On Tue, Jan 5, 2021 at 1:55 AM Dennis Zhou [off-list ref] wrote:

On Mon, Jan 04, 2021 at 04:46:51PM -0700, Nathan Chancellor wrote:

quoted

On Thu, Dec 31, 2020 at 09:28:52PM +0000, Dennis Zhou wrote:

quoted

Hi Nathan,

quoted

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.

quoted

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
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