Add hooks to MaxMemoryPreload to allow pausing the monitoring for a piece of code

Author: makortel

PerfTools/MaxMemoryPreload/README.md

@@ -16,6 +16,33 @@ LD_PRELOAD="libPerfToolsAllocMonitorPreload.so libPerfToolsMaxMemoryPreload.so"
 
 the order is important.
 
+### Pausing the monitoring
+
+It is possible to temporarily pause the monitoring by instrumenting the target application by definining the following functions
+```cpp
+// in some header,
+void pauseMaxMemoryPreload();
+void unpauseMaxMemoryPreload();
+
+// in an implementation source file
+void pauseMaxMemoryPreload() {
+}
+void unpauseMaxMemoryPreload() {
+}
+```
+and then using these in code
+```cpp
+  ...
+  pauseMaxMemoryPreload();
+  // code that should be excluded from the monitoring
+  unpauseMaxMemoryPreload();
+  ...
+```
+
+The trick is that by default these functions are defined in the application, and the functions do nothing. The `libPerfToolsMaxMemoryPreload.so` provides also the same functions that actually pause the data collection, and the LD_PRELOADing makes the application to call the functions within `libPerfToolsMaxMemoryPreload.so`.
+
+It is recommended to not pause the monitoring within a multithreaded section, because that could result in unexpected results, because the pausing setting is global.
+
 ## Reporting
 When the application ends, the monitor will report the following to standard error:
 

PerfTools/MaxMemoryPreload/src/preload.cc

@@ -26,6 +26,16 @@
 // static data member definitions
 //
 
+// Hooks the target application can be instrumented with to pause and
+// unpause the MaxMemoryPreload. Pausing the monitoring during a
+// multithreaded execution can result in unexpected results, because
+// the setting is global.
+namespace {
+  std::atomic<bool> paused = false;
+}
+void pauseMaxMemoryPreload() { paused = true; }
+void unpauseMaxMemoryPreload() { paused = false; }
+
 namespace {
   class MonitorAdaptor : public cms::perftools::AllocMonitorBase {
   public:
@@ -34,6 +44,9 @@ namespace {
 
   private:
     void allocCalled(size_t iRequested, size_t iActual, void const*) final {
+      if (paused)
+        return;
+
       nAllocations_.fetch_add(1, std::memory_order_acq_rel);
       requested_.fetch_add(iRequested, std::memory_order_acq_rel);
 
@@ -48,6 +61,9 @@ namespace {
       }
     }
     void deallocCalled(size_t iActual, void const*) final {
+      if (paused)
+        return;
+
       nDeallocations_.fetch_add(1, std::memory_order_acq_rel);
       auto present = presentActual_.load(std::memory_order_acquire);
       if (present >= iActual) {