perf stat: Support inherit events during fork() for bperf

bperf has a nice ability to share PMUs, but it still does not support
inherit events during fork(), resulting in some deviations in its stat
results compared with perf.

perf stat result:
$ ./perf stat -e cycles,instructions -- ./perf test -w sqrtloop
Performance counter stats for './perf test -w sqrtloop':

2,316,038,116 cycles
2,859,350,725 instructions

1.009603637 seconds time elapsed

1.004196000 seconds user
0.003950000 seconds sys

bperf stat result:
$ ./perf stat --bpf-counters -e cycles,instructions -- \
./perf test -w sqrtloop

Performance counter stats for './perf test -w sqrtloop':

18,762,093 cycles
23,487,766 instructions

1.008913769 seconds time elapsed

1.003248000 seconds user
0.004069000 seconds sys

In order to support event inheritance, two new bpf programs are added
to monitor the fork and exit of tasks respectively. When a task is
created, add it to the filter map to enable counting, and reuse the
`accum_key` of its parent task to count together with the parent task.
When a task exits, remove it from the filter map to disable counting.

After support:
$ ./perf stat --bpf-counters -e cycles,instructions -- \
./perf test -w sqrtloop

Performance counter stats for './perf test -w sqrtloop':

2,316,252,189 cycles
2,859,946,547 instructions

1.009422314 seconds time elapsed

1.003597000 seconds user
0.004270000 seconds sys

Signed-off-by: Tengda Wu <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Namhyung Kim <[e-mail]>

tools/perf/builtin-stat.c

···
 	} else if (big_num_opt == 0) /* User passed --no-big-num */
 		stat_config.big_num = false;
 
+	target.inherit = !stat_config.no_inherit;
 	err = target__validate(&target);
 	if (err) {
 		target__strerror(&target, err, errbuf, BUFSIZ);

tools/perf/util/bpf_counter.c

···
 }
 
 static	struct perf_cpu_map *all_cpu_map;
+static __u32 filter_entry_cnt;
 
 static int bperf_reload_leader_program(struct evsel *evsel, int attr_map_fd,
 				       struct perf_event_attr_map_entry *entry)
···
 	return err;
 }
 
+static int bperf_attach_follower_program(struct bperf_follower_bpf *skel,
+					 enum bperf_filter_type filter_type,
+					 bool inherit)
+{
+	struct bpf_link *link;
+	int err = 0;
+
+	if ((filter_type == BPERF_FILTER_PID ||
+	    filter_type == BPERF_FILTER_TGID) && inherit)
+		/* attach all follower bpf progs to enable event inheritance */
+		err = bperf_follower_bpf__attach(skel);
+	else {
+		link = bpf_program__attach(skel->progs.fexit_XXX);
+		if (IS_ERR(link))
+			err = PTR_ERR(link);
+	}
+
+	return err;
+}
+
 static int bperf__load(struct evsel *evsel, struct target *target)
 {
 	struct perf_event_attr_map_entry entry = {0xffffffff, 0xffffffff};
 	int attr_map_fd, diff_map_fd = -1, err;
 	enum bperf_filter_type filter_type;
-	__u32 filter_entry_cnt, i;
+	__u32 i;
 
 	if (bperf_check_target(evsel, target, &filter_type, &filter_entry_cnt))
 		return -1;
···
 	/* set up reading map */
 	bpf_map__set_max_entries(evsel->follower_skel->maps.accum_readings,
 				 filter_entry_cnt);
-	/* set up follower filter based on target */
-	bpf_map__set_max_entries(evsel->follower_skel->maps.filter,
-				 filter_entry_cnt);
 	err = bperf_follower_bpf__load(evsel->follower_skel);
 	if (err) {
 		pr_err("Failed to load follower skeleton\n");
···
 	for (i = 0; i < filter_entry_cnt; i++) {
 		int filter_map_fd;
 		__u32 key;
+		struct bperf_filter_value fval = { i, 0 };
 
 		if (filter_type == BPERF_FILTER_PID ||
 		    filter_type == BPERF_FILTER_TGID)
···
 			break;
 
 		filter_map_fd = bpf_map__fd(evsel->follower_skel->maps.filter);
-		bpf_map_update_elem(filter_map_fd, &key, &i, BPF_ANY);
+		bpf_map_update_elem(filter_map_fd, &key, &fval, BPF_ANY);
 	}
 
 	evsel->follower_skel->bss->type = filter_type;
+	evsel->follower_skel->bss->inherit = target->inherit;
 
-	err = bperf_follower_bpf__attach(evsel->follower_skel);
+	err = bperf_attach_follower_program(evsel->follower_skel, filter_type,
+					    target->inherit);
 
 out:
 	if (err && evsel->bperf_leader_link_fd >= 0)
···
 	bperf_sync_counters(evsel);
 	reading_map_fd = bpf_map__fd(skel->maps.accum_readings);
 
-	for (i = 0; i < bpf_map__max_entries(skel->maps.accum_readings); i++) {
+	for (i = 0; i < filter_entry_cnt; i++) {
 		struct perf_cpu entry;
 		__u32 cpu;
 

tools/perf/util/bpf_skel/bperf_follower.bpf.c

···
 #include <bpf/bpf_tracing.h>
 #include "bperf_u.h"
 
