Re: [PATCH] percpu: Drop CONFIG_DEBUG_FORCE_WEAK_PER_CPU

[Suren Baghdasaryan]

On Wed, Aug 12, 2026 at 12:55 PM Tejun Heo <tj@xxxxxxxxxx> wrote:
>
> alpha requires percpu variables in modules to be defined as weak so that the
> compiler generates GOT based external references for them. This puts two
> extra restrictions on percpu variable definitions. The symbol must be
> globally unique even when static and a static percpu variable can't be
> defined inside a function. DEBUG_FORCE_WEAK_PER_CPU exists to give generic
> code build coverage for these restrictions without building for alpha.
>
> MEM_ALLOC_PROFILING defines a static percpu counter at each allocation call
> site and thus can't be built with weak percpu definitions, so it depends on
> !DEBUG_FORCE_WEAK_PER_CPU. As allmodconfig enables DEBUG_FORCE_WEAK_PER_CPU,
> this knocks MEM_ALLOC_PROFILING out of allmodconfig build coverage.
>
> allmodconfig coverage for MEM_ALLOC_PROFILING is worth more than build
> coverage for restrictions which only matter to alpha module builds. Drop
> DEBUG_FORCE_WEAK_PER_CPU. Restriction violations will now show up only on
> alpha builds.
>
> Reported-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>

Reviewed-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>

Thanks Tejun and Andrew!

> ---
>  include/linux/percpu-defs.h |    7 +------
>  include/rv/da_monitor.h     |    2 +-
>  lib/Kconfig.debug           |   16 ----------------
>  3 files changed, 2 insertions(+), 23 deletions(-)
>
> --- a/include/linux/percpu-defs.h
> +++ b/include/linux/percpu-defs.h
> @@ -65,13 +65,8 @@
>   *
>   * Archs which need weak percpu definitions should set
>   * CONFIG_ARCH_MODULE_NEEDS_WEAK_PER_CPU when necessary.
> - *
> - * To ensure that the generic code observes the above two
> - * restrictions, if CONFIG_DEBUG_FORCE_WEAK_PER_CPU is set weak
> - * definition is used for all cases.
>   */
> -#if (defined(CONFIG_ARCH_MODULE_NEEDS_WEAK_PER_CPU) && defined(MODULE)) || \
> -       defined(CONFIG_DEBUG_FORCE_WEAK_PER_CPU)
> +#if defined(CONFIG_ARCH_MODULE_NEEDS_WEAK_PER_CPU) && defined(MODULE)
>  /*
>   * __pcpu_scope_* dummy variable is used to enforce scope.  It
>   * receives the static modifier when it's used in front of
> --- a/include/rv/da_monitor.h
> +++ b/include/rv/da_monitor.h
> @@ -24,7 +24,7 @@
>
>  /*
>   * Per-cpu variables require a unique name although static in some
> - * configurations (e.g. CONFIG_DEBUG_FORCE_WEAK_PER_CPU or alpha modules).
> + * configurations (e.g. alpha modules).
>   */
>  #define DA_MON_NAME CONCATENATE(da_mon_, MONITOR_NAME)
>
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -613,21 +613,6 @@ config BUILTIN_MODULE_RANGES
>          It also records an anchor symbol to determine the load address of the
>          section.
>
> -config DEBUG_FORCE_WEAK_PER_CPU
> -       bool "Force weak per-cpu definitions"
> -       depends on DEBUG_KERNEL
> -       help
> -         s390 and alpha require percpu variables in modules to be
> -         defined weak to work around addressing range issue which
> -         puts the following two restrictions on percpu variable
> -         definitions.
> -
> -         1. percpu symbols must be unique whether static or not
> -         2. percpu variables can't be defined inside a function
> -
> -         To ensure that generic code follows the above rules, this
> -         option forces all percpu variables to be defined as weak.
> -
>  config WARN_CONTEXT_ANALYSIS
>         bool "Compiler context-analysis warnings"
>         depends on CC_IS_CLANG && CLANG_VERSION >= 230000
> @@ -1053,7 +1038,6 @@ config MEM_ALLOC_PROFILING
>         default n
>         depends on MMU
>         depends on PROC_FS
> -       depends on !DEBUG_FORCE_WEAK_PER_CPU
>         select CODE_TAGGING
>         select PAGE_EXTENSION
>         select SLAB_OBJ_EXT