Reset TLS on Profiling Entrance (#999)

Summary:
Pull Request resolved: #999

D63924780 broke some tests because of pthread_atfork having strange properties with subsequent calls. To fix this, lets deviate from this method and just reset the TLS whenever we enter the a profiling context. This will ensure that we will start "fresh" for all the PID/TID related content upon every profile. The drawbacks are:

1. 1 Extra Cache Miss per Profile - This is negligible because the cache miss is during the prepare stage for auto-trace and schedule for on-demand. To add to this, the cache miss penalty is very small

2. No Reset if Fork during Profile - If someone were to fork in the middle of a profile the TLS won't get reset. However, there are many other issues that could happen due to a fork midway through a profile such as undefined behavior with cupti, distorted profiling window etc. We shouldn't worry about this case as of today.

Reviewed By: aaronenyeshi, briancoutinho

Differential Revision: D64120658

fbshipit-source-id: d7ed8462f76dfe6042f4a9a97979fa5010cccd2e

libkineto/include/ThreadUtil.h

@@ -27,4 +27,8 @@ std::string processName(int32_t pid);
 // and its parents.
 std::vector<std::pair<int32_t, std::string>> pidCommandPairsOfAncestors();
 
+// Resets all cached Thread local state, this must be done on
+// forks to prevent stale values from being retained.
+void resetTLS();
+
 } // namespace libkineto

libkineto/include/libkineto.h

@@ -117,6 +117,10 @@ class LibkinetoApi {
     suppressLibkinetoLogMessages();
   }
 
+  void resetKinetoTLS() {
+    resetTLS();
+  }
+
   // Provides access to profier configuration manaegement
   ConfigLoader& configLoader() {
     return configLoader_;

libkineto/src/ActivityProfilerProxy.cpp

@@ -12,6 +12,7 @@
 #include "ActivityProfilerController.h"
 #include "Config.h"
 #include "Logger.h"
+#include "ThreadUtil.h"
 
 namespace KINETO_NAMESPACE {
 
@@ -31,6 +32,7 @@ void ActivityProfilerProxy::init() {
 }
 
 void ActivityProfilerProxy::scheduleTrace(const std::string& configStr) {
+  resetTLS();
   Config config;
   config.parse(configStr);
   controller_->scheduleTrace(config);

libkineto/src/ConfigLoader.cpp

@@ -172,7 +172,9 @@ void ConfigLoader::stopThread() {
       std::lock_guard<std::mutex> lock(updateThreadMutex_);
       updateThreadCondVar_.notify_one();
     }
-    updateThread_->join();
+    if (updateThread_->joinable()) {
+      updateThread_->join();
+    }
     updateThread_ = nullptr;
   }
 }

libkineto/src/ThreadUtil.cpp

@@ -92,6 +92,14 @@ int32_t threadId() {
   return _tid;
 }
 
+// Resets all cached Thread local state, this must be done on
+// forks to prevent stale values from being retained.
+void resetTLS() {
+  _pid = 0;
+  _tid = 0;
+  _sysTid = 0;
+}
+
 namespace {
 static constexpr size_t kMaxThreadNameLength = 16;
 

libkineto/src/init.cpp

@@ -16,6 +16,7 @@
 #include "ConfigLoader.h"
 #include "DaemonConfigLoader.h"
 #include "DeviceUtil.h"
+#include "ThreadUtil.h"
 #ifdef HAS_CUPTI
 #include "CuptiActivityApi.h"
 #include "CuptiCallbackApi.h"