[PATCH] perf stat: Support old kernels for bperf cgroup counting

[Namhyung Kim]

The recent change in the cgroup will break the backward compatiblity in
the BPF program.  It should support both old and new kernels using BPF
CO-RE technique.

Like the task_struct->__state handling in the offcpu analysis, we can
check the field name in the cgroup struct.

Signed-off-by: Namhyung Kim <namhyung@xxxxxxxxxx>
---
Arnaldo, I think this should go through the cgroup tree since it depends
on the earlier change there.  I don't think it'd conflict with other
perf changes but please let me know if you see any trouble, thanks!

 tools/perf/util/bpf_skel/bperf_cgroup.bpf.c | 29 ++++++++++++++++++++-
 1 file changed, 28 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/bpf_skel/bperf_cgroup.bpf.c b/tools/perf/util/bpf_skel/bperf_cgroup.bpf.c
index 488bd398f01d..4fe61043de04 100644
--- a/tools/perf/util/bpf_skel/bperf_cgroup.bpf.c
+++ b/tools/perf/util/bpf_skel/bperf_cgroup.bpf.c
@@ -43,12 +43,39 @@ struct {
 	__uint(value_size, sizeof(struct bpf_perf_event_value));
 } cgrp_readings SEC(".maps");
 
+/* new kernel cgroup definition */
+struct cgroup___new {
+	int level;
+	struct cgroup *ancestors[];
+} __attribute__((preserve_access_index));
+
+/* old kernel cgroup definition */
+struct cgroup___old {
+	int level;
+	u64 ancestor_ids[];
+} __attribute__((preserve_access_index));
+
 const volatile __u32 num_events = 1;
 const volatile __u32 num_cpus = 1;
 
 int enabled = 0;
 int use_cgroup_v2 = 0;
 
+static inline __u64 get_cgroup_v1_ancestor_id(struct cgroup *cgrp, int level)
+{
+	/* recast pointer to capture new type for compiler */
+	struct cgroup___new *cgrp_new = (void *)cgrp;
+
+	if (bpf_core_field_exists(cgrp_new->ancestors)) {
+		return BPF_CORE_READ(cgrp_new, ancestors[level], kn, id);
+	} else {
+		/* recast pointer to capture old type for compiler */
+		struct cgroup___old *cgrp_old = (void *)cgrp;
+
+		return BPF_CORE_READ(cgrp_old, ancestor_ids[level]);
+	}
+}
+
 static inline int get_cgroup_v1_idx(__u32 *cgrps, int size)
 {
 	struct task_struct *p = (void *)bpf_get_current_task();
@@ -70,7 +97,7 @@ static inline int get_cgroup_v1_idx(__u32 *cgrps, int size)
 			break;
 
 		// convert cgroup-id to a map index
-		cgrp_id = BPF_CORE_READ(cgrp, ancestors[i], kn, id);
+		cgrp_id = get_cgroup_v1_ancestor_id(cgrp, i);
 		elem = bpf_map_lookup_elem(&cgrp_idx, &cgrp_id);
 		if (!elem)
 			continue;
-- 
2.37.3.968.ga6b4b080e4-goog