From: Pavel Begunkov
> Sent: 19 November 2020 23:25
>
> The block layer spends quite a while in iov_iter_npages(), but for the
> bvec case the number of pages is already known and stored in
> iter->nr_segs, so it can be returned immediately as an optimisation
>
> Perf for an io_uring benchmark with registered buffers (i.e. bvec) shows
> ~1.5-2.0% total cycle count spent in iov_iter_npages(), that's dropped
> by this patch to ~0.2%.
>
> Reviewed-by: Jens Axboe <axboe@xxxxxxxxx>
> Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
> ---
> lib/iov_iter.c | 10 +++++-----
> 1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> index 1635111c5bd2..0fa7ac330acf 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -1594,6 +1594,8 @@ int iov_iter_npages(const struct iov_iter *i, int maxpages)
> return 0;
> if (unlikely(iov_iter_is_discard(i)))
> return 0;
> + if (unlikely(iov_iter_is_bvec(i)))
> + return min_t(int, i->nr_segs, maxpages);
>
> if (unlikely(iov_iter_is_pipe(i))) {
Is it worth putting an extra condition around these three 'unlikely' cases.
ie:
if (unlikely((iov_iter_type(i) & (ITER_DISCARD | ITER_BVEC | ITER_PIPE)) {
if (iov_iter_is_discard(i))
return 0;
if (iov_iter_is_bvec(i))
return min_t(int, i->nr_segs, maxpages);
/* Must be ITER_PIPE */
David
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