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
