linux-kernel - Re: [PATCH] iov_iter: optimise bvec iov_iter

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Message-ID: <eb02fa7f-956e-ce7e-6a56-82318b2c6d2e@gmail.com>
Date:   Thu, 3 Dec 2020 11:48:56 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iov_iter: optimise bvec iov_iter_advance()

On 03/12/2020 09:14, Christoph Hellwig wrote:
>> @@ -1077,6 +1077,20 @@ void iov_iter_advance(struct iov_iter *i, size_t size)
>>  		i->count -= size;
>>  		return;
>>  	}
>> +	if (iov_iter_is_bvec(i)) {
>> +		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;
>> +		return;
>> +	}
>>  	iterate_and_advance(i, size, v, 0, 0, 0)
> 
> I like the idea, but whu not avoid the on-stack bvec_iter and just
> open code this entirely using a new helper?  E.g.

It's inlined and the on-stack iter is completely optimised out. Frankly,
I'd rather not open-code bvec_iter_advance(), at least for this chunk to
be findable from bvec.h, e.g. grep bvec_iter and bvec_for_each. Though,
I don't like myself that preamble/postamble.

> 
> static void bio_iov_iter_advance(struct iov_iter *i, size_t bytes)
> {
> 	unsigned int cnt;
> 
> 	i->count -= bytes;
> 
> 	bytes += i->iov_offset;
> 	for (cnt = 0; bytes && bytes >= i->bvec[cnt].bv_len; cnt++)
> 		bytes -= i->bvec[cnt].bv_len;
> 	i->iov_offset = bytes;
> 
> 	i->bvec += cnt;
> 	i->nr_segs -= cnt;
> }
> 

-- 
Pavel Begunkov