[PATCH] tools/perf: Fix usage of perf probe when libtraceevent is missing

[Athira Rajeev]

While parsing the tracepoint events in parse_events_add_tracepoint()
function, code checks for HAVE_LIBTRACEEVENT support. This is needed
since libtraceevent is necessary for tracepoint. But while adding
probe points, check for LIBTRACEEVENT is not done in case of perf probe.
Hence, in environment with missing libtraceevent-devel, it is
observed that adding a probe point shows below message though it
can't be used via perf record.

Example:
Adding probe point:
	./perf probe 'vfs_getname=getname_flags:72 pathname=result->name:string'
	Added new event:
	  probe:vfs_getname    (on getname_flags:72 with pathname=result->name:string)

	You can now use it in all perf tools, such as:

		perf record -e probe:vfs_getname -aR sleep 1

But trying perf record:
	./perf  record -e probe:vfs_getname -aR sleep 1
	event syntax error: 'probe:vfs_getname'
				\___ unsupported tracepoint
	libtraceevent is necessary for tracepoint support
	Run 'perf list' for a list of valid events

The builtin tool like perf record needs libtraceevent to
parse tracefs. But still the probe can be used by enabling
via tracefs. Patch fixes the probe usage message to the user
based on presence of libtraceevent. With the fix,

 # ./perf probe 'pmu:myprobe=schedule'
 Added new event:
   pmu:myprobe          (on schedule)

 perf is not linked with libtraceevent, to use the new probe you can use tracefs:

	cd /sys/kernel/tracing/
	echo 1 > events/pmu/myprobe/enable
	echo 1 > tracing_on
	cat trace_pipe
	Before removing the probe, echo 0 > events/pmu/myprobe/enable

Signed-off-by: Athira Rajeev <atrajeev at linux.vnet.ibm.com>
---
 tools/perf/builtin-probe.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/tools/perf/builtin-probe.c b/tools/perf/builtin-probe.c
index ed73d0b89ca2..e72f6cea76f7 100644
--- a/tools/perf/builtin-probe.c
+++ b/tools/perf/builtin-probe.c
@@ -383,9 +383,18 @@ static int perf_add_probe_events(struct perf_probe_event *pevs, int npevs)
 
 	/* Note that it is possible to skip all events because of blacklist */
 	if (event) {
+#ifndef HAVE_LIBTRACEEVENT
+		pr_info("\nperf is not linked with libtraceevent, to use the new probe you can use tracefs:\n\n");
+		pr_info("\tcd /sys/kernel/tracing/\n");
+		pr_info("\techo 1 > events/%s/%s/enable\n", group, event);
+		pr_info("\techo 1 > tracing_on\n");
+		pr_info("\tcat trace_pipe\n");
+		pr_info("\tBefore removing the probe, echo 0 > events/%s/%s/enable\n", group, event);
+#else
 		/* Show how to use the event. */
 		pr_info("\nYou can now use it in all perf tools, such as:\n\n");
 		pr_info("\tperf record -e %s:%s -aR sleep 1\n\n", group, event);
+#endif
 	}
 
 out_cleanup:
-- 
2.39.0