linux-kernel - Re: [PATCH 2/2] iov_iter: optimise iter type checking

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Date:   Sun, 13 Dec 2020 22:32:37 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] iov_iter: optimise iter type checking

On 11/12/2020 02:01, Al Viro wrote:
> On Thu, Nov 19, 2020 at 05:12:44PM +0000, Pavel Begunkov wrote:
>> On 19/11/2020 17:03, Christoph Hellwig wrote:
>>> On Thu, Nov 19, 2020 at 03:29:43PM +0000, Pavel Begunkov wrote:
>>>> The problem here is that iov_iter_is_*() helpers check types for
>>>> equality, but all iterate_* helpers do bitwise ands. This confuses
>>>> a compiler, so even if some cases were handled separately with
>>>> iov_iter_is_*(), it can't eliminate and skip unreachable branches in
>>>> following iterate*().
>>>
>>> I think we need to kill the iov_iter_is_* helpers, renumber to not do
>>> the pointless bitmask and just check for equality (might turn into a
>>> bunch of nice switch statements actually).
>>
>> There are uses like below though, and that would also add some overhead
>> on iov_iter_type(), so it's not apparent to me which version would be
>> cleaner/faster in the end. But yeah, we can experiment after landing
>> this patch.
>>
>> if (type & (ITER_BVEC|ITER_KVEC))
> 
> There are exactly 3 such places, and all of them would've been just as well
> with case ITER_BVEC: case ITER_KVEC: ... in a switch.
> 
> Hmm...  I wonder which would work better:
> 
> enum iter_type {
>         ITER_IOVEC = 0,
>         ITER_KVEC = 2,
>         ITER_BVEC = 4,
>         ITER_PIPE = 6,
>         ITER_DISCARD = 8,
> };
> iov_iter_type(iter)	(((iter)->type) & ~1)
> iov_iter_rw(iter)	(((iter)->type) & 1)
> 
> or
> 
> enum iter_type {
>         ITER_IOVEC,
>         ITER_KVEC,
>         ITER_BVEC,
>         ITER_PIPE,
>         ITER_DISCARD,
> };
> iov_iter_type(iter)	(((iter)->type) & (~0U>>1))
> // callers of iov_iter_rw() are almost all comparing with explicit READ or WRITE
> iov_iter_rw(iter)	(((iter)->type) & ~(~0U>>1) ? WRITE : READ)
> with places like iov_iter_kvec() doing
> 	i->type = ITER_KVEC | ((direction == WRITE) ? BIT(31) : 0);
> 
> Preferences?

For the bitmask version (with this patch) we have most of
iov_iter_type() completely optimised out. E.g. identical

iov_iter_type(i) & ITER_IOVEC <=> iter->type & ITER_IOVEC

It's also nice to have iov_iter_rw() to be just
(type & 1), operations with which can be optimised in a handful of ways.

Unless the compiler would be able to heavily optimise switches,
e.g. to out-of-memory/calculation-based jump tables, that I doubt,
I'd personally leave it be. Though, not like it should matter much.

-- 
Pavel Begunkov