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From: David Laight <[email protected]>
To: 'Pavel Begunkov' <[email protected]>,
	"[email protected]" <[email protected]>
Cc: Jens Axboe <[email protected]>,
	Christoph Hellwig <[email protected]>,
	Matthew Wilcox <[email protected]>,
	Ming Lei <[email protected]>,
	Johannes Weiner <[email protected]>,
	Alexander Viro <[email protected]>,
	"Darrick J . Wong" <[email protected]>,
	"Martin K . Petersen" <[email protected]>,
	Jonathan Corbet <[email protected]>,
	"[email protected]" <[email protected]>,
	"[email protected]" <[email protected]>,
	"[email protected]" <[email protected]>,
	"[email protected]" <[email protected]>,
	"[email protected]" <[email protected]>,
	"[email protected]" <[email protected]>,
	"[email protected]" <[email protected]>
Subject: RE: [PATCH v1 2/6] iov_iter: optimise bvec iov_iter_advance()
Date: Tue, 15 Dec 2020 09:37:53 +0000	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <5c9c22dbeecad883ca29b31896c262a8d2a77132.1607976425.git.asml.silence@gmail.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 <[email protected]>
> ---
>  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

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)


  reply	other threads:[~2020-12-15  9:43 UTC|newest]

Thread overview: 38+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-15  0:20 [PATCH v1 0/6] no-copy bvec Pavel Begunkov
2020-12-15  0:20 ` [PATCH v1 1/6] target/file: allocate the bvec array as part of struct target_core_file_cmd Pavel Begunkov
2020-12-15  0:20 ` [PATCH v1 2/6] iov_iter: optimise bvec iov_iter_advance() Pavel Begunkov
2020-12-15  9:37   ` David Laight [this message]
2020-12-15 11:23     ` Pavel Begunkov
2020-12-15 13:54       ` David Laight
2020-12-15 13:56         ` Pavel Begunkov
2020-12-22 14:03   ` Christoph Hellwig
2020-12-15  0:20 ` [PATCH v1 3/6] bio: deduplicate adding a page into bio Pavel Begunkov
2020-12-22 14:04   ` Christoph Hellwig
2020-12-15  0:20 ` [PATCH v1 4/6] block/psi: remove PSI annotations from direct IO Pavel Begunkov
2020-12-15  0:56   ` Dave Chinner
2020-12-15  1:03     ` Pavel Begunkov
2020-12-15  1:33       ` Dave Chinner
2020-12-15 11:41         ` Pavel Begunkov
2020-12-22 14:07   ` Christoph Hellwig
2020-12-15  0:20 ` [PATCH v1 5/6] bio: add a helper calculating nr segments to alloc Pavel Begunkov
2020-12-15  1:00   ` Dave Chinner
2020-12-15  1:07     ` Pavel Begunkov
2020-12-15  1:09     ` Dave Chinner
2020-12-22 14:07   ` Christoph Hellwig
2020-12-15  0:20 ` [PATCH v1 6/6] block/iomap: don't copy bvec for direct IO Pavel Begunkov
2020-12-15  1:09   ` Dave Chinner
2020-12-15  1:15     ` Pavel Begunkov
2020-12-22 14:15   ` Christoph Hellwig
2020-12-15  1:41 ` [PATCH v1 0/6] no-copy bvec Ming Lei
2020-12-15 11:14   ` Pavel Begunkov
2020-12-15 12:03     ` Ming Lei
2020-12-15 14:05       ` Pavel Begunkov
2020-12-22 14:11         ` Christoph Hellwig
2020-12-23 12:52           ` Pavel Begunkov
2020-12-23 15:51             ` Christoph Hellwig
2020-12-23 16:04               ` James Bottomley
2020-12-23 20:23                 ` Douglas Gilbert
2020-12-23 20:32                   ` Pavel Begunkov
2020-12-24  6:41                     ` Christoph Hellwig
2020-12-24 16:45                       ` Douglas Gilbert
2020-12-24 17:30                   ` James Bottomley

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