[PATCH bpf-next v3 2/3] perf bpf_counter: Track valid BPF counter snapshots

[Mykyta Yatsenko <mykyta.yatsenko5@gmail.com>]

From: Mykyta Yatsenko <yatsenko@meta.com>

The perf profiler uses a zero counter value to reject an invalid entry
snapshot. However, zero is valid when the PMU has not scheduled the event.

To reproduce the issue, fill the PMU with other events and profile a
long-running BPF program for iTLB misses. In this test, the entry read
returned 0.

Use an armed flag to track successful entry reads. Reset the flag before
each new read. The fexit path clears the flag when it consumes the
snapshot.

Fixes: fa853c4b839e ("perf stat: Enable counting events for BPF programs")
Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
---
 tools/perf/util/bpf_skel/bpf_prog_profiler.bpf.c | 26 ++++++++++++++++--------
 1 file changed, 18 insertions(+), 8 deletions(-)

diff --git a/tools/perf/util/bpf_skel/bpf_prog_profiler.bpf.c b/tools/perf/util/bpf_skel/bpf_prog_profiler.bpf.c
index 97037d3b3d9f..7bc0db26e005 100644
--- a/tools/perf/util/bpf_skel/bpf_prog_profiler.bpf.c
+++ b/tools/perf/util/bpf_skel/bpf_prog_profiler.bpf.c
@@ -4,6 +4,11 @@
 #include <bpf/bpf_helpers.h>
 #include <bpf/bpf_tracing.h>
 
+struct profile_reading {
+	struct bpf_perf_event_value value;
+	bool armed;
+};
+
 /* map of perf event fds, num_cpu * num_metric entries */
 struct {
 	__uint(type, BPF_MAP_TYPE_PERF_EVENT_ARRAY);
@@ -15,7 +20,7 @@ struct {
 struct {
 	__uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
 	__uint(key_size, sizeof(__u32));
-	__uint(value_size, sizeof(struct bpf_perf_event_value));
+	__uint(value_size, sizeof(struct profile_reading));
 	__uint(max_entries, 1);
 } fentry_readings SEC(".maps");
 
@@ -33,7 +38,7 @@ SEC("fentry/XXX")
 int BPF_PROG(fentry_XXX)
 {
 	__u32 key = bpf_get_smp_processor_id();
-	struct bpf_perf_event_value *ptr;
+	struct profile_reading *ptr;
 	__u32 zero = 0;
 	long err;
 
@@ -42,9 +47,12 @@ int BPF_PROG(fentry_XXX)
 	if (!ptr)
 		return 0;
 
-	err = bpf_perf_event_read_value(&events, key, ptr, sizeof(*ptr));
+	ptr->armed = false;
+	err = bpf_perf_event_read_value(&events, key, &ptr->value,
+					sizeof(ptr->value));
 	if (err)
 		return 0;
+	ptr->armed = true;
 
 	return 0;
 }
@@ -52,17 +60,19 @@ int BPF_PROG(fentry_XXX)
 static inline void
 fexit_update_maps(struct bpf_perf_event_value *after)
 {
-	struct bpf_perf_event_value *before, diff;
+	struct profile_reading *before;
+	struct bpf_perf_event_value diff;
 	__u32 zero = 0;
 
 	before = bpf_map_lookup_elem(&fentry_readings, &zero);
 	/* only account samples with a valid fentry_reading */
-	if (before && before->counter) {
+	if (before && before->armed) {
 		struct bpf_perf_event_value *accum;
 
-		diff.counter = after->counter - before->counter;
-		diff.enabled = after->enabled - before->enabled;
-		diff.running = after->running - before->running;
+		before->armed = false;
+		diff.counter = after->counter - before->value.counter;
+		diff.enabled = after->enabled - before->value.enabled;
+		diff.running = after->running - before->value.running;
 
 		accum = bpf_map_lookup_elem(&accum_readings, &zero);
 		if (accum) {

-- 
2.53.0-Meta