[PATCH v2 01/10] perf trace: Fix perf trace -p

Howard Chu
2024-08-15T01:36:41Z

perf trace -p <PID> work on a syscall that is unaugmented, but doesn't
work on a syscall that's augmented (when it calls perf_event_output() in
BPF).

Let's take open() as an example. open() is augmented in perf trace.

Before:
```
perf $ perf trace -e open -p 3792392
? ( ): ... [continued]: open()) = -1 ENOENT (No such file or directory)
? ( ): ... [continued]: open()) = -1 ENOENT (No such file or directory)
```

We can see there's no output.

After:
```
perf $ perf trace -e open -p 3792392
0.000 ( 0.123 ms): a.out/3792392 open(filename: "DINGZHEN", flags: WRONLY) = -1 ENOENT (No such file or directory)
1000.398 ( 0.116 ms): a.out/3792392 open(filename: "DINGZHEN", flags: WRONLY) = -1 ENOENT (No such file or directory)
```

Reason:

bpf_perf_event_output() will fail when you specify a pid in perf trace (EOPNOTSUPP).

When using perf trace -p 114, before perf_event_open(), we'll have PID
= 114, and CPU = -1.

This is bad for bpf-output event, because the ring buffer won't accept
output from BPF's perf_event_output(), making it fail. I'm still trying
to find out why.

If we open bpf-output for every cpu, instead of setting it to -1, like
this:
PID = <PID>, CPU = 0
PID = <PID>, CPU = 1
PID = <PID>, CPU = 2
PID = <PID>, CPU = 3
...

Everything works.

You can test it with this script:
```
#include <unistd.h>
#include <sys/syscall.h>

int main()
{
int i1 = 1, i2 = 2, i3 = 3, i4 = 4;
char s1[] = "DINGZHEN", s2[] = "XUEBAO";

while (1) {
syscall(SYS_open, s1, i1, i2);
sleep(1);
}

return 0;
}
```

save, compile
```
gcc open.c
```

perf trace
```
perf trace -e open <path-to-the-executable>
```

Signed-off-by: Howard Chu <howardchu95@gmail.com>
---
tools/perf/util/evlist.c | 14 +++++++++++++-
tools/perf/util/evlist.h | 1 +
2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c
index c0379fa1ccfe..f14b7e6ff1dc 100644
--- a/tools/perf/util/evlist.c
+++ b/tools/perf/util/evlist.c
@@ -1067,7 +1067,7 @@ int evlist__create_maps(struct evlist *evlist, struct target *target)
if (!threads)
return -1;

- if (target__uses_dummy_map(target))
+ if (target__uses_dummy_map(target) && !evlist__has_bpf_output(evlist))
cpus = perf_cpu_map__new_any_cpu();
else
cpus = perf_cpu_map__new(target->cpu_list);
@@ -2627,3 +2627,15 @@ void evlist__uniquify_name(struct evlist *evlist)
}
}
}
+
+bool evlist__has_bpf_output(struct evlist *evlist)
+{
+ struct evsel *evsel;
+
+ evlist__for_each_entry(evlist, evsel) {
+ if (evsel__is_bpf_output(evsel))
+ return true;
+ }
+
+ return false;
+}
diff --git a/tools/perf/util/evlist.h b/tools/perf/util/evlist.h
index b46f1a320783..bcc1c6984bb5 100644
--- a/tools/perf/util/evlist.h
+++ b/tools/perf/util/evlist.h
@@ -447,5 +447,6 @@ int evlist__scnprintf_evsels(struct evlist *evlist, size_t size, char *bf);
void evlist__check_mem_load_aux(struct evlist *evlist);
void evlist__warn_user_requested_cpus(struct evlist *evlist, const char *cpu_list);
void evlist__uniquify_name(struct evlist *evlist);
+bool evlist__has_bpf_output(struct evlist *evlist);

#endif /* __PERF_EVLIST_H */
--
2.45.2