The control knobs set before loading BPF programs should be declared as
'const volatile' so that it can be optimized by the BPF core.

Committer testing:

  root@x1:~# perf stat --bpf-counters -e cpu_core/cycles/,cpu_core/instructions/ sleep 1

   Performance counter stats for 'sleep 1':

           2,442,583      cpu_core/cycles/
           2,494,425      cpu_core/instructions/

         1.002687372 seconds time elapsed

         0.001126000 seconds user
         0.001166000 seconds sys

  root@x1:~# perf trace -e bpf --max-events 10 perf stat --bpf-counters -e cpu_core/cycles/,cpu_core/instructions/ sleep 1
       0.000 ( 0.019 ms): perf/2944119 bpf(cmd: OBJ_GET, uattr: 0x7fffdf5cdd40, size: 20)            = 5
       0.021 ( 0.002 ms): perf/2944119 bpf(cmd: OBJ_GET_INFO_BY_FD, uattr: 0x7fffdf5cdcd0, size: 16) = 0
       0.030 ( 0.005 ms): perf/2944119 bpf(cmd: MAP_LOOKUP_ELEM, uattr: 0x7fffdf5ceda0, size: 32)    = 0
       0.037 ( 0.004 ms): perf/2944119 bpf(cmd: LINK_GET_FD_BY_ID, uattr: 0x7fffdf5ced80, size: 12)  = -1 ENOENT (No such file or directory)
       0.189 ( 0.004 ms): perf/2944119 bpf(cmd: 36, uattr: 0x7fffdf5cec10, size: 8)                  = -1 EOPNOTSUPP (Operation not supported)
       0.201 ( 0.095 ms): perf/2944119 bpf(cmd: PROG_LOAD, uattr: 0x7fffdf5ce940, size: 148)         = 10
       0.305 ( 0.026 ms): perf/2944119 bpf(cmd: PROG_LOAD, uattr: 0x7fffdf5cea00, size: 148)         = 10
       0.347 ( 0.012 ms): perf/2944119 bpf(cmd: BTF_LOAD, uattr: 0x7fffdf5ce8e0, size: 40)           = 10
       0.364 ( 0.004 ms): perf/2944119 bpf(cmd: BTF_LOAD, uattr: 0x7fffdf5ce950, size: 40)           = 10
       0.376 ( 0.006 ms): perf/2944119 bpf(cmd: BTF_LOAD, uattr: 0x7fffdf5ce730, size: 40)           = 10
  root@x1:~#
   Performance counter stats for 'sleep 1':

             271,221      cpu_core/cycles/
             139,150      cpu_core/instructions/

         1.002881677 seconds time elapsed

         0.001318000 seconds user
         0.001314000 seconds sys

  root@x1:~#
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20240902200515.2103769-2-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>