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From: Pavel Begunkov <asml.silence@gmail.com>
To: Christoph Hellwig <hch@infradead.org>
Cc: linux-block@vger.kernel.org, Jens Axboe <axboe@kernel.dk>
Subject: Re: [PATCH 3/5] block: avoid extra iter advance with async iocb
Date: Mon, 25 Oct 2021 12:10:14 +0100	[thread overview]
Message-ID: <87d9f109-c00e-7e79-36c3-61f153db3561@gmail.com> (raw)
In-Reply-To: <YXZd3ETk4P+OBWXD@infradead.org>

On 10/25/21 08:33, Christoph Hellwig wrote:
> On Sat, Oct 23, 2021 at 05:21:34PM +0100, Pavel Begunkov wrote:
>> --- a/block/fops.c
>> +++ b/block/fops.c
>> @@ -352,11 +352,21 @@ static ssize_t __blkdev_direct_IO_async(struct kiocb *iocb,
>>   	bio->bi_end_io = blkdev_bio_end_io_async;
>>   	bio->bi_ioprio = iocb->ki_ioprio;
>>   
>> -	ret = bio_iov_iter_get_pages(bio, iter);
>> -	if (unlikely(ret)) {
>> -		bio->bi_status = BLK_STS_IOERR;
>> -		bio_endio(bio);
>> -		return ret;
>> +	if (!iov_iter_is_bvec(iter)) {
>> +		ret = bio_iov_iter_get_pages(bio, iter);
>> +		if (unlikely(ret)) {
>> +			bio->bi_status = BLK_STS_IOERR;
>> +			bio_endio(bio);
>> +			return ret;
>> +		}
> 
> Nit: I generally find it much nicer to read if simple if statements
> don't use pointless negations.
> 
>> +	} else {
>> +		/*
>> +		 * Users don't rely on the iterator being in any particular
>> +		 * state for async I/O returning -EIOCBQUEUED, hence we can
>> +		 * avoid expensive iov_iter_advance(). Bypass
>> +		 * bio_iov_iter_get_pages() and set the bvec directly.
>> +		 */
>> +		bio_iov_bvec_set(bio, iter);
> 
> So if this optimization is so useful, please also do it for
> non-bvec iov_iters, which is what 99% of the applications actually
> use.

It's an async path, so mainly io_uring or aio, I don't think there
is much profit in doing that for iov, especially behind iov -> bvec
translation with page referencing.

Could've been done nonetheless, but what I think about looks too
ugly because of the loop inside of bio_iov_iter_get_pages(). Don't
think the sketch below is viable, any better ideas?


ssize_t __bio_iov_iter_get_pages() {
	...
	/* not advancing */
	return size;
}

do {
	if (bio_op(bio) == REQ_OP_ZONE_APPEND)
		ret = __bio_iov_append_get_pages(bio, iter);
	else
		ret = __bio_iov_iter_get_pages(bio, iter);
	if (ret < 0)
		break;
	iov_iter_advance(ret);
} while (iov_iter_count(iter) && !bio_full(bio, 0));

and copy paste into fops.c

do {
	if (bio_op(bio) == REQ_OP_ZONE_APPEND)
		ret = __bio_iov_append_get_pages(bio, iter);
	else
		ret = __bio_iov_iter_get_pages(bio, iter);

	if (ret < 0 || iov_iter_count(iter) == ret || bio_full(bio, 0))
		break;
	iov_iter_advance(ret);
} while (1);



-- 
Pavel Begunkov

  reply	other threads:[~2021-10-25 11:12 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-10-23 16:21 [PATCH 0/5] block optimisations Pavel Begunkov
2021-10-23 16:21 ` [PATCH 1/5] block: add single bio async direct IO helper Pavel Begunkov
2021-10-23 16:21 ` [PATCH 2/5] block: refactor bio_iov_bvec_set() Pavel Begunkov
2021-10-25  7:31   ` Christoph Hellwig
2021-10-23 16:21 ` [PATCH 3/5] block: avoid extra iter advance with async iocb Pavel Begunkov
2021-10-25  7:33   ` Christoph Hellwig
2021-10-25 11:10     ` Pavel Begunkov [this message]
2021-10-23 16:21 ` [PATCH 4/5] block: kill unused polling bits in __blkdev_direct_IO() Pavel Begunkov
2021-10-23 16:46   ` Pavel Begunkov
2021-10-24 15:09     ` Jens Axboe
2021-10-24 17:17       ` Pavel Begunkov
2021-10-24 18:11         ` Jens Axboe
2021-10-25  7:35   ` Christoph Hellwig
2021-10-25 10:12     ` Pavel Begunkov
2021-10-25 10:27       ` Pavel Begunkov
2021-10-27  6:47         ` Christoph Hellwig
2021-10-23 16:21 ` [PATCH 5/5] block: add async version of bio_set_polled Pavel Begunkov
2021-10-25  7:36   ` Christoph Hellwig
2021-10-25 10:20     ` Pavel Begunkov
2021-10-25  7:28 ` [PATCH 0/5] block optimisations Christoph Hellwig
2021-10-26 23:30 ` (subset) " Jens Axboe

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