RE: [PATCH v1 2/6] iov_iter: optimise bvec iov_iter_advance()

[David Laight <David.Laight@ACULAB.COM>]

From: Pavel Begunkov
> Sent: 15 December 2020 00:20
> 
> iov_iter_advance() is heavily used, but implemented through generic
> iteration. As bvecs have a specifically crafted advance() function, i.e.
> bvec_iter_advance(), which is faster and slimmer, use it instead.
> 
> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> ---
>  lib/iov_iter.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> index 1635111c5bd2..5b186dc2c9ea 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -1067,6 +1067,21 @@ static void pipe_advance(struct iov_iter *i, size_t size)
>  	pipe_truncate(i);
>  }
> 
> +static void iov_iter_bvec_advance(struct iov_iter *i, size_t size)
> +{
> +	struct bvec_iter bi;
> +
> +	bi.bi_size = i->count;
> +	bi.bi_bvec_done = i->iov_offset;
> +	bi.bi_idx = 0;
> +	bvec_iter_advance(i->bvec, &bi, size);
> +
> +	i->bvec += bi.bi_idx;
> +	i->nr_segs -= bi.bi_idx;
> +	i->count = bi.bi_size;
> +	i->iov_offset = bi.bi_bvec_done;
> +}
> +
>  void iov_iter_advance(struct iov_iter *i, size_t size)
>  {
>  	if (unlikely(iov_iter_is_pipe(i))) {
> @@ -1077,6 +1092,10 @@ void iov_iter_advance(struct iov_iter *i, size_t size)
>  		i->count -= size;
>  		return;
>  	}
> +	if (iov_iter_is_bvec(i)) {
> +		iov_iter_bvec_advance(i, size);
> +		return;
> +	}
>  	iterate_and_advance(i, size, v, 0, 0, 0)
>  }

This seems to add yet another comparison before what is probably
the common case on an IOVEC (ie normal userspace buffer).

Can't the call to bver_iter_advance be dropped into the 'advance'
path for BVEC's inside iterate_and_advance?

iterate_and_advance itself has three 'unlikely' conditional tests
that may be mis-predicted taken before the 'likely' path.
One is for DISCARD which is checked twice on the object I just
looked at - the test in iov_iter_advance() is pointless.

	David

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