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

src/cc/api/BPFTable.cc

@@ -306,14 +306,13 @@ StatusTuple BPFPerfBuffer::close_all_cpu() {
   return StatusTuple(0);
 }
 
-void BPFPerfBuffer::poll(int timeout_ms) {
+int BPFPerfBuffer::poll(int timeout_ms) {
   if (epfd_ < 0)
-    return;
+    return -1;
   int cnt = epoll_wait(epfd_, ep_events_.get(), cpu_readers_.size(), timeout_ms);
-  if (cnt <= 0)
-    return;
   for (int i = 0; i < cnt; i++)
     perf_reader_event_read(static_cast<perf_reader*>(ep_events_[i].data.ptr));
+  return cnt;
 }
 
 BPFPerfBuffer::~BPFPerfBuffer() {

src/cc/api/BPFTable.h

@@ -310,7 +310,7 @@ class BPFPerfBuffer : public BPFTableBase<int, int> {
   StatusTuple open_all_cpu(perf_reader_raw_cb cb, perf_reader_lost_cb lost_cb,
                            void* cb_cookie, int page_cnt);
   StatusTuple close_all_cpu();
-  void poll(int timeout_ms);
+  int poll(int timeout_ms);
 
  private:
   StatusTuple open_on_cpu(perf_reader_raw_cb cb, perf_reader_lost_cb lost_cb,