+#define MAX_ENTRIES 102400
+
 struct {
 	__uint(type, BPF_MAP_TYPE_PERCPU_ARRAY);
 	__uint(key_size, sizeof(__u32));
···
 struct {
 	__uint(type, BPF_MAP_TYPE_HASH);
 	__uint(key_size, sizeof(__u32));
-	__uint(value_size, sizeof(__u32));
+	__uint(value_size, sizeof(struct bperf_filter_value));
+	__uint(max_entries, MAX_ENTRIES);
+	__uint(map_flags, BPF_F_NO_PREALLOC);
 } filter SEC(".maps");
 
 enum bperf_filter_type type = 0;
 int enabled = 0;
+int inherit;
 
 SEC("fexit/XXX")
 int BPF_PROG(fexit_XXX)
 {
 	struct bpf_perf_event_value *diff_val, *accum_val;
 	__u32 filter_key, zero = 0;
-	__u32 *accum_key;
+	__u32 accum_key;
+	struct bperf_filter_value *fval;
 
 	if (!enabled)
 		return 0;
 
 	switch (type) {
 	case BPERF_FILTER_GLOBAL:
-		accum_key = &zero;
+		accum_key = zero;
 		goto do_add;
 	case BPERF_FILTER_CPU:
 		filter_key = bpf_get_smp_processor_id();
···
 		filter_key = bpf_get_current_pid_tgid() & 0xffffffff;
 		break;
 	case BPERF_FILTER_TGID:
-		filter_key = bpf_get_current_pid_tgid() >> 32;
+		/* Use pid as the filter_key to exclude new task counts
+		 * when inherit is disabled. Don't worry about the existing
+		 * children in TGID losing their counts, bpf_counter has
+		 * already added them to the filter map via perf_thread_map
+		 * before this bpf prog runs.
+		 */
+		filter_key = inherit ?
+			     bpf_get_current_pid_tgid() >> 32 :
+			     bpf_get_current_pid_tgid() & 0xffffffff;
 		break;
 	default:
 		return 0;
 	}
 
-	accum_key = bpf_map_lookup_elem(&filter, &filter_key);
-	if (!accum_key)
+	fval = bpf_map_lookup_elem(&filter, &filter_key);
+	if (!fval)
 		return 0;
+
+	accum_key = fval->accum_key;
+	if (fval->exited)
+		bpf_map_delete_elem(&filter, &filter_key);
 
 do_add:
 	diff_val = bpf_map_lookup_elem(&diff_readings, &zero);
 	if (!diff_val)
 		return 0;
 
-	accum_val = bpf_map_lookup_elem(&accum_readings, accum_key);
+	accum_val = bpf_map_lookup_elem(&accum_readings, &accum_key);
 	if (!accum_val)
 		return 0;
 
 	accum_val->counter += diff_val->counter;
 	accum_val->enabled += diff_val->enabled;
 	accum_val->running += diff_val->running;
+
+	return 0;
+}
+
+/* The program is only used for PID or TGID filter types. */
+SEC("tp_btf/task_newtask")
+int BPF_PROG(on_newtask, struct task_struct *task, __u64 clone_flags)
+{
+	__u32 parent_key, child_key;
+	struct bperf_filter_value *parent_fval;
+	struct bperf_filter_value child_fval = { 0 };
+
+	if (!enabled)
+		return 0;
+
+	switch (type) {
+	case BPERF_FILTER_PID:
+		parent_key = bpf_get_current_pid_tgid() & 0xffffffff;
+		child_key = task->pid;
+		break;
+	case BPERF_FILTER_TGID:
+		parent_key = bpf_get_current_pid_tgid() >> 32;
+		child_key = task->tgid;
+		if (child_key == parent_key)
+			return 0;
+		break;
+	default:
+		return 0;
+	}
+
+	/* Check if the current task is one of the target tasks to be counted */
+	parent_fval = bpf_map_lookup_elem(&filter, &parent_key);
+	if (!parent_fval)
+		return 0;
+
+	/* Start counting for the new task by adding it into filter map,
+	 * inherit the accum key of its parent task so that they can be
+	 * counted together.
+	 */
+	child_fval.accum_key = parent_fval->accum_key;
+	child_fval.exited = 0;
+	bpf_map_update_elem(&filter, &child_key, &child_fval, BPF_NOEXIST);
+
+	return 0;
+}
+
+/* The program is only used for PID or TGID filter types. */
+SEC("tp_btf/sched_process_exit")
+int BPF_PROG(on_exittask, struct task_struct *task)
+{
+	__u32 pid;
+	struct bperf_filter_value *fval;
+
+	if (!enabled)
+		return 0;
+
+	/* Stop counting for this task by removing it from filter map.
+	 * For TGID type, if the pid can be found in the map, it means that
+	 * this pid belongs to the leader task. After the task exits, the
+	 * tgid of its child tasks (if any) will be 1, so the pid can be
+	 * safely removed.
+	 */
+	pid = task->pid;
+	fval = bpf_map_lookup_elem(&filter, &pid);
+	if (fval)
+		fval->exited = 1;
 
 	return 0;
 }

tools/perf/util/bpf_skel/bperf_u.h

···
 	BPERF_FILTER_TGID,
 };
 
+struct bperf_filter_value {
+	__u32 accum_key;
+	__u8 exited;
+};
+
 #endif /* __BPERF_STAT_U_H */

tools/perf/util/target.h

···
 	bool	     default_per_cpu;
 	bool	     per_thread;
 	bool	     use_bpf;
+	bool	     inherit;
 	int	     initial_delay;
 	const char   *attr_map;
 };