_kbuf that should be simple math, like in Nitesh's patch. For
is_kbuf, iterate them.

diff --git a/io_uring/rsrc.c b/io_uring/rsrc.c
index bef66e733a77..e482ea1e22a9 100644
--- a/io_uring/rsrc.c
+++ b/io_uring/rsrc.c
@@ -1036,6 +1036,7 @@ static int io_import_fixed(int ddir, struct iov_iter *iter,
 			   struct io_mapped_ubuf *imu,
 			   u64 buf_addr, size_t len)
 {
+	const struct bio_vec *bvec;
 	unsigned int folio_shift;
 	size_t offset;
 	int ret;
@@ -1052,9 +1053,10 @@ static int io_import_fixed(int ddir, struct iov_iter *iter,
 	 * Might not be a start of buffer, set size appropriately
 	 * and advance us to the beginning.
 	 */
+	bvec = imu->bvec;
 	offset = buf_addr - imu->ubuf;
 	folio_shift = imu->folio_shift;
-	iov_iter_bvec(iter, ddir, imu->bvec, imu->nr_bvecs, offset + len);
+	iov_iter_bvec(iter, ddir, bvec, imu->nr_bvecs, offset + len);
 
 	if (offset) {
 		/*
@@ -1073,7 +1075,6 @@ static int io_import_fixed(int ddir, struct iov_iter *iter,
 		 * since we can just skip the first segment, which may not
 		 * be folio_size aligned.
 		 */
-		const struct bio_vec *bvec = imu->bvec;
 
 		/*
 		 * Kernel buffer bvecs, on the other hand, don't necessarily
@@ -1099,6 +1100,27 @@ static int io_import_fixed(int ddir, struct iov_iter *iter,
 		}
 	}
 
+	/*
+	 * Offset trimmed front segments too, if any, now trim the tail.
+	 * For is_kbuf we'll iterate them as they may be different sizes,
+	 * otherwise we can just do straight up math.
+	 */
+	if (len + offset < imu->len) {
+		bvec = iter->bvec;
+		if (imu->is_kbuf) {
+			while (len > bvec->bv_len) {
+				len -= bvec->bv_len;
+				bvec++;
+			}
+			iter->nr_segs = bvec - iter->bvec;
+		} else {
+			size_t vec_len;
+
+			vec_len = bvec->bv_offset + iter->iov_offset +
+					iter->count + ((1UL << folio_shift) - 1);
+			iter->nr_segs = vec_len >> folio_shift;
+		}
+	}
 	return 0;
 }
 

-- 
Jens Axboe