FIX Re: [PATCH v7 3/3] perf-stat: enable counting events for BPF programs

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Wed, Jan 20, 2021 at 10:50:13AM -0300, Arnaldo Carvalho de Melo escreveu:
> So sizeof(struct bpf_perf_event_value) == 24 and it is a per-cpu array, the
> machine has 24 cpus, why is the kernel thinking it has more and end up zeroing
> entries after the 24 cores? Some percpu map subtlety (or obvious thing ;-\) I'm
> missing?
> 
> Checking lookups into per cpu maps in sample code now...
 
(gdb) run stat -b 244 -I 1000 -e cycles
Starting program: /root/bin/perf stat -b 244 -I 1000 -e cycles
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
libbpf: elf: skipping unrecognized data section(9) .eh_frame
libbpf: elf: skipping relo section(15) .rel.eh_frame for section(9) .eh_frame

Breakpoint 1, bpf_program_profiler__read (evsel=0xce02c0) at util/bpf_counter.c:217
217		if (list_empty(&evsel->bpf_counter_list))
(gdb) p num_
num_cpu              num_groups           num_leaps            num_print_iv         num_stmts            num_transitions      num_warnings_issued
num_cpu_bpf          num_ifs              num_print_interval   num_srcfiles         num_to_str           num_types
(gdb) p num_cpu
$1 = 24
(gdb) p num_cpu_bpf
$2 = 32
(gdb)

Humm, why?

But then libbpf and the sample/bpf/ code use it this way:


diff --git a/tools/perf/util/bpf_counter.c b/tools/perf/util/bpf_counter.c
index 8c977f038f497fc1..7dd3d57aba4f620c 100644
--- a/tools/perf/util/bpf_counter.c
+++ b/tools/perf/util/bpf_counter.c
@@ -207,7 +207,8 @@ static int bpf_program_profiler__enable(struct evsel *evsel)
 static int bpf_program_profiler__read(struct evsel *evsel)
 {
 	int num_cpu = evsel__nr_cpus(evsel);
-	struct bpf_perf_event_value values[num_cpu];
+	int num_cpu_bpf = libbpf_num_possible_cpus();
+	struct bpf_perf_event_value values[num_cpu > num_cpu_bpf ? num_cpu : num_cpu_bpf];
 	struct bpf_counter *counter;
 	int reading_map_fd;
 	__u32 key = 0;

-------------------------------------------------------------

[root@five ~]# cat /sys/devices/system/cpu/possible
0-31
[root@five ~]#

I bet that in your test systems evsel__nr_cpus(evsel) matches
/sys/devices/system/cpu/possible and thus you don't see the problem.

evsel__nr_cpus(evsel) uses what is in:

[acme@five perf]$ cat /sys/devices/system/cpu/online
0-23
[acme@five perf]$

So that is the reason for the problem and the fix is to use
libbpf_num_possible_cpus(), I'll bolt that into the patch that
introduced that code.

- Arnaldo