[PATCH] perf evsel: Return exact sub event which failed with EPERM for wildcards

[Jin Yao]

The kernel has a special check for irq_vectors trace event.

TRACE_EVENT_PERF_PERM(irq_work_exit,
	is_sampling_event(p_event) ? -EPERM : 0);

The perf-record is failed for irq_vectors event if using a wildcard.

root@skl:/tmp# perf record -a -e irq_vectors:* sleep 2
Error:
You may not have permission to collect system-wide stats.

Consider tweaking /proc/sys/kernel/perf_event_paranoid,
which controls use of the performance events system by
unprivileged users (without CAP_SYS_ADMIN).

The current value is 2:

  -1: Allow use of (almost) all events by all users
>= 0: Disallow raw tracepoint access by users without CAP_IOC_LOCK
>= 1: Disallow CPU event access by users without CAP_SYS_ADMIN
>= 2: Disallow kernel profiling by users without CAP_SYS_ADMIN

To make this setting permanent, edit /etc/sysctl.conf too, e.g.:

        kernel.perf_event_paranoid = -1

This patch prints out the exact sub event that failed with EPERM
for wildcards to help user understanding easily.

For example,

root@skl:/tmp# perf record -a -e irq_vectors:* sleep 2
Error:
No permission to enable irq_vectors:irq_work_exit event.

You may not have permission to collect system-wide stats.
......

Signed-off-by: Jin Yao <yao.jin@xxxxxxxxxxxxxxx>
---
 tools/perf/util/evsel.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
index 9dc7e2d..8f5d86b 100644
--- a/tools/perf/util/evsel.c
+++ b/tools/perf/util/evsel.c
@@ -2457,11 +2457,17 @@ int perf_evsel__open_strerror(struct perf_evsel *evsel, struct target *target,
 			      int err, char *msg, size_t size)
 {
 	char sbuf[STRERR_BUFSIZE];
+	int printed = 0;
 
 	switch (err) {
 	case EPERM:
 	case EACCES:
-		return scnprintf(msg, size,
+		if (err == EPERM)
+			printed = scnprintf(msg, size,
+				"No permission to enable %s event.\n\n",
+				perf_evsel__name(evsel));
+
+		return scnprintf(msg + printed, size - printed,
 		 "You may not have permission to collect %sstats.\n\n"
 		 "Consider tweaking /proc/sys/kernel/perf_event_paranoid,\n"
 		 "which controls use of the performance events system by\n"
-- 
2.7.4