On Mon, Jan 25, 2021 at 3:07 AM Arnd Bergmann <arnd@xxxxxxxxxx> wrote: > > On Tue, Jan 5, 2021 at 1:55 AM Dennis Zhou <dennis@xxxxxxxxxx> 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@xxxxxxxx> > 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@xxxxxxxx> > > 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 > > -- > You received this message because you are subscribed to the Google Groups "Clang Built Linux" group. > To unsubscribe from this group and stop receiving emails from it, send an email to clang-built-linux+unsubscribe@xxxxxxxxxxxxxxxx. > To view this discussion on the web visit https://groups.google.com/d/msgid/clang-built-linux/CAK8P3a2ZWfNeXKSm8K_SUhhwkor17jFo3xApLXjzfPqX0eUDUA%40mail.gmail.com. -- Thanks, ~Nick Desaulniers
