[PATCH v4 08/14] LoongArch: Select FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY

[George Guo <dongtai.guo@linux.dev>]

From: George Guo <guodongtai@kylinos.cn>

arch/loongarch/Makefile uses -fpatchable-function-entry=2 when
CONFIG_DYNAMIC_FTRACE is set, but nothing selects
FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY, so kbuild falls back to
FTRACE_MCOUNT_USE_RECORDMCOUNT and runs recordmcount on every object.
The pass is useless here: there are no _mcount calls to find, and the
real ftrace sites come from the __patchable_function_entries section
anyway.

It is not just wasted time.  recordmcount needs a non-weak symbol in
each text section, to use as the base of the __mcount_loc relocations
it writes (see find_secsym_ndx()).  GAS emits a section symbol for
every section, so GCC objects always have one.  Clang's integrated
assembler drops unreferenced section symbols, so a clang object must
use the function's own symbol -- but find_secsym_ndx() skips weak
symbols.  So with -ffunction-sections (which klp-build uses), a __weak
function like sched_clock is alone in its section with no usable
symbol, and recordmcount fails:

  Cannot find symbol for section 5: .text.sched_clock.
  kernel/sched/build_utility.o: failed

Select FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY like arm64 and riscv
do, which disables the recordmcount pass altogether.

Reported-by: Joe Lawrence <joe.lawrence@redhat.com>
Suggested-by: Joe Lawrence <joe.lawrence@redhat.com>
Signed-off-by: George Guo <guodongtai@kylinos.cn>
---
 arch/loongarch/Kconfig | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
index 1dbf51ba9d6a..a1549003c871 100644
--- a/arch/loongarch/Kconfig
+++ b/arch/loongarch/Kconfig
@@ -88,6 +88,7 @@ config LOONGARCH
 	select CPU_PM
 	select EDAC_SUPPORT
 	select EFI
+	select FTRACE_MCOUNT_USE_PATCHABLE_FUNCTION_ENTRY if DYNAMIC_FTRACE
 	select GENERIC_ATOMIC64 if 32BIT
 	select GENERIC_CLOCKEVENTS
 	select GENERIC_CMOS_UPDATE
-- 
2.53.0