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

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Message-ID: <5b65de70-19db-4572-d122-df65191ab098@gmail.com>
Date:   Mon, 14 Dec 2020 17:25:37 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     David Laight <David.Laight@...LAB.COM>,
        Al Viro <viro@...iv.linux.org.uk>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Jens Axboe <axboe@...nel.dk>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] iov_iter: optimise iter type checking

On 14/12/2020 10:28, David Laight wrote:
> From: Pavel Begunkov
>> Sent: 13 December 2020 22:33
>>
>> 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.
> 
> The advantage of the bit-masks is that the 'usual' options can
> be tested for together. So the code can be (for example):

Well, you can do that for the non-bitwise case as well.
In a simpler form but should be enough.

enum { ITER_IOVEC = 1, ITER_BVEC = 2, ... }
if (type <= ITER_BVEC) {
	if (iovec) ...
	if (bvec) ...
} else { ... }


> 	if (likely(iter->type & (ITER_IOVEC | ITER_PIPE) {
> 		if (likely((iter->type & ITER_IOVEC)) {
> 			... code for iovec
> 		} else [
> 			... code for pipe
> 		}
> 	} else if (iter->type & ITER_BVEC) {
> 		... code for bvec
> 	} else if (iter->type & ITER_KVEC) {
> 		.. code for kvec
> 	} else {
> 		.. must be discard
> 	}

-- 
Pavel Begunkov