[PATCH v2] perf sched timehist: Improve error message when analyzing wrong file

[David Ahern <dsahern@gmail.com>]

Arnaldo reported an unhelpful error message when running perf sched
timehist on a file that did not contain sched tracepoints:

    [root@jouet ~]# perf sched timehist
    No trace sample to read. Did you call 'perf record -R'?

    [root@jouet ~]# perf evlist -v
    cycles:ppp: size: 112, { sample_period, sample_freq }: 4000, sample_type: IP|TID|TIME|CALLCHAIN|CPU|PERIOD, disabled: 1, inherit: 1, mmap: 1, comm: 1, freq: 1, task: 1, precise_ip: 3, sample_id_all: 1, exclude_guest: 1, mmap2: 1, comm_exec: 1

Change the has_traces check to look for the sched_switch event. Analysis
for perf sched timehist requires at least this event.

Now when analyzing a file without sched tracepoints you get:

    root@f21-vbox:/tmp$ perf sched timehist
    No sched_switch events found. Have you run 'perf sched record'?

Signed-off-by: David Ahern <dsahern@gmail.com>
---
 tools/perf/builtin-sched.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/tools/perf/builtin-sched.c b/tools/perf/builtin-sched.c
index 870d94cd20ba..84c39f177bef 100644
--- a/tools/perf/builtin-sched.c
+++ b/tools/perf/builtin-sched.c
@@ -2629,8 +2629,12 @@ static int perf_sched__timehist(struct perf_sched *sched)
 	if (perf_session__set_tracepoints_handlers(session, handlers))
 		goto out;
 
-	if (!perf_session__has_traces(session, "record -R"))
+	/* sched_switch event at a minimum needs to exist */
+	if (!perf_evlist__find_tracepoint_by_name(session->evlist,
+						  "sched:sched_switch")) {
+		pr_err("No sched_switch events found. Have you run 'perf sched record'?\n");
 		goto out;
+	}
 
 	if (sched->show_migrations &&
 	    perf_session__set_tracepoints_handlers(session, migrate_handlers))
-- 
2.7.4 (Apple Git-66)