Thread filter - add a table with linear probing

Remove usage of set to ensure we are signal safe.

Author: r1viollet

ddprof-lib/src/main/cpp/flightRecorder.cpp

@@ -1063,8 +1063,8 @@ void Recording::writeThreads(Buffer *buf) {
   threads.insert(_tid);
 
   for (int i = 0; i < CONCURRENCY_LEVEL; ++i) {
-    // I can not use merge : cpp 17
-    threads.insert(_thread_ids[i][old_index].begin(), _thread_ids[i][old_index].end());
+    // Collect thread IDs from the fixed-size table into the main set
+    _thread_ids[i][old_index].collect(threads);
     _thread_ids[i][old_index].clear();
   }
 

ddprof-lib/src/main/cpp/flightRecorder.h

@@ -35,6 +35,7 @@
 #include "mutex.h"
 #include "objectSampler.h"
 #include "threadFilter.h"
+#include "threadIdTable.h"
 #include "vmEntry.h"
 
 const u64 MAX_JLONG = 0x7fffffffffffffffULL;
@@ -128,9 +129,9 @@ class Recording {
   static char *_java_command;
 
   RecordingBuffer _buf[CONCURRENCY_LEVEL];
-  // we have several sets to avoid lock contention
-  // we have a second dimension to allow a switch in the active set
-  std::unordered_set<int> _thread_ids[CONCURRENCY_LEVEL][2];
+  // we have several tables to avoid lock contention
+  // we have a second dimension to allow a switch in the active table
+  ThreadIdTable _thread_ids[CONCURRENCY_LEVEL][2];
   std::atomic<int> _active_index{0};  // 0 or 1 globally
 
   int _fd;

ddprof-lib/src/main/cpp/javaApi.cpp

@@ -156,6 +156,28 @@ Java_com_datadoghq_profiler_JavaProfiler_filterThread_1remove(JNIEnv *env,
   thread_filter->remove(slot_id);
 }
 
+// Backward compatibility for existing code
+extern "C" DLLEXPORT void JNICALL
+Java_com_datadoghq_profiler_JavaProfiler_filterThread0(JNIEnv *env,
+                                                       jobject unused,
+                                                       jboolean enable) {
+  ProfiledThread *current = ProfiledThread::current();
+  int tid = current->tid();
+  if (unlikely(tid < 0)) {
+    return;
+  }
+  ThreadFilter *thread_filter = Profiler::instance()->threadFilter();
+  int slot_id = current->filterSlotId();
+  if (unlikely(slot_id == -1)) {
+    return;
+  }
+  
+  if (enable) {
+    thread_filter->add(tid, slot_id);
+  } else {
+    thread_filter->remove(slot_id);
+  }
+}
 
 extern "C" DLLEXPORT jobject JNICALL
 Java_com_datadoghq_profiler_JavaProfiler_getContextPage0(JNIEnv *env,

ddprof-lib/src/main/cpp/profiler.cpp

@@ -106,8 +106,9 @@ void Profiler::onThreadStart(jvmtiEnv *jvmti, JNIEnv *jni, jthread thread) {
   ProfiledThread *current = ProfiledThread::current();
   int tid = current->tid();
   if (_thread_filter.enabled()) {
-    current->setFilterSlotId(_thread_filter.registerThread());
-    _thread_filter.remove(tid);
+    int slot_id = _thread_filter.registerThread();
+    current->setFilterSlotId(slot_id);
+    _thread_filter.remove(slot_id);  // Remove from filtering initially
   }
   updateThreadName(jvmti, jni, thread, true);
 

ddprof-lib/src/main/java/com/datadoghq/profiler/JavaProfiler.java

@@ -448,6 +448,8 @@ public Map<String, Long> getDebugCounters() {
 
     private native void filterThread_add();
     private native void filterThread_remove();
+    // Backward compatibility for existing code
+    private native void filterThread0(boolean enable);
 
     private static native int getTid0();
     private static native ByteBuffer getContextPage0(int tid);