perf ftrace latency: 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 ftrace latency --use-bpf -T schedule
^C# DURATION | COUNT | GRAPH |
0 - 1 us | 0 | |
1 - 2 us | 0 | |
2 - 4 us | 0 | |
4 - 8 us | 0 | |
8 - 16 us | 1 | |
16 - 32 us | 5 | |
32 - 64 us | 2 | |
64 - 128 us | 6 | |
128 - 256 us | 7 | |
256 - 512 us | 5 | |
512 - 1024 us | 22 | # |
1 - 2 ms | 36 | ## |
2 - 4 ms | 68 | ##### |
4 - 8 ms | 22 | # |
8 - 16 ms | 91 | ####### |
16 - 32 ms | 11 | |
32 - 64 ms | 26 | ## |
64 - 128 ms | 213 | ################# |
128 - 256 ms | 19 | # |
256 - 512 ms | 14 | # |
512 - 1024 ms | 5 | |
1 - ... s | 8 | |
root@x1:~#

root@x1:~# perf trace -e bpf perf ftrace latency --use-bpf -T schedule
0.000 ( 0.015 ms): perf/2944525 bpf(cmd: 36, uattr: 0x7ffe80de7b40, size: 8) = -1 EOPNOTSUPP (Operation not supported)
0.025 ( 0.102 ms): perf/2944525 bpf(cmd: PROG_LOAD, uattr: 0x7ffe80de7870, size: 148) = 8
0.136 ( 0.026 ms): perf/2944525 bpf(cmd: PROG_LOAD, uattr: 0x7ffe80de7930, size: 148) = 8
0.174 ( 0.026 ms): perf/2944525 bpf(cmd: PROG_LOAD, uattr: 0x7ffe80de77e0, size: 148) = 8
0.205 ( 0.010 ms): perf/2944525 bpf(uattr: 0x7ffe80de7990, size: 80) = 8
0.227 ( 0.011 ms): perf/2944525 bpf(cmd: BTF_LOAD, uattr: 0x7ffe80de7810, size: 40) = 8
0.244 ( 0.004 ms): perf/2944525 bpf(cmd: BTF_LOAD, uattr: 0x7ffe80de7880, size: 40) = 8
0.257 ( 0.006 ms): perf/2944525 bpf(cmd: BTF_LOAD, uattr: 0x7ffe80de7660, size: 40) = 8
0.265 ( 0.058 ms): perf/2944525 bpf(cmd: PROG_LOAD, uattr: 0x7ffe80de7730, size: 148) = 9
0.330 ( 0.004 ms): perf/2944525 bpf(cmd: BTF_LOAD, uattr: 0x7ffe80de78e0, size: 40) = 8
0.337 ( 0.003 ms): perf/2944525 bpf(cmd: BTF_LOAD, uattr: 0x7ffe80de7890, size: 40) = 8
0.343 ( 0.004 ms): perf/2944525 bpf(cmd: BTF_LOAD, uattr: 0x7ffe80de7880, size: 40) = 8
0.349 ( 0.003 ms): perf/2944525 bpf(cmd: BTF_LOAD, uattr: 0x7ffe80de78b0, size: 40) = 8
0.355 ( 0.004 ms): perf/2944525 bpf(cmd: BTF_LOAD, uattr: 0x7ffe80de7890, size: 40) = 8
0.361 ( 0.003 ms): perf/2944525 bpf(cmd: BTF_LOAD, uattr: 0x7ffe80de78b0, size: 40) = 8
0.367 ( 0.003 ms): perf/2944525 bpf(cmd: BTF_LOAD, uattr: 0x7ffe80de7880, size: 40) = 8
0.373 ( 0.014 ms): perf/2944525 bpf(cmd: BTF_LOAD, uattr: 0x7ffe80de7a00, size: 40) = 8
0.390 ( 0.358 ms): perf/2944525 bpf(uattr: 0x7ffe80de7950, size: 80) = 9
0.763 ( 0.014 ms): perf/2944525 bpf(uattr: 0x7ffe80de7950, size: 80) = 9
0.783 ( 0.011 ms): perf/2944525 bpf(uattr: 0x7ffe80de7950, size: 80) = 9
0.798 ( 0.017 ms): perf/2944525 bpf(uattr: 0x7ffe80de7950, size: 80) = 9
0.819 ( 0.003 ms): perf/2944525 bpf(uattr: 0x7ffe80de7700, size: 80) = 9
0.824 ( 0.047 ms): perf/2944525 bpf(cmd: PROG_LOAD, uattr: 0x7ffe80de76c0, size: 148) = 10
0.878 ( 0.008 ms): perf/2944525 bpf(uattr: 0x7ffe80de7950, size: 80) = 9
0.891 ( 0.014 ms): perf/2944525 bpf(cmd: MAP_UPDATE_ELEM, uattr: 0x7ffe80de79e0, size: 32) = 0
0.910 ( 0.103 ms): perf/2944525 bpf(cmd: PROG_LOAD, uattr: 0x7ffe80de7880, size: 148) = 9
1.016 ( 0.143 ms): perf/2944525 bpf(cmd: PROG_LOAD, uattr: 0x7ffe80de7880, size: 148) = 10
3.777 ( 0.068 ms): perf/2944525 bpf(cmd: PROG_LOAD, uattr: 0x7ffe80de7570, size: 148) = 12
3.848 ( 0.003 ms): perf/2944525 bpf(cmd: LINK_CREATE, uattr: 0x7ffe80de7550, size: 64) = -1 EBADF (Bad file descriptor)
3.859 ( 0.006 ms): perf/2944525 bpf(cmd: LINK_CREATE, uattr: 0x7ffe80de77c0, size: 64) = 12
6.504 ( 0.010 ms): perf/2944525 bpf(cmd: LINK_CREATE, uattr: 0x7ffe80de77c0, size: 64) = 14
^C# DURATION | COUNT | GRAPH |
0 - 1 us | 0 | |
1 - 2 us | 0 | |
2 - 4 us | 1 | |
4 - 8 us | 3 | |
8 - 16 us | 3 | |
16 - 32 us | 11 | |
32 - 64 us | 9 | |
64 - 128 us | 17 | |
128 - 256 us | 30 | # |
256 - 512 us | 20 | |
512 - 1024 us | 42 | # |
1 - 2 ms | 151 | ###### |
2 - 4 ms | 106 | #### |
4 - 8 ms | 18 | |
8 - 16 ms | 149 | ###### |
16 - 32 ms | 30 | # |
32 - 64 ms | 17 | |
64 - 128 ms | 360 | ############### |
128 - 256 ms | 52 | ## |
256 - 512 ms | 18 | |
512 - 1024 ms | 28 | # |
1 - ... s | 5 | |
root@x1:~#

