linux-kernel - RE: [PATCH v2 1/2] iov_iter: optimise iov_iter

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Date:   Fri, 20 Nov 2020 13:29:14 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Pavel Begunkov' <asml.silence@...il.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>
CC:     Jens Axboe <axboe@...nel.dk>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2 1/2] iov_iter: optimise iov_iter_npages for bvec

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@...nel.dk>
> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
> ---
>  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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