From: Pavel Begunkov <asml.silence@gmail.com>
To: David Laight <David.Laight@ACULAB.COM>,
"linux-block@vger.kernel.org" <linux-block@vger.kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>,
Christoph Hellwig <hch@infradead.org>,
Matthew Wilcox <willy@infradead.org>,
Ming Lei <ming.lei@redhat.com>,
Johannes Weiner <hannes@cmpxchg.org>,
Alexander Viro <viro@zeniv.linux.org.uk>,
"Darrick J . Wong" <darrick.wong@oracle.com>,
"Martin K . Petersen" <martin.petersen@oracle.com>,
Jonathan Corbet <corbet@lwn.net>,
"linux-xfs@vger.kernel.org" <linux-xfs@vger.kernel.org>,
"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
"io-uring@vger.kernel.org" <io-uring@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"target-devel@vger.kernel.org" <target-devel@vger.kernel.org>,
"linux-scsi@vger.kernel.org" <linux-scsi@vger.kernel.org>,
"linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>
Subject: Re: [PATCH v1 2/6] iov_iter: optimise bvec iov_iter_advance()
Date: Tue, 15 Dec 2020 11:23:34 +0000 [thread overview]
Message-ID: <d151f81e-ec56-59c0-d2a0-ffd4a269fec1@gmail.com> (raw)
In-Reply-To: <262132648a8f4e7a9d1c79003ea74b3f@AcuMS.aculab.com>
On 15/12/2020 09:37, David Laight wrote:
> 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.
>>
>> lib/iov_iter.c | 19 +++++++++++++++++++
[...]
>> 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).
If Al finally takes the patch for iov_iter_is_*() helpers it would
be completely optimised out.
>
> Can't the call to bver_iter_advance be dropped into the 'advance'
> path for BVEC's inside iterate_and_advance?
It iterates by page/segment/etc., why would you want to do
bver_iter_advance(i->count) there?
>
> 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.
And again, both second checks, including for discards, would be
optimised out by the iov_iter_is_* patch.
--
Pavel Begunkov
next prev parent reply other threads:[~2020-12-15 11:27 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
2020-12-15 11:23 ` Pavel Begunkov [this message]
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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