Re: [PATCH 1/3] block: fix bio_may_need_split() by using bvec iterator way

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

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On 19/12/25 12:08AM, Ming Lei wrote:
>On Thu, Dec 18, 2025 at 08:46:47PM +0530, Nitesh Shetty wrote:
>> On 18/12/25 05:45PM, Ming Lei wrote:
>> > On Thu, Dec 18, 2025 at 01:37:37AM -0800, Christoph Hellwig wrote:
>> > > On Thu, Dec 18, 2025 at 05:31:42PM +0800, Ming Lei wrote:
>> > > > ->bi_vcnt doesn't make sense for cloned bio, which is perfectly fine
>> > > > passed to bio_may_need_split().
>> > > >
>> > > > So fix bio_may_need_split() by not taking ->bi_vcnt directly, instead
>> > > > checking with help from bio size and bvec->len.
>> > > >
>> > > > Meantime retrieving the 1st bvec via __bvec_iter_bvec().
>> > >
>> > > That totally misses the point.  The ->bi_vcnt is a fast and lose
>> > > check to see if we need the fairly expensive iterators to do the
>> > > real check.
>> >
>> > It is just __bvec_iter_bvec(), whatever it should be in cache sooner or
>> > later.
>> >
>> >
>> Functionality wise overall patch looks fine to me, but as Christoph
>> stated there is slight performance(IOPS) penalty.
>> Here is my benchmarking numbers[1], I suspect Jens setup might show
>> more regression.
>>
>> Regards,
>> Nitesh
>>
>>
>> [1]
>> ===============================
>> a. two optane nvme device setup:
>> ----------
>> base case:
>> ----------
>> sudo taskset -c 0,1 /home/nitesh/src/private/fio/t/io_uring -b512 \
>> -d128 -c32 -s32 -p1 -F1 -B1 -n2 -r4 /dev/nvme0n1 /dev/nvme1n1
>> submitter=0, tid=206586, file=/dev/nvme0n1, nfiles=1, node=-1
>> submitter=1, tid=206587, file=/dev/nvme1n1, nfiles=1, node=-1
>> polled=1, fixedbufs=1, register_files=1, buffered=0, QD=128
>> Engine=io_uring, sq_ring=128, cq_ring=128
>> polled=1, fixedbufs=1, register_files=1, buffered=0, QD=128
>> Engine=io_uring, sq_ring=128, cq_ring=128
>> IOPS=6.45M, BW=3.15GiB/s, IOS/call=32/31
>> IOPS=6.47M, BW=3.16GiB/s, IOS/call=32/32
>> IOPS=6.47M, BW=3.16GiB/s, IOS/call=32/32
>> Exiting on timeout
>> Maximum IOPS=6.47M
>>
>> ----------------
>> with this patch:
>> ----------------
>> sudo taskset -c 0,1 /home/nitesh/src/private/fio/t/io_uring -b512 \
>> -d128 -c32 -s32 -p1 -F1 -B1 -n2 -r4 /dev/nvme0n1 /dev/nvme1n1
>> submitter=0, tid=6352, file=/dev/nvme0n1, nfiles=1, node=-1
>> polled=1, fixedbufs=1, register_files=1, buffered=0, QD=128
>> Engine=io_uring, sq_ring=128, cq_ring=128
>> submitter=1, tid=6353, file=/dev/nvme1n1, nfiles=1, node=-1
>> IOPS=6.30M, BW=3.08GiB/s, IOS/call=32/31
>> IOPS=6.35M, BW=3.10GiB/s, IOS/call=32/31
>> IOPS=6.37M, BW=3.11GiB/s, IOS/call=32/32
>> Exiting on timeout
>> Maximum IOPS=6.37M
>>
>> =============================
>> b. two null-blk device setup:
>> ------------------
>> null device setup:
>> ------------------
>> sudo modprobe null_blk queue_mode=2 gb=10 bs=512 nr_devices=2 irqmode=2 \
>> completion_nsec=1000000 hw_queue_depth=256 memory_backed=0 discard=0 \
>> use_per_node_hctx=1
>>
>> ----------
>> base case:
>> ----------
>> sudo taskset -c 0,1 /home/nitesh/src/private/fio/t/io_uring -b512 \
>> -d128 -c32 -s32 -p1 -F1 -B1 -n2 -r4 /dev/nullb0 /dev/nullb1
>> submitter=0, tid=6743, file=/dev/nullb0, nfiles=1, node=-1
>> submitter=1, tid=6744, file=/dev/nullb1, nfiles=1, node=-1
>> polled=1, fixedbufs=1, register_files=1, buffered=0, QD=128
>> Engine=io_uring, sq_ring=128, cq_ring=128
>> IOPS=7.89M, BW=3.85GiB/s, IOS/call=32/31
>> IOPS=7.96M, BW=3.89GiB/s, IOS/call=32/32
>> IOPS=7.99M, BW=3.90GiB/s, IOS/call=32/32
>> Exiting on timeout
>> Maximum IOPS=7.99M
>>
>> -------------------
>> with this patchset:
>> -------------------
>> sudo taskset -c 0,1 /home/nitesh/src/private/fio/t/io_uring -b512 \
>> -d128 -c32 -s32 -p1 -F1 -B1 -n2 -r4 /dev/nullb0 /dev/nullb1
>> submitter=0, tid=35633, file=/dev/nullb0, nfiles=1, node=-1
>> submitter=1, tid=35634, file=/dev/nullb1, nfiles=1, node=-1
>> polled=1, fixedbufs=1, register_files=1, buffered=0, QD=128
>> Engine=io_uring, sq_ring=128, cq_ring=128
>> polled=1, fixedbufs=1, register_files=1, buffered=0, QD=128
>> Engine=io_uring, sq_ring=128, cq_ring=128
>> IOPS=7.79M, BW=3.80GiB/s, IOS/call=32/31
>> IOPS=7.86M, BW=3.84GiB/s, IOS/call=32/32
>> IOPS=7.89M, BW=3.85GiB/s, IOS/call=32/32
>> Exiting on timeout
>> Maximum IOPS=7.89M
>
>Thanks for the perf test!
>
>This patch only adds bio->bi_iter memory footprint, which is supposed
>to hit from L1, maybe because `bi_io_vec` is in the 2nd cacheline, can
>you see any difference with the following change?
>
>
>diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
>index 5dc061d318a4..1c4570b37436 100644
>--- a/include/linux/blk_types.h
>+++ b/include/linux/blk_types.h
>@@ -240,6 +240,7 @@ struct bio {
>                /* for plugged zoned writes only: */
>                unsigned int            __bi_nr_segments;
>        };
>+       struct bio_vec          *bi_io_vec;     /* the actual vec list */
>        bio_end_io_t            *bi_end_io;
>        void                    *bi_private;
> #ifdef CONFIG_BLK_CGROUP
>@@ -275,8 +276,6 @@ struct bio {
>
>        atomic_t                __bi_cnt;       /* pin count */
>
>-       struct bio_vec          *bi_io_vec;     /* the actual vec list */
>-
>        struct bio_set          *bi_pool;
> };
>
With above patch perf numbers match the base case.

Thanks,
Nitesh Shetty

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