Re: [PATCH 00/17] Add memberof(), split some headers, and slightly simplify code

Alejandro Colomar (man-pages)
2021-11-19T16:12:27Z

Hi Arnd,

On 11/19/21 16:57, Arnd Bergmann wrote:
>
> From what I can tell, linux/stddef.h is tiny, I don't think it's really
> worth optimizing this part. I have spent some time last year
> trying to untangle some of the more interesting headers, but ended
> up not completing this as there are some really hard problems
> once you start getting to the interesting bits.

In this case it was not about being worth it or not,
but that the fact that adding memberof() would break,
unless I use 0 instead of NULL for the implementation of memberof(),
which I'm against,
or I split stddef.

If I don't do either of those,
I'm creating a circular dependency,
and it doesn't compile.

>
> The approach I tried was roughly:
>
> - For each header in the kernel, create a preprocessed version
> that includes all the indirect includes, from that start a set
> of lookup tables that record which header is eventually included
> by which ones, and the size of each preprocessed header in
> bytes
>
> - For a given kernel configuration (e.g. defconfig or allmodconfig)
> that I'm most interested in, look at which files are built, and what
> the direct includes are in the source files.
>
> - Sort the headers by the product of the number of direct includes
> and the preprocessed size: the largest ones are those that are
> worth looking at first.
>
> - use graphviz to visualize the directed graph showing the includes
> between the top 100 headers in that list. You get something like
> I had in [1], or the version afterwards at [2].
>
> - split out unneeded indirect includes from the headers in the center
> of that graph, typically by splitting out struct definitions.
>
> - repeat.
>
> The main problem with this approach is that as soon as you start
> actually reducing the unneeded indirect includes, you end up with
> countless .c files that no longer build because they are missing a
> direct include for something that was always included somewhere
> deep underneath, so I needed a second set of scripts to add
> direct includes to every .c file.
>
> On the plus side, I did see something on the order of a 30%
> compile speed improvement with clang, which is insane
> given that this only removed dead definitions.

Huh!

I'd like to see the kernel some day
not having _any_ hidden dependencies.

For the moment,
since my intent is familiarizing with kernel programming,
and not necessarily improving performance considerably
(at least not in the first rounds of changes),
I prefer starting where it more directly affects
what I initially intended to change in the kernel,
which in this case was adding memberof().

>
>> But I'll note that linux/fs.h, linux/sched.h, linux/mm.h are
>> interesting headers for further splitting.
>>
>>
>> BTW, I also have a longstanding doubt about
>> how header files are organized in the kernel,
>> and which headers can and cannot be included
>> from which other files.
>>
>> For example I see that files in samples or scripts or tools,
>> that redefine many things such as offsetof() or ARRAY_SIZE(),
>> and I don't know if there's a good reason for that,
>> or if I should simply remove all that stuff and
>> include <linux/offsetof.h> everywhere I see offsetof() being used.
>
> The main issue here is that user space code should not
> include anything outside of include/uapi/ and arch/*/include/uapi/

Okay. That's good to know.

So everything can use uapi code,
and uapi code can only use uapi code,
right?

Every duplicate definition of something outside of uapi
should/could be removed.

>
> offsetof() is defined in include/linux/stddef.h, so this is by
> definition not accessible here. It appears that there is also
> an include/uapi/linux/stddef.h that is really strange because
> it includes linux/compiler_types.h, which in turn is outside
> of uapi/. This should probably be fixed.

I see.
Then,
perhaps it would be better to define offsetof() _only_ inside uapi/,
and use that definition from everywhere else,
and therefore remove the non-uapi version,
right?

Thanks,
Alex

--
Alejandro Colomar
Linux man-pages comaintainer; https://www.kernel.org/doc/man-pages/