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Subject[PATCH v4] perf tools: Get a perf cgroup more portably in BPF
Date
The perf_event_cgrp_id can be different on other configurations.
To be more portable as CO-RE, it needs to get the cgroup subsys id
using the bpf_core_enum_value() helper.

Suggested-by: Ian Rogers <irogers@google.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
---
v4 changes)
* add a missing check in the off_cpu

v3 changes)
* check compiler features for enum value

v2 changes)
* fix off_cpu.bpf.c too
* get perf_subsys_id only once

tools/perf/util/bpf_skel/bperf_cgroup.bpf.c | 11 ++++++++++-
tools/perf/util/bpf_skel/off_cpu.bpf.c | 18 ++++++++++++++----
2 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/tools/perf/util/bpf_skel/bperf_cgroup.bpf.c b/tools/perf/util/bpf_skel/bperf_cgroup.bpf.c
index 292c430768b5..8e7520e273db 100644
--- a/tools/perf/util/bpf_skel/bperf_cgroup.bpf.c
+++ b/tools/perf/util/bpf_skel/bperf_cgroup.bpf.c
@@ -48,6 +48,7 @@ const volatile __u32 num_cpus = 1;

int enabled = 0;
int use_cgroup_v2 = 0;
+int perf_subsys_id = -1;

static inline int get_cgroup_v1_idx(__u32 *cgrps, int size)
{
@@ -58,7 +59,15 @@ static inline int get_cgroup_v1_idx(__u32 *cgrps, int size)
int level;
int cnt;

- cgrp = BPF_CORE_READ(p, cgroups, subsys[perf_event_cgrp_id], cgroup);
+ if (perf_subsys_id == -1) {
+#if __has_builtin(__builtin_preserve_enum_value)
+ perf_subsys_id = bpf_core_enum_value(enum cgroup_subsys_id,
+ perf_event_cgrp_id);
+#else
+ perf_subsys_id = perf_event_cgrp_id;
+#endif
+ }
+ cgrp = BPF_CORE_READ(p, cgroups, subsys[perf_subsys_id], cgroup);
level = BPF_CORE_READ(cgrp, level);

for (cnt = 0; i < MAX_LEVELS; i++) {
diff --git a/tools/perf/util/bpf_skel/off_cpu.bpf.c b/tools/perf/util/bpf_skel/off_cpu.bpf.c
index c4ba2bcf179f..38e3b287dbb2 100644
--- a/tools/perf/util/bpf_skel/off_cpu.bpf.c
+++ b/tools/perf/util/bpf_skel/off_cpu.bpf.c
@@ -94,6 +94,8 @@ const volatile bool has_prev_state = false;
const volatile bool needs_cgroup = false;
const volatile bool uses_cgroup_v1 = false;

+int perf_subsys_id = -1;
+
/*
* Old kernel used to call it task_struct->state and now it's '__state'.
* Use BPF CO-RE "ignored suffix rule" to deal with it like below:
@@ -119,11 +121,19 @@ static inline __u64 get_cgroup_id(struct task_struct *t)
{
struct cgroup *cgrp;

- if (uses_cgroup_v1)
- cgrp = BPF_CORE_READ(t, cgroups, subsys[perf_event_cgrp_id], cgroup);
- else
- cgrp = BPF_CORE_READ(t, cgroups, dfl_cgrp);
+ if (!uses_cgroup_v1)
+ return BPF_CORE_READ(t, cgroups, dfl_cgrp, kn, id);
+
+ if (perf_subsys_id == -1) {
+#if __has_builtin(__builtin_preserve_enum_value)
+ perf_subsys_id = bpf_core_enum_value(enum cgroup_subsys_id,
+ perf_event_cgrp_id);
+#else
+ perf_subsys_id = perf_event_cgrp_id;
+#endif
+ }

+ cgrp = BPF_CORE_READ(t, cgroups, subsys[perf_subsys_id], cgroup);
return BPF_CORE_READ(cgrp, kn, id);
}

--
2.37.3.998.g577e59143f-goog
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