Basic Linux and Windows Thread Prioritisation for worker threads

CMakeLists.txt

@@ -191,6 +191,7 @@ check_function_exists(pthread_attr_setcpupercent_np HAVE_PTHREAD_ATTR_SETCPUPERC
 check_function_exists(pthread_yield_np HAVE_PTHREAD_YIELD_NP)
 check_function_exists(pthread_main_np HAVE_PTHREAD_MAIN_NP)
 check_function_exists(pthread_workqueue_setdispatch_np HAVE_PTHREAD_WORKQUEUE_SETDISPATCH_NP)
+check_function_exists(pthread_setname_np HAVE_PTHREAD_SETNAME_NP)
 check_function_exists(strlcpy HAVE_STRLCPY)
 check_function_exists(sysconf HAVE_SYSCONF)
 check_function_exists(arc4random HAVE_ARC4RANDOM)

cmake/config.h.in

@@ -208,6 +208,9 @@
 /* Define to 1 if you have the `_pthread_workqueue_init' function. */
 #cmakedefine HAVE__PTHREAD_WORKQUEUE_INIT
 
+/* Define to 1 if you have the `pthread_setname_np' function. */
+#cmakedefine01 HAVE_PTHREAD_SETNAME_NP
+
 /* Define to use non-portable pthread TSD optimizations for Mac OS X) */
 #cmakedefine USE_APPLE_TSD_OPTIMIZATIONS
 

dispatch/queue.h

@@ -173,12 +173,12 @@ DISPATCH_DECL_SUBCLASS(dispatch_queue_main, dispatch_queue_serial);
  *
  * @discussion
  * Dispatch concurrent queues are lightweight objects to which regular and
- * barrier workitems may be submited. Barrier workitems are invoked in
+ * barrier workitems may be submitted. Barrier workitems are invoked in
  * exclusion of any other kind of workitem in FIFO order.
  *
  * Regular workitems can be invoked concurrently for the same concurrent queue,
  * in any order. However, regular workitems will not be invoked before any
- * barrier workitem submited ahead of them has been invoked.
+ * barrier workitem submitted ahead of them has been invoked.
  *
  * In other words, if a serial queue is equivalent to a mutex in the Dispatch
  * world, a concurrent queue is equivalent to a reader-writer lock, where

src/init.c

@@ -383,9 +383,9 @@ dispatch_get_global_queue(intptr_t priority, uintptr_t flags)
 	}
 	dispatch_qos_t qos = _dispatch_qos_from_queue_priority(priority);
 #if !HAVE_PTHREAD_WORKQUEUE_QOS
-	if (qos == QOS_CLASS_MAINTENANCE) {
+	if (qos == DISPATCH_QOS_MAINTENANCE) {
 		qos = DISPATCH_QOS_BACKGROUND;
-	} else if (qos == QOS_CLASS_USER_INTERACTIVE) {
+	} else if (qos == DISPATCH_QOS_USER_INTERACTIVE) {
 		qos = DISPATCH_QOS_USER_INITIATED;
 	}
 #endif

src/queue.c

@@ -23,6 +23,33 @@
 #include "protocol.h" // _dispatch_send_wakeup_runloop_thread
 #endif
 
+#if defined(__linux__)
+#include <errno.h>
+#include <sys/resource.h>
+#endif
+
+#if defined(_WIN32)
+// Wrapper around SetThreadDescription for UTF-8 strings
+void _dispatch_win32_set_thread_description(HANDLE hThread, const char *description) {
+    int wcsize = MultiByteToWideChar(CP_UTF8, 0, description, -1, NULL, 0);
+    if (wcsize == 0) {
+        return;
+    }
+
+    wchar_t* wcstr = (wchar_t*)malloc(wcsize * sizeof(wchar_t));
+    if (wcstr == NULL) {
+        return;
+    }
+
+    int result = MultiByteToWideChar(CP_UTF8, 0, description, -1, wcstr, wcsize);
+    if (result != 0) {
+        SetThreadDescription(hThread, wcstr);
+    }
+
+    free(wcstr);
+}
+#endif
+
 static inline void _dispatch_root_queues_init(void);
 static void _dispatch_lane_barrier_complete(dispatch_lane_class_t dqu,
 		dispatch_qos_t qos, dispatch_wakeup_flags_t flags);
@@ -6216,10 +6243,61 @@ _dispatch_worker_thread(void *context)
 	_dispatch_sigmask();
 #endif
 	_dispatch_introspection_thread_add();
+	dispatch_priority_t pri = dq->dq_priority;
+	pthread_priority_t pp = _dispatch_get_priority();
+
+#if defined(__linux__)
+	// The Linux kernel does not have a direct analogue to the QoS-based
+	// thread policy engine found in XNU.
+	//
+	// We cannot use 'pthread_setschedprio', because all threads with default
+	// scheduling policy (SCHED_OTHER) have the same pthread 'priority'.
+	// For both CFS, which was introduced in Linux 2.6.23, and its successor
+	// EEVDF (since 6.6) 'sched_get_priority_max' and 'sched_get_priority_min'
+	// will just return 0.
+	//
+	// However, as outlined in "man 2 setpriority", the nice value is a
+	// per‐thread attribute: different threads in the same process can have
+	// different nice values. We can thus setup the thread's initial priority
+	// by converting the QoS class and relative priority to a 'nice' value.
+	pp = _dispatch_priority_to_pp_strip_flags(pri);
+	int nice = _dispatch_pp_to_nice(pp);
+
+	#if HAVE_PTHREAD_SETNAME_NP 
+	// pthread thread names are restricted to just 16 characters
+	// including NUL. It does not make sense to pass the queue's
+	// label as a name.
+	pthread_setname_np(pthread_self(), "DispatchWorker");
+	#endif
+
+	errno = 0;
+	int rc = setpriority(PRIO_PROCESS, 0, nice);
+	if (rc != -1 || errno == 0) {
+		_dispatch_thread_setspecific(dispatch_priority_key, (void *)(uintptr_t)pp);
+	} else {
+		_dispatch_log("Failed to set thread priority for worker thread: pqc=%p errno=%d\n", pqc, errno);
+	}
+#elif defined(_WIN32)
+	pp = _dispatch_priority_to_pp_strip_flags(pri);
+	int win_priority = _dispatch_pp_to_win32_priority(pp);
+
+	HANDLE current = GetCurrentThread();
+
+	// Set thread description to the label of the root queue
+	if (dq->dq_label) {
+		_dispatch_win32_set_thread_description(current, dq->dq_label);
+	}
+
+	int rc = SetThreadPriority(current, win_priority);
+	if (rc) {
+		_dispatch_thread_setspecific(dispatch_priority_key, (void *)(uintptr_t)pp);
+	} else {
+		DWORD dwError = GetLastError();
+		_dispatch_log("Failed to set thread priority for worker thread: pqc=%p win_priority=%d dwError=%lu\n", pqc, win_priority, dwError);
+	}
+#endif
 
