linux-kernel - Re: [ANNOUNCE] "Fast Kernel Headers" Tree -v2

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Date:   Thu, 13 Jan 2022 10:20:22 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jonathan Corbet <corbet@....net>
Subject: Re: [ANNOUNCE] "Fast Kernel Headers" Tree -v2

On Thu, Jan 13, 2022 at 9:27 AM Ingo Molnar <mingo@...nel.org> wrote:
> * Arnd Bergmann <arnd@...db.de> wrote:
> > On Mon, Jan 10, 2022 at 11:03 PM Arnd Bergmann <arnd@...db.de> wrote:
> > > On Sat, Jan 8, 2022 at 5:26 PM Ingo Molnar <mingo@...nel.org> wrote:
> > > I've started building randconfig kernels for arm64 and x86, and fixing
> > > up things that come up, a few things I have noticed out so far:
> >
> > I have run into a couple more specific issues:
> >
> > * net/smc/smc_ib.c:824:26: error: implicit declaration of function
> > 'cache_line_size' [-Werror=implicit-function-declaration]
> > cache_line_size is generally provided by linux/cache.h, which includes
> > asm/cache.h.
> > This works on arm64, but not on x86, where asm/cache.h would have to include
> > asm/cpufeature.h, and but it would be good to avoid that because of the implicit
> > linux/percpu.h and linux/bitops.h inclusions. Also, if I add the
> > include, I get this
> > build failure instead: include/linux/smp_types.h:88:33: error:
> > requested alignment '20'
> > is not a positive power of 2
>
> Note that this particular one should be fixed in the WIP branch, which is
> at:
>
>    git://git.kernel.org/pub/scm/linux/kernel/git/mingo/tip.git sched/headers

Ok.

> > * arm64 has a couple of issues around asm/memory.h, linux/mm_types.h and
> > asm/page.h that can cause loops. I think my latest version has it figured
> > out, but there is probably room for optimization.
>
> Yeah, this is like the 5th attempt at finding a robust solution. :-/

It will likely come back in another form when more architectures get
converted then. I'm currently looking at reviving my own metrics scripts
from 2020 to see if I can improve arch/arm64 further, after that I was
planning to look at arch/arm/

> > * There is no general way to get the get_order() definition, other than
> > including asm/page.h from .c files. On arm64, this shows up in a couple
> > of files after the cleanup. Only xtensa and ia64 define their own version
> > of get_order(), and I think we should just remove those and move the
> > generic version to linux/getorder.h, where any file using it can pick it
> > up. For randconfig builds, I had to add asm/page.h to
> > net/xdp/xsk_queue.c, mm/memtest.c and
> > drivers/target/iscsi/iscsi_target_nego.c, after I removed the indirect
> > include from arch/arm64/include/asm/mmu.h in the previous step.
>
> Would including <linux/mm_page_address.h> be sufficient? That already has
> an <asm/page.h> inclusion and is vaguely related.

Sure, works for me.

> I tried to avoid as many low level headers as possible from the main types
> headers - and the get_order() functionality also brings in bitops
> definitions, which I'm still hoping to be able to reduce from its current
> ~95% utilization in a distro kernel ...

Agreed, I think reducing bitops.h and atomic.h usage is fairly important,
I think these are even bigger on arm64 than on x86.

> We could add <linux/page_api.h> as well, as a standardized header. We
> already have page_types.h and et_order() is a page types API.

More generally speaking, do you have a plan for how to document which
header to include for getting a particular symbol that is provided by
a header we don't want to include directly? I think iwyu has a particular
notation for it, but when I looked at using that in 2020 I decided it wouldn't
scale to the size of the kernel. I did my own shell script with a long
list of regex patterns, but I'm not convinced about that approach either.

       Arnd