util/queue: add ability to kill a subset of threads

for ARB_parallel_shader_compile

src/util/u_queue.c

@@ -33,7 +33,8 @@
 #include "util/u_thread.h"
 #include "u_process.h"
 
-static void util_queue_killall_and_wait(struct util_queue *queue);
+static void
+util_queue_kill_threads(struct util_queue *queue, unsigned keep_num_threads);
 
 /****************************************************************************
  * Wait for all queues to assert idle when exit() is called.
@@ -54,7 +55,7 @@ atexit_handler(void)
    mtx_lock(&exit_mutex);
    /* Wait for all queues to assert idle. */
    LIST_FOR_EACH_ENTRY(iter, &queue_list, head) {
-      util_queue_killall_and_wait(iter);
+      util_queue_kill_threads(iter, 0);
    }
    mtx_unlock(&exit_mutex);
 }
@@ -266,10 +267,11 @@ util_queue_thread_func(void *input)
       assert(queue->num_queued >= 0 && queue->num_queued <= queue->max_jobs);
 
       /* wait if the queue is empty */
-      while (!queue->kill_threads && queue->num_queued == 0)
+      while (thread_index < queue->num_threads && queue->num_queued == 0)
          cnd_wait(&queue->has_queued_cond, &queue->lock);
 
-      if (queue->kill_threads) {
+      /* only kill threads that are above "num_threads" */
+      if (thread_index >= queue->num_threads) {
          mtx_unlock(&queue->lock);
          break;
       }
@@ -290,17 +292,19 @@ util_queue_thread_func(void *input)
       }
    }
 
-   /* signal remaining jobs before terminating */
+   /* signal remaining jobs if all threads are being terminated */
    mtx_lock(&queue->lock);
-   for (unsigned i = queue->read_idx; i != queue->write_idx;
-        i = (i + 1) % queue->max_jobs) {
-      if (queue->jobs[i].job) {
-         util_queue_fence_signal(queue->jobs[i].fence);
-         queue->jobs[i].job = NULL;
+   if (queue->num_threads == 0) {
+      for (unsigned i = queue->read_idx; i != queue->write_idx;
+           i = (i + 1) % queue->max_jobs) {
+         if (queue->jobs[i].job) {
+            util_queue_fence_signal(queue->jobs[i].fence);
+            queue->jobs[i].job = NULL;
+         }
       }
+      queue->read_idx = queue->write_idx;
+      queue->num_queued = 0;
    }
-   queue->read_idx = queue->write_idx;
-   queue->num_queued = 0;
    mtx_unlock(&queue->lock);
    return 0;
 }
@@ -425,25 +429,37 @@ fail:
 }
 
 static void
-util_queue_killall_and_wait(struct util_queue *queue)
+util_queue_kill_threads(struct util_queue *queue, unsigned keep_num_threads)
 {
    unsigned i;
 
    /* Signal all threads to terminate. */
+   mtx_lock(&queue->finish_lock);
+
+   if (keep_num_threads >= queue->num_threads) {
+      mtx_unlock(&queue->finish_lock);
+      return;
+   }
+
    mtx_lock(&queue->lock);
-   queue->kill_threads = 1;
+   unsigned old_num_threads = queue->num_threads;
+   /* Setting num_threads is what causes the threads to terminate.
+    * Then cnd_broadcast wakes them up and they will exit their function.
+    */
+   queue->num_threads = keep_num_threads;
    cnd_broadcast(&queue->has_queued_cond);
    mtx_unlock(&queue->lock);
 
-   for (i = 0; i < queue->num_threads; i++)
+   for (i = keep_num_threads; i < old_num_threads; i++)
       thrd_join(queue->threads[i], NULL);
-   queue->num_threads = 0;
+
+   mtx_unlock(&queue->finish_lock);
 }
 
 void
 util_queue_destroy(struct util_queue *queue)
 {
-   util_queue_killall_and_wait(queue);
+   util_queue_kill_threads(queue, 0);
    remove_from_atexit_list(queue);
 
    cnd_destroy(&queue->has_space_cond);
@@ -464,7 +480,7 @@ util_queue_add_job(struct util_queue *queue,
    struct util_queue_job *ptr;
 
    mtx_lock(&queue->lock);
-   if (queue->kill_threads) {
+   if (queue->num_threads == 0) {
       mtx_unlock(&queue->lock);
       /* well no good option here, but any leaks will be
        * short-lived as things are shutting down..

src/util/u_queue.h

@@ -201,15 +201,14 @@ struct util_queue_job {
 /* Put this into your context. */
 struct util_queue {
    char name[14]; /* 13 characters = the thread name without the index */
-   mtx_t finish_lock; /* only for util_queue_finish */
+   mtx_t finish_lock; /* for util_queue_finish and protects threads/num_threads */
    mtx_t lock;
    cnd_t has_queued_cond;
    cnd_t has_space_cond;
    thrd_t *threads;
    unsigned flags;
    int num_queued;
-   unsigned num_threads;
-   int kill_threads;
+   unsigned num_threads; /* decreasing this number will terminate threads */
    int max_jobs;
    int write_idx, read_idx; /* ring buffer pointers */
    struct util_queue_job *jobs;