perf ftrace: Detect workload failure

Currently there's no error message prompted if we failed to start
workload.  And we still get some trace which is confusing. Let's tell
users what happened.

Committer testing:

Before:

    # perf ftrace nonsense |& head
     5)               |  switch_mm_irqs_off() {
     5)   0.400 us    |    load_new_mm_cr3();
     5)   3.261 us    |  }
     ------------------------------------------
     5)    <idle>-0    =>   <...>-3494
     ------------------------------------------

     5)               |  finish_task_switch() {
     5)   ==========> |
     5)               |    smp_irq_work_interrupt() {
    # type nonsense
    -bash: type: nonsense: not found
    #

After:

  # perf ftrace nonsense |& head
  workload failed: No such file or directory
  # type nonsense
  -bash: type: nonsense: not found
  #
Signed-off-by: Changbin Du <changbin.du@gmail.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt (VMware) <rostedt@goodmis.org>
Link: http://lore.kernel.org/lkml/20200510150628.16610-3-changbin.du@gmail.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-ftrace.c

@@ -45,6 +45,7 @@ struct filter_entry {
 	char			name[];
 };
 
+static volatile int workload_exec_errno;
 static bool done;
 
 static void sig_handler(int sig __maybe_unused)
@@ -63,7 +64,7 @@ static void ftrace__workload_exec_failed_signal(int signo __maybe_unused,
 						siginfo_t *info __maybe_unused,
 						void *ucontext __maybe_unused)
 {
-	/* workload_exec_errno = info->si_value.sival_int; */
+	workload_exec_errno = info->si_value.sival_int;
 	done = true;
 }
 
@@ -383,6 +384,14 @@ static int __cmd_ftrace(struct perf_ftrace *ftrace, int argc, const char **argv)
 
 	write_tracing_file("tracing_on", "0");
 
+	if (workload_exec_errno) {
+		const char *emsg = str_error_r(workload_exec_errno, buf, sizeof(buf));
+		/* flush stdout first so below error msg appears at the end. */
+		fflush(stdout);
+		pr_err("workload failed: %s\n", emsg);
+		goto out_close_fd;
+	}
+
 	/* read remaining buffer contents */
 	while (true) {
 		int n = read(trace_fd, buf, sizeof(buf));
@@ -397,7 +406,7 @@ static int __cmd_ftrace(struct perf_ftrace *ftrace, int argc, const char **argv)
 out_reset:
 	reset_tracing_files(ftrace);
 out:
-	return done ? 0 : -1;
+	return (done && !workload_exec_errno) ? 0 : -1;
 }
 
 static int perf_ftrace_config(const char *var, const char *value, void *cb)