[tip:perf/core] perf sched timehist: Improve error message when analyzing wrong file

[tip-bot for David Ahern <tipbot@zytor.com>]

Commit-ID:  f45bf8d3939d8f32f06ec12e56d2b8651c5437b4
Gitweb:     http://git.kernel.org/tip/f45bf8d3939d8f32f06ec12e56d2b8651c5437b4
Author:     David Ahern <dsahern@gmail.com>
AuthorDate: Tue, 29 Nov 2016 13:39:48 -0700
Committer:  Arnaldo Carvalho de Melo <acme@redhat.com>
CommitDate: Wed, 7 Dec 2016 12:00:32 -0300

perf sched timehist: Improve error message when analyzing wrong file

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>
Reported-and-Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1480451988-43673-1-git-send-email-dsahern@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.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 870d94c..84c39f1 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))