linux-kernel - Re: [PATCH 0/2] optimise sqe-to-req flags

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Message-ID: <b507d39e-ec2b-5c9f-0fd0-6ab1b0491cad@kernel.dk>
Date:   Fri, 17 Jan 2020 17:16:57 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Pavel Begunkov <asml.silence@...il.com>, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] optimise sqe-to-req flags

On 1/17/20 4:14 PM, Pavel Begunkov wrote:
> On 18/01/2020 01:49, Jens Axboe wrote:
>> On 1/17/20 3:41 PM, Pavel Begunkov wrote:
>>> *lost the cover-letter, but here we go*
>>>
>>> The main idea is to optimise code like the following by directly
>>> copying sqe flags:
>>>
>>> if (sqe_flags & IOSQE_IO_HARDLINK)
>>> 	req->flags |= REQ_F_HARDLINK;
>>>
>>> The first patch is a minor cleanup, and the second one do the
>>> trick. No functional changes.
>>>
>>> The other thing to consider is whether to use such flags as 
>>> REQ_F_LINK = IOSQE_IO_LINK, or directly use IOSQE_IO_LINK instead.
>>
>> I think we should keep the names separate. I think it looks fine, though
>> I do wish that we could just have both in an enum and not have to do
>> weird naming. We sometimes do:
>>
>> enum {
>> 	__REQ_F_FOO
>> };
>>
>> #define REQ_F_FOO	(1U << __REQ_F_FOO)
>>
> 
> I thought it will be too bulky as it needs retyping the same name many
> times.  Though, it solves numbering problem and is less error-prone
> indeed. Let me to play with it a bit.

It's less error prone once the change is done, though the change will be
bigger. I think that's the right tradeoff.

> BTW, there is another issue from development perspective -- it's
> harder to find from where a flag is came. E.g. search for REQ_F_FOO
> won't give you a place, where it was set. SQE_INHERITED_FLAGS is close
> in the code to its usage exactly
> for this reason.

Since it's just that one spot, add a comment with the flag names or get
rid of the SQE_INHERITED_FLAGS define. That'll make it easy to find.

>> and with that we could have things Just Work in terms of numbering, if
>> we keep the overlapped values at the start. Would need IOSQE_* to also
>> use an enum, ala:
>>
>> enum {
>> 	__IOSQE_FIXED_FILE,
>> 	__IOSQE_IO_DRAIN,
>> 	...
>> };
>>
> 
> I tried to not modify the userspace header. Wouldn't it be better to
> add _BIT postfix instead of the underscores?

No strong preference, I usually do the underscores, but not a big deal
to me. There's also BIT_* helpers to make the masks, we should use
those.

-- 
Jens Axboe