Return number of CPUs with data for BPFPerfBufferTable::poll()

src/cc/api/BPF.cc

@@ -495,11 +495,11 @@ BPFPerfBuffer* BPF::get_perf_buffer(const std::string& name) {
   return (it == perf_buffers_.end()) ? nullptr : it->second;
 }
 
-void BPF::poll_perf_buffer(const std::string& name, int timeout_ms) {
+int BPF::poll_perf_buffer(const std::string& name, int timeout_ms) {
   auto it = perf_buffers_.find(name);
   if (it == perf_buffers_.end())
-    return;
-  it->second->poll(timeout_ms);
+    return -1;
+  return it->second->poll(timeout_ms);
 }
 
 StatusTuple BPF::load_func(const std::string& func_name, bpf_prog_type type,

src/cc/api/BPF.h

@@ -155,7 +155,11 @@ class BPF {
   BPFPerfBuffer* get_perf_buffer(const std::string& name);
   // Poll an opened Perf Buffer of given name with given timeout, using callback
   // provided when opening. Do nothing if such open Perf Buffer doesn't exist.
-  void poll_perf_buffer(const std::string& name, int timeout_ms = -1);
+  // Returns: 
+  //   -1 on error or if perf buffer with such name doesn't exist; 
+  //   0, if no data was available before timeout; 
+  //   number of CPUs that have new data, otherwise.
+  int poll_perf_buffer(const std::string& name, int timeout_ms = -1);
 
   StatusTuple load_func(const std::string& func_name, enum bpf_prog_type type,
                         int& fd);