fix(profiling): remove slow getpid call from memalloc path [backport 2.18] (#11849)

Backport 6bfe77e from #11848 to 2.18.

memalloc uses getpid to detect whether the process has forked, so that
we can unlock the memalloc lock in the child process (if it isn't
already locked). Unfortunately the getpid call is quite slow. From the
man page: "calls to getpid() always invoke the actual system call,
rather than returning a cached value." Furthermore, we _always_ attempt
to take the lock for allocations, even if we aren't going to sample
them. So this is basically adding a syscall to every allocation.

Move this logic out of the allocation path. Switch to using
pthread_atfork handlers to ensure that the lock is held prior to
forking, and unlock it in the parent and child after forking. This
(maybe) has the added benefit of making sure the data structures are in
a consistent state in the child process after forking. Unclear if that's
an issue prior to this change, though. I may be missing some code that
resets the profiler on fork anyway?

Author: github-actions[bot]

ddtrace/profiling/collector/_memalloc.c

@@ -57,6 +57,28 @@ static alloc_tracker_t* global_alloc_tracker;
 static void
 memalloc_init(void);
 
+static void
+memalloc_prefork(void)
+{
+    // Lock the mutex prior to forking. This ensures that the memory profiler
+    // data structures will be in a consistent state in the child process.
+    // The rest of the memalloc calls do trylock so we don't run the risk
+    // of deadlocking if some other fork handler allocates
+    memlock_lock(&g_memalloc_lock);
+}
+
+static void
+memalloc_postfork_parent(void)
+{
+    memlock_unlock(&g_memalloc_lock);
+}
+
+static void
+memalloc_postfork_child(void)
+{
+    memlock_unlock(&g_memalloc_lock);
+}
+
 #ifdef _MSC_VER
 #pragma section(".CRT$XCU", read)
 __declspec(allocate(".CRT$XCU")) void (*memalloc_init_func)(void) = memalloc_init;
@@ -81,6 +103,9 @@ memalloc_init()
         }
     }
     memlock_init(&g_memalloc_lock, crash_on_mutex_pass);
+#ifndef _WIN32
+    pthread_atfork(memalloc_prefork, memalloc_postfork_parent, memalloc_postfork_child);
+#endif
 }
 
 static void

ddtrace/profiling/collector/_memalloc_heap.c

@@ -36,6 +36,25 @@ static heap_tracker_t global_heap_tracker;
 static void
 memheap_init(void);
 
+static void
+memheap_prefork(void)
+{
+    // See memalloc_prefork for an explanation of why this is here
+    memlock_lock(&g_memheap_lock);
+}
+
+static void
+memheap_postfork_parent(void)
+{
+    memlock_unlock(&g_memheap_lock);
+}
+
+static void
+memheap_postfork_child(void)
+{
+    memlock_unlock(&g_memheap_lock);
+}
+
 #ifdef _MSC_VER
 #pragma section(".CRT$XCU", read)
 __declspec(allocate(".CRT$XCU")) void (*memheap_init_func)(void) = memheap_init;
@@ -60,6 +79,9 @@ memheap_init()
         }
     }
     memlock_init(&g_memheap_lock, crash_on_mutex_pass);
+#ifndef _WIN32
+    pthread_atfork(memheap_prefork, memheap_postfork_parent, memheap_postfork_child);
+#endif
 }
 
 static uint32_t

ddtrace/profiling/collector/_memalloc_reentrant.h

@@ -125,19 +125,6 @@ memlock_trylock(memlock_t* lock)
     if (!lock)
         return false;
 
-#ifdef __linux__
-    // On Linux, we need to make sure we didn't just fork
-    // pthreads will guarantee the lock is consistent, but we at least need to clear it
-    static pid_t my_pid = 0;
-    if (my_pid == 0) {
-        my_pid = getpid();
-    } else if (my_pid != getpid()) {
-        // We've forked, so we need to free the lock
-        memlock_unlock(lock);
-        my_pid = getpid();
-    }
-#endif
-
 #ifdef _WIN32
     bool result = WAIT_OBJECT_0 == WaitForSingleObject(lock->mutex, 0); // 0ms timeout -> no wait
 #else
@@ -153,6 +140,19 @@ memlock_trylock(memlock_t* lock)
     return result;
 }
 
+static inline void
+memlock_lock(memlock_t* lock)
+{
+    if (!lock)
+        return;
+
+#ifdef _WIN32
+    WaitForSingleObject(lock->mutex, INFINITE);
+#else
+    pthread_mutex_lock(&lock->mutex);
+#endif
+}
+
 // Cleanup function
 static inline bool
 memlock_destroy(memlock_t* lock)

releasenotes/notes/profiling-remove-getpid-from-memalloc-74f54043accdfc9e.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    profiling: Removed a system call from the memory allocation profiler, used to detect forks,
+    which ran on every allocation and resulted in a significant slowdown.