Signed-off-by: Namhyung Kim <[e-mail]>
Tested-by: Arnaldo Carvalho de Melo <[e-mail]>
Cc: Adrian Hunter <[e-mail]>
Cc: Ian Rogers <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Kan Liang <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Song Liu <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/bpf_ftrace.c

···
 	if (ftrace->target.cpu_list) {
 		ncpus = perf_cpu_map__nr(ftrace->evlist->core.user_requested_cpus);
 		bpf_map__set_max_entries(skel->maps.cpu_filter, ncpus);
+		skel->rodata->has_cpu = 1;
 	}
 
 	if (target__has_task(&ftrace->target) || target__none(&ftrace->target)) {
 		ntasks = perf_thread_map__nr(ftrace->evlist->core.threads);
 		bpf_map__set_max_entries(skel->maps.task_filter, ntasks);
+		skel->rodata->has_task = 1;
 	}
+
+	skel->rodata->use_nsec = ftrace->use_nsec;
 
 	set_max_rlimit();
 
···
 		u32 cpu;
 		u8 val = 1;
 
-		skel->bss->has_cpu = 1;
 		fd = bpf_map__fd(skel->maps.cpu_filter);
 
 		for (i = 0; i < ncpus; i++) {
···
 		u32 pid;
 		u8 val = 1;
 
-		skel->bss->has_task = 1;
 		fd = bpf_map__fd(skel->maps.task_filter);
 
 		for (i = 0; i < ntasks; i++) {
···
 			bpf_map_update_elem(fd, &pid, &val, BPF_ANY);
 		}
 	}
-
-	skel->bss->use_nsec = ftrace->use_nsec;
 
 	skel->links.func_begin = bpf_program__attach_kprobe(skel->progs.func_begin,
 							    false, func->name);

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

···
 
 
 int enabled = 0;
-int has_cpu = 0;
-int has_task = 0;
-int use_nsec = 0;
+
+const volatile int has_cpu = 0;
+const volatile int has_task = 0;
+const volatile int use_nsec = 0;
 
 SEC("kprobe/func")
 int BPF_PROG(func_begin)