hazptr: Detect frequent calls to hazptr_tc::fill

Summary: Detect frequent calls to hazptr_tc::fill. This is usually indicative of unnecessary overhead, either due to insufficient thread cache capacity for legitimate use cases, or due to unnecessary allocation of extra hazard pointers in user code.

Reviewed By: yfeldblum

Differential Revision: D8606484

fbshipit-source-id: e75223f8bc3e180a1ee651ee2989522d6b0a89df

folly/synchronization/HazptrThrLocal.h

@@ -27,6 +27,7 @@
 #include <glog/logging.h>
 
 #include <atomic>
+#include <chrono>
 
 /**
  *  Thread local classes and singletons
@@ -74,6 +75,9 @@ class hazptr_tc {
   hazptr_tc_entry<Atom> entry_[kCapacity];
   uint8_t count_{0};
   bool local_{false}; // for debug mode only
+  // the following two members are for detecting capacity problems
+  uint16_t num_fills_{0};
+  std::chrono::steady_clock::time_point first_fill_time_;
 
  public:
   ~hazptr_tc() {
@@ -125,6 +129,7 @@ class hazptr_tc {
 
   FOLLY_NOINLINE void fill(uint8_t num) {
     DCHECK_LE(count_ + num, capacity());
+    fill_should_not_be_called_frequently();
     auto& domain = default_hazptr_domain<Atom>();
     for (uint8_t i = 0; i < num; ++i) {
       auto hprec = domain.hprec_acquire();
@@ -139,6 +144,27 @@ class hazptr_tc {
     }
   }
 
+  FOLLY_NOINLINE void fill_should_not_be_called_frequently() {
+    constexpr uint16_t max_fills = 10;
+    constexpr auto period = std::chrono::milliseconds(1);
+    if (num_fills_++ == 0) {
+      first_fill_time_ = std::chrono::steady_clock::now();
+    } else if (num_fills_ > max_fills) {
+      auto now = std::chrono::steady_clock::now();
+      auto dur = now - first_fill_time_;
+      using std::chrono::nanoseconds;
+      CHECK_GT(nanoseconds(dur).count(), nanoseconds(period).count())
+          << "[*** INVESTIGATE: "
+          << "Frequent calls to hazptr_tc::fill may indicate "
+          << "unnecessary overhead either due to "
+          << "insufficient thread cache capacity, "
+          << "or due to unnecessary allocation of extra hazard pointers "
+          << "triggered by user code. ***]";
+      num_fills_ = 1;
+      first_fill_time_ = now;
+    }
+  }
+
   bool local() const noexcept { // for debugging only
     return local_;
   }