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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Mon, 25 Jan 2021 10:27:11 -0800
Message-ID: <CAKwvOdnxnooqtyeSem63V_P5980jc0Z2PDG=0iM8ixeYTSaTCg@mail.gmail.com>
Subject: Re: [PATCH] percpu: fix clang modpost warning in pcpu_build_alloc_info()
To:     Arnd Bergmann <arnd@kernel.org>
Cc:     Dennis Zhou <dennis@kernel.org>,
        Nathan Chancellor <natechancellor@gmail.com>,
        Tejun Heo <tj@kernel.org>, Christoph Lameter <cl@linux.com>,
        Linux-MM <linux-mm@kvack.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        kbuild-all@lists.01.org, kernel test robot <lkp@intel.com>
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On Mon, Jan 25, 2021 at 3:07 AM Arnd Bergmann <arnd@kernel.org> wrote:
>
> On Tue, Jan 5, 2021 at 1:55 AM Dennis Zhou <dennis@kernel.org> wrote:
> >
> > On Mon, Jan 04, 2021 at 04:46:51PM -0700, Nathan Chancellor wrote:
> > > On Thu, Dec 31, 2020 at 09:28:52PM +0000, Dennis Zhou wrote:
> > > >
> >
> > Hi Nathan,
> >
> > >
> > > Hi Dennis,
> > >
> > > I did a bisect of the problematic config against defconfig and it points
> > > out that CONFIG_GCOV_PROFILE_ALL is in the bad config but not the good
> > > config, which makes some sense as that will mess with clang's inlining
> > > heuristics. It does not appear to be the single config that makes a
> > > difference but it gives some clarity.
> > >
> >
> > Ah, thanks. To me it's kind of a corner case that I don't have a lot of
> > insight into. __init code is pretty limited and this warning is really
> > at the compilers whim. However, in this case only clang throws this
> > warning.
> >
> > > I do not personally have any strong opinions around the patch but is it
> > > really that much wasted memory to just annotate mask with __refdata?
> >
> > It's really not much memory, 1 bit per max # of cpus. The reported
> > config is on the extreme side compiling with 8k NR_CPUS, so 1kb. I'm
> > just not in love with the idea of adding a patch to improve readability
> > and it cost idle memory to resolve a compile time warning.
> >
> > If no one else chimes in in the next few days, I'll probably just apply
> > it and go from there. If another issue comes up I'll drop this and tag
> > it as __refdata.
>
> I've come across this one again in linux-next today, and found that
> I had an old patch for it already, that I had never submitted:
>
> From 7d6f40414490092b86f1a64d8c42426ee350da1a Mon Sep 17 00:00:00 2001
> From: Arnd Bergmann <arnd@arndb.de>
> Date: Mon, 7 Dec 2020 23:24:20 +0100
> Subject: [PATCH] mm: percpu: fix section mismatch warning
>
> Building with arm64 clang sometimes (fairly rarely) shows a
> warning about the pcpu_build_alloc_info() function:
>
> WARNING: modpost: vmlinux.o(.text+0x21697c): Section mismatch in
> reference from the function cpumask_clear_cpu() to the variable
> .init.data:pcpu_build_alloc_info.mask
> The function cpumask_clear_cpu() references
> the variable __initdata pcpu_build_alloc_info.mask.
> This is often because cpumask_clear_cpu lacks a __initdata
> annotation or the annotation of pcpu_build_alloc_info.mask is wrong.
>
> What appears to be going on here is that the compiler decides to not
> inline the cpumask_clear_cpu() function that is marked 'inline' but not
> 'always_inline', and it then produces a specialized version of it that
> references the static mask unconditionally as an optimization.
>
> Marking cpumask_clear_cpu() as __always_inline would fix it, as would
> removing the __initdata annotation on the variable.  I went for marking
> the function as __attribute__((flatten)) instead because all functions

I had to look this one up; it's new to me!
https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes
https://awesomekling.github.io/Smarter-C++-inlining-with-attribute-flatten/

Seems pretty cool/flexible to control inlining on the caller side!

At the least though, we should avoid open coding the function attributes.  See
include/linux/compiler_attributes.h

Testing quickly in godbolt, __flatten__ has been supported since at
least clang 3.5 and gcc 4.4, FWIW (so it doesn't need a
__has_attribute guard).

> called from it are really meant to be inlined here, and it prevents
> the same problem happening here again. This is unlikely to be a problem
> elsewhere because there are very few function-local static __initdata
> variables in the kernel.
>
> Fixes: 6c207504ae79 ("percpu: reduce the number of cpu distance comparisons")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
>
> diff --git a/mm/percpu.c b/mm/percpu.c
> index 5ede8dd407d5..527181c46b08 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -2662,10 +2662,9 @@ early_param("percpu_alloc", percpu_alloc_setup);
>   * On success, pointer to the new allocation_info is returned.  On
>   * failure, ERR_PTR value is returned.
>   */
> -static struct pcpu_alloc_info * __init pcpu_build_alloc_info(
> -                               size_t reserved_size, size_t dyn_size,
> -                               size_t atom_size,
> -                               pcpu_fc_cpu_distance_fn_t cpu_distance_fn)
> +static struct pcpu_alloc_info * __init __attribute__((flatten))
> +pcpu_build_alloc_info(size_t reserved_size, size_t dyn_size, size_t atom_size,
> +                     pcpu_fc_cpu_distance_fn_t cpu_distance_fn)
>  {
>         static int group_map[NR_CPUS] __initdata;
>         static int group_cnt[NR_CPUS] __initdata;
>
>
> Not sure if this would be any better than your patch.
>
>        Arnd
>
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-- 
Thanks,
~Nick Desaulniers