[PATCH RFC v2 riscv/for-next 1/5] riscv: align ftrace to 4 Byte boundary and increase ftrace prologue size

[Guo Ren]

On Tue, Sep 13, 2022 at 5:44 PM Andy Chiu <andy.chiu at sifive.com> wrote:
>
> We are introducing a new ftrace mechanism in order to phase out
> stop_machine() and enable kernel preemption. The new mechanism requires
> ftrace patchable function entries to be 24 bytes and aligned to 4 Byte
> boundaries.
>
> Before applying this patch, the size of the kernel code, with 122465 of
> ftrace entries, was at 12.46 MB. Under the same configuration, the size
> has increased to 12.99 MB after applying this patch set.
>
> However, we found the -falign-functions alone was not strong enoungh to
> make all functions align as required. In fact, cold functions are not
> aligned after turning on optimizations. We consider this is a bug in GCC
> and turn off guess-branch-probility as a workaround to align all
> functions.
Disable pgo static optimization would reduce the code performance. I
think we need to fix that problem in GCC, and pgo is a default-enabled
optimization.

>
> GCC bug id: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88345
>
> Signed-off-by: Andy Chiu <andy.chiu at sifive.com>
> Reviewed-by: Greentime Hu <greentime.hu at sifive.com>
> ---
>  arch/riscv/Makefile | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> index 3fa8ef336822..fd8069f59a59 100644
> --- a/arch/riscv/Makefile
> +++ b/arch/riscv/Makefile
> @@ -11,7 +11,7 @@ LDFLAGS_vmlinux :=
>  ifeq ($(CONFIG_DYNAMIC_FTRACE),y)
>         LDFLAGS_vmlinux := --no-relax
>         KBUILD_CPPFLAGS += -DCC_USING_PATCHABLE_FUNCTION_ENTRY
> -       CC_FLAGS_FTRACE := -fpatchable-function-entry=8
> +       CC_FLAGS_FTRACE := -fpatchable-function-entry=12  -falign-functions=4 -fno-guess-branch-probability
Another fixup should be covered, eg:

+ifeq ($(CONFIG_RISCV_ISA_C),y)
+       CC_FLAGS_FTRACE := -fpatchable-function-entry=12
+else
+       CC_FLAGS_FTRACE := -fpatchable-function-entry=6
+endif

>  endif
>
>  ifeq ($(CONFIG_CMODEL_MEDLOW),y)
> --
> 2.36.0
>


-- 
Best Regards
 Guo Ren