Hi Thomas,

FYI, the error/warning still remains.

tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
head:   5695e51619745d4fe3ec2506a2f0cd982c5e27a4
commit: 1cf12e08bc4d50a76b80c42a3109c53d8794a0c9 sched/hotplug: Consolidate task migration on CPU unplug
date:   4 months ago
config: mips-randconfig-r004-20210228 (attached as .config)
compiler: clang version 13.0.0 (https://github.com/llvm/llvm-project 83bc7815c4235786111aa2abf7193292e4a602f5)
reproduce (this is a W=1 build):
        wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
        chmod +x ~/bin/make.cross
        # install mips cross compiling tool for clang build
        # apt-get install binutils-mips-linux-gnu
        # https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=1cf12e08bc4d50a76b80c42a3109c53d8794a0c9
        git remote add linus https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
        git fetch --no-tags linus master
        git checkout 1cf12e08bc4d50a76b80c42a3109c53d8794a0c9
        # save the attached .config to linux build tree
        COMPILER_INSTALL_PATH=$HOME/0day COMPILER=clang make.cross ARCH=mips 

If you fix the issue, kindly add following tag as appropriate
Reported-by: kernel test robot <lkp@intel.com>

All warnings (new ones prefixed by >>):

   kernel/sched/core.c:2376:6: warning: no previous prototype for function 'sched_set_stop_task'
   void sched_set_stop_task(int cpu, struct task_struct
   ^
   kernel/sched/core.c:2376:1: note: declare 'static' if the function is not intended to be used outside of this translation unit
   void sched_set_stop_task(int cpu, struct task_struct
   ^
   static
   kernel/sched/core.c:4196:20: warning: unused function 'sched_tick_stop'
   static inline void sched_tick_stop(int cpu) { }
   ^
>> kernel/sched/core.c:6854:20: warning: unused function 'balance_hotplug_wait'
   static inline void balance_hotplug_wait(void)
   ^
   fatal error: error in backend: Nested variants found in inline asm string: '.if ( 0x00 ) != -1)) 0x00 ) != -1)) : ($( static struct ftrace_branch_data __attribute__((__aligned__(4))) __attribute__((__section__("_ftrace_branch"))) __if_trace = $( .func = __func__, .file = "arch/mips/include/asm/barrier.h", .line = 27, $); 0x00 ) != -1)) : $))) ) && ( (1 << 0) ); .set push; .set mips64r2; .rept 1; sync 0x00; .endr; .set pop; .else; ; .endif'
   clang-13: error: clang frontend command failed with exit code 70 (use -v to see invocation)
   clang version 13.0.0 (git://gitmirror/llvm_project 83bc7815c4235786111aa2abf7193292e4a602f5)
   Target: mipsel-unknown-linux-gnu
   Thread model: posix
   InstalledDir: /opt/cross/clang-83bc7815c4/bin
   clang-13: note: diagnostic msg:
   Makefile arch drivers fs include kernel mm scripts security source usr


vim +/balance_hotplug_wait +6854 kernel/sched/core.c

2558aacff85866 Peter Zijlstra  2020-09-11  6853  
f2469a1fb43f85 Thomas Gleixner 2020-09-14 @6854  static inline void balance_hotplug_wait(void)
f2469a1fb43f85 Thomas Gleixner 2020-09-14  6855  {
f2469a1fb43f85 Thomas Gleixner 2020-09-14  6856  }
f2469a1fb43f85 Thomas Gleixner 2020-09-14  6857  

:::::: The code at line 6854 was first introduced by commit
:::::: f2469a1fb43f85d243ce72638367fb6e15c33491 sched/core: Wait for tasks being pushed away on hotplug

:::::: TO: Thomas Gleixner <tglx@linutronix.de>
:::::: CC: Peter Zijlstra <peterz@infradead.org>

---
0-DAY CI Kernel Test Service, Intel Corporation
https://lists.01.org/hyperkitty/list/kbuild-all@lists.01.org