 	const int64_t timeout = 5ull * NSEC_PER_SEC;
-	pthread_priority_t pp = _dispatch_get_priority();
-	dispatch_priority_t pri = dq->dq_priority;
 
 	// If the queue is neither
 	// - the manager
@@ -6258,6 +6336,14 @@ _dispatch_worker_thread(void *context)
 	(void)os_atomic_inc2o(dq, dgq_thread_pool_size, release);
 	_dispatch_root_queue_poke(dq, 1, 0);
 	_dispatch_release(dq); // retained in _dispatch_root_queue_poke_slow
+
+#if defined(_WIN32)
+	// Make sure to properly end the background processing mode
+	if (win_priority == THREAD_MODE_BACKGROUND_BEGIN) {
+		SetThreadPriority(current, THREAD_MODE_BACKGROUND_END);
+	}
+#endif
+
 	return NULL;
 }
 #if defined(_WIN32)

src/shims/priority.h

@@ -210,6 +210,71 @@ _dispatch_qos_to_pp(dispatch_qos_t qos)
 	return pp | _PTHREAD_PRIORITY_PRIORITY_MASK;
 }
 
+
+#if defined(__linux__)
+// These presets roughly match the `android.os.Process' constants
+// used for `setThreadPriority()'.
+//
+// Be aware that with the Completely Fair Scheduler (CFS) the weight is computed
+// as 1024 / (1.25) ^ (nice) where nice is in the range -20 to 19.
+// This means that nice is not a linear scale.
+#define DISPATCH_NICE_BACKGROUND  10
+#define DISPATCH_NICE_UTILITY     2
+#define DISPATCH_NICE_DEFAULT     0
+// Note that you might not have permission to increase the priority
+// of a thread beyond the default priority.
+#define DISPATCH_NICE_USER_INITIATED -2
+#define DISPATCH_NICE_USER_INTERACTIVE -4
+
+DISPATCH_ALWAYS_INLINE
+static inline int _dispatch_pp_to_nice(pthread_priority_t pp)
+{
+	// FIXME: What about relative priorities?
+	uint32_t qos = _dispatch_qos_from_pp(pp);
+
+	switch (qos) {
+		case DISPATCH_QOS_BACKGROUND:
+			return DISPATCH_NICE_BACKGROUND;
+		case DISPATCH_QOS_UTILITY:
+			return DISPATCH_NICE_UTILITY;
+		case DISPATCH_QOS_DEFAULT:
+			return DISPATCH_NICE_DEFAULT;
+		case DISPATCH_QOS_USER_INITIATED:
+			return DISPATCH_NICE_USER_INITIATED;
+		case DISPATCH_QOS_USER_INTERACTIVE:
+			return DISPATCH_NICE_USER_INTERACTIVE;
+	}
+
+	return DISPATCH_NICE_DEFAULT;
+}
+#endif // defined(__linux__)
+
+#if defined(_WIN32)
+DISPATCH_ALWAYS_INLINE
+static inline int _dispatch_pp_to_win32_priority(pthread_priority_t pp) {
+	uint32_t qos = _dispatch_qos_from_pp(pp);
+
+	switch (qos) {
+		case DISPATCH_QOS_BACKGROUND:
+			// Make sure to end background mode before exiting the thread!
+			return THREAD_MODE_BACKGROUND_BEGIN;
+		case DISPATCH_QOS_UTILITY:
+			return THREAD_PRIORITY_BELOW_NORMAL;
+		case DISPATCH_QOS_DEFAULT:
+			return THREAD_PRIORITY_NORMAL;
+			// User input threads should be THREAD_PRIORITY_NORMAL, to
+			// avoid unintentionally starving the system
+		case DISPATCH_QOS_USER_INITIATED:
+			return THREAD_PRIORITY_NORMAL;
+		case DISPATCH_QOS_USER_INTERACTIVE:
+			return THREAD_PRIORITY_NORMAL;
+	}
+
+	return THREAD_PRIORITY_NORMAL;
+}
+#endif // defined(_WIN32)
+
+
 // including maintenance
 DISPATCH_ALWAYS_INLINE
 static inline bool