[PATCH v1 3/4] perf off-cpu: Fix cgroup_filter map sizing

[Ian Rogers <irogers@google.com>]

off_cpu_prepare() assumed that if evlist__first(evlist)->cgrp is set,
the number of cgroups is evlist__nr_entries(evlist) - 1 (excluding the
dummy bpf-output event). This can under-size cgroup_filter when
non-cgroup events precede cgroup events or when no dummy bpf-output
event is added.

Count the number of evsels in evlist that have a cgroup set and use that
count to size and populate cgroup_filter.

Fixes: 685439a7a037 ("perf record: Add cgroup support for off-cpu profiling")
Signed-off-by: Ian Rogers <irogers@google.com>
---
 tools/perf/util/bpf_off_cpu.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/tools/perf/util/bpf_off_cpu.c b/tools/perf/util/bpf_off_cpu.c
index 16948761cc45..71933a71c8d1 100644
--- a/tools/perf/util/bpf_off_cpu.c
+++ b/tools/perf/util/bpf_off_cpu.c
@@ -151,9 +151,10 @@ int off_cpu_prepare(struct evlist *evlist, struct target *target,
 		    struct record_opts *opts)
 {
 	int err, fd, i;
-	int ncpus = 1, ntasks = 1, ncgrps = 1;
+	int ncpus = 1, ntasks = 1, ncgrps = 0;
 	struct strlist *pid_slist = NULL;
 	struct str_node *pos;
+	struct evsel *evsel;
 
 	if (off_cpu_config(evlist) < 0) {
 		pr_err("Failed to config off-cpu BPF event\n");
@@ -208,8 +209,12 @@ int off_cpu_prepare(struct evlist *evlist, struct target *target,
 		skel->rodata->uses_tgid = 1;
 	}
 
-	if (evlist__first(evlist)->cgrp) {
-		ncgrps = evlist__nr_entries(evlist) - 1; /* excluding a dummy */
+	evlist__for_each_entry(evlist, evsel) {
+		if (evsel->cgrp)
+			ncgrps++;
+	}
+
+	if (ncgrps > 0) {
 		bpf_map__set_max_entries(skel->maps.cgroup_filter, ncgrps);
 
 		if (!cgroup_is_v2("perf_event"))
@@ -274,8 +279,7 @@ int off_cpu_prepare(struct evlist *evlist, struct target *target,
 		}
 	}
 
-	if (evlist__first(evlist)->cgrp) {
-		struct evsel *evsel;
+	if (ncgrps > 0) {
 		u8 val = 1;
 
 		fd = bpf_map__fd(skel->maps.cgroup_filter);
-- 
2.56.0.313.g2479322e68-goog