linux-kernel - Re: [PATCH v1 2/6] iov_iter: optimise bvec iov_iter

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Date:   Tue, 15 Dec 2020 11:23:34 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     David Laight <David.Laight@...LAB.COM>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>
Cc:     Jens Axboe <axboe@...nel.dk>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Ming Lei <ming.lei@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        Jonathan Corbet <corbet@....net>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "io-uring@...r.kernel.org" <io-uring@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "target-devel@...r.kernel.org" <target-devel@...r.kernel.org>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH v1 2/6] iov_iter: optimise bvec iov_iter_advance()

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