Hi Song,

I love your patch! Yet something to improve:

[auto build test ERROR on bpf-next/master]

url:    https://github.com/0day-ci/linux/commits/Song-Liu/bpf-introduce-bpf_get_branch_snapshot/20210831-054455
base:   https://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next.git master
config: x86_64-randconfig-a003-20210831 (attached as .config)
compiler: gcc-9 (Debian 9.3.0-22) 9.3.0
reproduce (this is a W=1 build):
        # https://github.com/0day-ci/linux/commit/09548da8ea3e9a75d0d969b18c148ce17ed1c97d
        git remote add linux-review https://github.com/0day-ci/linux
        git fetch --no-tags linux-review Song-Liu/bpf-introduce-bpf_get_branch_snapshot/20210831-054455
        git checkout 09548da8ea3e9a75d0d969b18c148ce17ed1c97d
        # save the attached .config to linux build tree
        mkdir build_dir
        make W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash

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

All errors (new ones prefixed by >>):

   ld: kernel/bpf/trampoline.o: in function `__bpf_prog_enter':
>> kernel/bpf/trampoline.c:578: undefined reference to `bpf_perf_branch_snapshot'


vim +578 kernel/bpf/trampoline.c

   551	
   552	/* The logic is similar to bpf_prog_run(), but with an explicit
   553	 * rcu_read_lock() and migrate_disable() which are required
   554	 * for the trampoline. The macro is split into
   555	 * call __bpf_prog_enter
   556	 * call prog->bpf_func
   557	 * call __bpf_prog_exit
   558	 *
   559	 * __bpf_prog_enter returns:
   560	 * 0 - skip execution of the bpf prog
   561	 * 1 - execute bpf prog
   562	 * [2..MAX_U64] - execute bpf prog and record execution time.
   563	 *     This is start time.
   564	 */
   565	u64 notrace __bpf_prog_enter(struct bpf_prog *prog)
   566		__acquires(RCU)
   567	{
   568	#ifdef CONFIG_PERF_EVENTS
   569		/* Calling migrate_disable costs two entries in the LBR. To save
   570		 * some entries, we call perf_snapshot_branch_stack before
   571		 * migrate_disable to save some entries. This is OK because we
   572		 * care about the branch trace before entering the BPF program.
   573		 * If migrate happens exactly here, there isn't much we can do to
   574		 * preserve the data.
   575		 */
   576		if (prog->call_get_branch)
   577			static_call(perf_snapshot_branch_stack)(
 > 578				this_cpu_ptr(&bpf_perf_branch_snapshot));
   579	#endif
   580		rcu_read_lock();
   581		migrate_disable();
   582		if (unlikely(__this_cpu_inc_return(*(prog->active)) != 1)) {
   583			inc_misses_counter(prog);
   584			return 0;
   585		}
   586		return bpf_prog_start_time();
   587	}
   588	

---
0-DAY CI Kernel Test Service, Intel Corporation
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