perf stat: Constify control data for BPF

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>

tools/perf/util/bpf_counter_cgroup.c

@@ -61,6 +61,9 @@ static int bperf_load_program(struct evlist *evlist)
 	skel->rodata->num_cpus = total_cpus;
 	skel->rodata->num_events = evlist->core.nr_entries / nr_cgroups;
 
+	if (cgroup_is_v2("perf_event") > 0)
+		skel->rodata->use_cgroup_v2 = 1;
+
 	BUG_ON(evlist->core.nr_entries % nr_cgroups != 0);
 
 	/* we need one copy of events per cpu for reading */
@@ -82,9 +85,6 @@ static int bperf_load_program(struct evlist *evlist)
 		goto out;
 	}
 
-	if (cgroup_is_v2("perf_event") > 0)
-		skel->bss->use_cgroup_v2 = 1;
-
 	err = -1;
 
 	cgrp_switch = evsel__new(&cgrp_switch_attr);

tools/perf/util/bpf_skel/bperf_cgroup.bpf.c

@@ -57,9 +57,9 @@ struct cgroup___old {
 
 const volatile __u32 num_events = 1;
 const volatile __u32 num_cpus = 1;
+const volatile int use_cgroup_v2 = 0;
 
 int enabled = 0;
-int use_cgroup_v2 = 0;
 int perf_subsys_id = -1;
 
 static inline __u64 get_cgroup_v1_ancestor_id(struct cgroup *cgrp, int level)