From: Jens Axboe axboe@kernel.dk
commit bd11b3a391e3df6fa958facbe4b3f9f4cca9bd49 upstream.
Hrvoje reports that when a large fixed buffer is registered and IO is being done to the latter pages of said buffer, the IO submission time is much worse:
reading to the start of the buffer: 11238 ns reading to the end of the buffer: 1039879 ns
In fact, it's worse by two orders of magnitude. The reason for that is how io_uring figures out how to setup the iov_iter. We point the iter at the first bvec, and then use iov_iter_advance() to fast-forward to the offset within that buffer we need.
However, that is abysmally slow, as it entails iterating the bvecs that we setup as part of buffer registration. There's really no need to use this generic helper, as we know it's a BVEC type iterator, and we also know that each bvec is PAGE_SIZE in size, apart from possibly the first and last. Hence we can just use a shift on the offset to find the right index, and then adjust the iov_iter appropriately. After this fix, the timings are:
reading to the start of the buffer: 10135 ns reading to the end of the buffer: 1377 ns
Or about an 755x improvement for the tail page.
Reported-by: Hrvoje Zeba zeba.hrvoje@gmail.com Tested-by: Hrvoje Zeba zeba.hrvoje@gmail.com Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- fs/io_uring.c | 39 +++++++++++++++++++++++++++++++++++++-- 1 file changed, 37 insertions(+), 2 deletions(-)
--- a/fs/io_uring.c +++ b/fs/io_uring.c @@ -1001,8 +1001,43 @@ static int io_import_fixed(struct io_rin */ offset = buf_addr - imu->ubuf; iov_iter_bvec(iter, rw, imu->bvec, imu->nr_bvecs, offset + len); - if (offset) - iov_iter_advance(iter, offset); + + if (offset) { + /* + * Don't use iov_iter_advance() here, as it's really slow for + * using the latter parts of a big fixed buffer - it iterates + * over each segment manually. We can cheat a bit here, because + * we know that: + * + * 1) it's a BVEC iter, we set it up + * 2) all bvecs are PAGE_SIZE in size, except potentially the + * first and last bvec + * + * So just find our index, and adjust the iterator afterwards. + * If the offset is within the first bvec (or the whole first + * bvec, just use iov_iter_advance(). This makes it easier + * since we can just skip the first segment, which may not + * be PAGE_SIZE aligned. + */ + const struct bio_vec *bvec = imu->bvec; + + if (offset <= bvec->bv_len) { + iov_iter_advance(iter, offset); + } else { + unsigned long seg_skip; + + /* skip first vec */ + offset -= bvec->bv_len; + seg_skip = 1 + (offset >> PAGE_SHIFT); + + iter->bvec = bvec + seg_skip; + iter->nr_segs -= seg_skip; + iter->count -= (seg_skip << PAGE_SHIFT); + iter->iov_offset = offset & ~PAGE_MASK; + if (iter->iov_offset) + iter->count -= iter->iov_offset; + } + }
/* don't drop a reference to these pages */ iter->type |= ITER_BVEC_FLAG_NO_REF;