funccount: Switch to BPF array instead of hash

Because we know the number of probes in advance before
attaching them, we can simply preinitialize a fixed-size
array instead of using a BPF map. This avoids potential
deadlocks/hangs/race conditions with the Python program
and internally in the kernel. See also iovisor#415, iovisor#665, iovisor#233
for more discussion.

tools/funccount.py

@@ -170,16 +170,18 @@ def load(self):
  trace_count_text = """
 int PROBE_FUNCTION(void *ctx) {
  FILTER
- u64 loc = LOCATION, zero = 0;
- u64 *val = counts.lookup_or_init(&loc, &zero);
+ int loc = LOCATION;
+ u64 *val = counts.lookup(&loc);
+ if (!val) {
+ return 0; // Should never happen, # of locations is known
+ }
  (*val)++;
  return 0;
 }
  """
  bpf_text = """#include <uapi/linux/ptrace.h>
 
-BPF_HASH(counts, u64, u64); // map location number to number of calls
-
+BPF_TABLE("array", int, u64, counts, NUMLOCATIONS);
  """
 
  # We really mean the tgid from the kernel's perspective, which is in
@@ -192,11 +194,22 @@ def load(self):
  trace_count_text = trace_count_text.replace('FILTER', '')
 
  bpf_text += self._generate_functions(trace_count_text)
+ bpf_text = bpf_text.replace("NUMLOCATIONS",
+ str(len(self.trace_functions)))
  if debug:
  print(bpf_text)
 
  self.bpf = BPF(text=bpf_text,
  usdt_contexts=[self.usdt] if self.usdt else [])
+ self.clear() # Initialize all array items to zero
+
+ def counts(self):
+ return self.bpf["counts"]
+
+ def clear(self):
+ counts = self.bpf["counts"]
+ for location, _ in list(self.trace_functions.items()):
+ counts[counts.Key(location)] = counts.Leaf()
 
 class Tool(object):
  def __init__(self):
@@ -253,18 +266,19 @@ def run(self):
  print("%-8s\n" % strftime("%H:%M:%S"), end="")
 
  print("%-36s %8s" % ("FUNC", "COUNT"))
- counts = self.probe.bpf["counts"]
+ counts = self.probe.counts()
  for k, v in sorted(counts.items(),
  key=lambda counts: counts[1].value):
  if v.value == 0:
  continue
  print("%-36s %8d" %
  (self.probe.trace_functions[k.value], v.value))
- counts.clear()
 
  if exiting:
  print("Detaching...")
  exit()
+ else:
+ self.probe.clear()
 
 if __name__ == "__main__":
  try: