[PATCH] 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

Add a helper to determine if a session has a specific event name,
and then use that helper for timehist. For perf sched timehist the
analysis requires at least sched_switch event, so check for it.

Now when analyzing a file without sched tracepoints you get:
    root@f21-vbox:/tmp$ perf sched timehist
    sched_switch event does not exist in file; nothing to analyze

Signed-off-by: David Ahern <dsahern@gmail.com>
---
 tools/perf/builtin-sched.c |  5 ++++-
 tools/perf/util/session.c  | 12 ++++++++++++
 tools/perf/util/session.h  |  2 ++
 3 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/tools/perf/builtin-sched.c b/tools/perf/builtin-sched.c
index 870d94cd20ba..7c3c8176f51f 100644
--- a/tools/perf/builtin-sched.c
+++ b/tools/perf/builtin-sched.c
@@ -2629,8 +2629,11 @@ 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_session__has_event(session, "sched:sched_switch")) {
+		pr_err("sched_switch event does not exist in file; nothing to analyze\n");
 		goto out;
+	}
 
 	if (sched->show_migrations &&
 	    perf_session__set_tracepoints_handlers(session, migrate_handlers))
diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
index f268201048a0..9bb3e03dcd8e 100644
--- a/tools/perf/util/session.c
+++ b/tools/perf/util/session.c
@@ -1897,6 +1897,18 @@ bool perf_session__has_traces(struct perf_session *session, const char *msg)
 	return false;
 }
 
+bool perf_session__has_event(struct perf_session *session, const char *name)
+{
+	struct perf_evsel *evsel;
+
+	evlist__for_each_entry(session->evlist, evsel) {
+		if (!strcmp(perf_evsel__name(evsel), name))
+			return true;
+	}
+
+	return false;
+}
+
 int maps__set_kallsyms_ref_reloc_sym(struct map **maps,
 				     const char *symbol_name, u64 addr)
 {
diff --git a/tools/perf/util/session.h b/tools/perf/util/session.h
index 4bd758553450..8609eca38c7f 100644
--- a/tools/perf/util/session.h
+++ b/tools/perf/util/session.h
@@ -64,6 +64,8 @@ int perf_session__resolve_callchain(struct perf_session *session,
 
 bool perf_session__has_traces(struct perf_session *session, const char *msg);
 
+bool perf_session__has_event(struct perf_session *session, const char *name);
+
 void perf_event__attr_swap(struct perf_event_attr *attr);
 
 int perf_session__create_kernel_maps(struct perf_session *session);
-- 
2.7.4 (Apple Git-66)