perf offcpu: Track child processes

When -p option used or a workload is given, it needs to handle child
processes. The perf_event can inherit those task events
automatically. We can add a new BPF program in task_newtask
tracepoint to track child processes.

Before:
$ sudo perf record --off-cpu -- perf bench sched messaging
$ sudo perf report --stat | grep -A1 offcpu
offcpu-time stats:
SAMPLE events: 1

After:
$ sudo perf record -a --off-cpu -- perf bench sched messaging
$ sudo perf report --stat | grep -A1 offcpu
offcpu-time stats:
SAMPLE events: 856

Signed-off-by: Namhyung Kim <[e-mail]>
Cc: Blake Jones <[e-mail]>
Cc: Hao Luo <[e-mail]>
Cc: Ian Rogers <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Milian Wolff <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Song Liu <[e-mail]>
Cc: [e-mail]
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/bpf_off_cpu.c

···
 #include "bpf_skel/off_cpu.skel.h"
 
 #define MAX_STACKS  32
+#define MAX_PROC  4096
 /* we don't need actual timestamp, just want to put the samples at last */
 #define OFF_CPU_TIMESTAMP  (~0ull << 32)
 
···
 
 			ntasks++;
 		}
+
+		if (ntasks < MAX_PROC)
+			ntasks = MAX_PROC;
+
 		bpf_map__set_max_entries(skel->maps.task_filter, ntasks);
 	} else if (target__has_task(target)) {
 		ntasks = perf_thread_map__nr(evlist->core.threads);
 		bpf_map__set_max_entries(skel->maps.task_filter, ntasks);
+	} else if (target__none(target)) {
+		bpf_map__set_max_entries(skel->maps.task_filter, MAX_PROC);
 	}
 
 	if (evlist__first(evlist)->cgrp) {

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

···
 #define TASK_INTERRUPTIBLE	0x0001
 #define TASK_UNINTERRUPTIBLE	0x0002
 
+/* create a new thread */
+#define CLONE_THREAD  0x10000
+
 #define MAX_STACKS   32
 #define MAX_ENTRIES  102400
 
···
 		/* prevent to reuse the timestamp later */
 		pelem->timestamp = 0;
 	}
+
+	return 0;
+}
+
+SEC("tp_btf/task_newtask")
+int on_newtask(u64 *ctx)
+{
+	struct task_struct *task;
+	u64 clone_flags;
+	u32 pid;
+	u8 val = 1;
+
+	if (!uses_tgid)
+		return 0;
+
+	task = (struct task_struct *)bpf_get_current_task();
+
+	pid = BPF_CORE_READ(task, tgid);
+	if (!bpf_map_lookup_elem(&task_filter, &pid))
+		return 0;
+
+	task = (struct task_struct *)ctx[0];
+	clone_flags = ctx[1];
+
+	pid = task->tgid;
+	if (!(clone_flags & CLONE_THREAD))
+		bpf_map_update_elem(&task_filter, &pid, &val, BPF_NOEXIST);
 
 	return 0;
 }