cpudist: create sufficiently large hash table to avoid missing tasks

This change fixes the cpudist tool to avoid issue when too many tasks
are running.

Fixes iovisor#2567 -- cpudist stop working when there are too many fork

tools/cpudist.py

@@ -64,7 +64,7 @@
 } pid_key_t;
 
 
-BPF_HASH(start, u32, u64);
+BPF_HASH(start, u32, u64, MAX_PID);
 STORAGE
 
 static inline void store_start(u32 tgid, u32 pid, u64 ts)
@@ -142,7 +142,7 @@
  pid = "pid"
  section = "tid"
  bpf_text = bpf_text.replace('STORAGE',
- 'BPF_HISTOGRAM(dist, pid_key_t);')
+ 'BPF_HISTOGRAM(dist, pid_key_t, MAX_PID);')
  bpf_text = bpf_text.replace('STORE',
  'pid_key_t key = {.id = ' + pid + ', .slot = bpf_log2l(delta)}; ' +
  'dist.increment(key);')
@@ -156,7 +156,9 @@
  if args.ebpf:
  exit()
 
-b = BPF(text=bpf_text)
+max_pid = int(open("/proc/sys/kernel/pid_max").read())
+
+b = BPF(text=bpf_text, cflags=["-DMAX_PID=%d" % max_pid])
 b.attach_kprobe(event="finish_task_switch", fn_name="sched_switch")
 
 print("Tracing %s-CPU time... Hit Ctrl-C to end." %