Task scheduler: Remove per-pool threads limit

This feature was adding extra complexity to task scheduling
which required yet extra variables to be worried about to be
modified in atomic manner, which resulted in following issues:

- More complex code to maintain, which increases risks of
  something going wrong when we modify the code.

- Extra barriers and/or locks during task scheduling, which
  causes extra threading overhead.

- Unable to use some other implementation (such as TBB) even for
  the comparison tests.

Notes about other changes.

There are two places where we really had to use that limit.

One of them is the single threaded dependency graph. This will
now construct a single-threaded scheduler at evaluation time.
This shouldn't be a problem because it only happens when using
debugging command line arguments and the code simply don't
run in regular Blender operation.

The code seems a bit duplicated here across old and new
depsgraph, but think it's OK since the old depsgraph is already
gone in 2.8 branch and i don't see where else we might want
to use such a single-threaded scheduler.

When/if we'll want to do so, we can move it to a centralized
single-threaded scheduler in threads.c.

OpenGL render was a bit more tricky to port, but basically we
are using conditional variables to wait background thread to
do all the job.

2 files changed, 10 insertions, 36 deletions

diff --git a/source/blender/blenlib/BLI_task.h b/source/blender/blenlib/BLI_task.h
index d27bf4dad20..bc695d174fa 100644
--- a/source/blender/blenlib/BLI_task.h
+++ b/source/blender/blenlib/BLI_task.h
@@ -96,9 +96,6 @@ void BLI_task_pool_work_and_wait(TaskPool *pool);
 /* cancel all tasks, keep worker threads running */
 void BLI_task_pool_cancel(TaskPool *pool);
 
-/* set number of threads allowed to be used by this pool */
-void BLI_pool_set_num_threads(TaskPool *pool, int num_threads);
-
 /* for worker threads, test if canceled */
 bool BLI_task_pool_canceled(TaskPool *pool);
 
diff --git a/source/blender/blenlib/intern/task.c b/source/blender/blenlib/intern/task.c
index 5d16fd9229c..2bf1ee26507 100644
--- a/source/blender/blenlib/intern/task.c
+++ b/source/blender/blenlib/intern/task.c
@@ -106,8 +106,6 @@ struct TaskPool {
 	TaskScheduler *scheduler;
 
 	volatile size_t num;
-	size_t num_threads;
-	size_t currently_running_tasks;
 	ThreadMutex num_mutex;
 	ThreadCondition num_cond;
 
@@ -236,7 +234,6 @@ static void task_pool_num_decrease(TaskPool *pool, size_t done)
 	BLI_assert(pool->num >= done);
 
 	pool->num -= done;
-	atomic_sub_and_fetch_z(&pool->currently_running_tasks, done);
 
 	if (pool->num == 0)
 		BLI_condition_notify_all(&pool->num_cond);
@@ -290,17 +287,10 @@ static bool task_scheduler_thread_wait_pop(TaskScheduler *scheduler, Task **task
 				continue;
 			}
 
-			if (atomic_add_and_fetch_z(&pool->currently_running_tasks, 1) <= pool->num_threads ||
-			    pool->num_threads == 0)
-			{
-				*task = current_task;
-				found_task = true;
-				BLI_remlink(&scheduler->queue, *task);
-				break;
-			}
-			else {
-				atomic_sub_and_fetch_z(&pool->currently_running_tasks, 1);
-			}
+			*task = current_task;
+			found_task = true;
+			BLI_remlink(&scheduler->queue, *task);
+			break;
 		}
 		if (!found_task)
 			BLI_condition_wait(&scheduler->queue_cond, &scheduler->queue_mutex);
@@ -502,8 +492,6 @@ static TaskPool *task_pool_create_ex(TaskScheduler *scheduler, void *userdata, c
 
 	pool->scheduler = scheduler;
 	pool->num = 0;
-	pool->num_threads = 0;
-	pool->currently_running_tasks = 0;
 	pool->do_cancel = false;
 	pool->run_in_background = is_background;
 
@@ -648,16 +636,12 @@ void BLI_task_pool_work_and_wait(TaskPool *pool)
 		/* find task from this pool. if we get a task from another pool,
 		 * we can get into deadlock */
 
-		if (pool->num_threads == 0 ||
-		    pool->currently_running_tasks < pool->num_threads)
-		{
-			for (task = scheduler->queue.first; task; task = task->next) {
-				if (task->pool == pool) {
-					work_task = task;
-					found_task = true;
-					BLI_remlink(&scheduler->queue, task);
-					break;
-				}
+		for (task = scheduler->queue.first; task; task = task->next) {
+			if (task->pool == pool) {
+				work_task = task;
+				found_task = true;
+				BLI_remlink(&scheduler->queue, task);
+				break;
 			}
 		}
 
@@ -666,7 +650,6 @@ void BLI_task_pool_work_and_wait(TaskPool *pool)
 		/* if found task, do it, otherwise wait until other tasks are done */
 		if (found_task) {
 			/* run task */
-			atomic_add_and_fetch_z(&pool->currently_running_tasks, 1);
 			work_task->run(pool, work_task->taskdata, 0);
 
 			/* delete task */
@@ -687,12 +670,6 @@ void BLI_task_pool_work_and_wait(TaskPool *pool)
 	BLI_mutex_unlock(&pool->num_mutex);
 }
 
-void BLI_pool_set_num_threads(TaskPool *pool, int num_threads)
-{
-	/* NOTE: Don't try to modify threads while tasks are running! */
-	pool->num_threads = num_threads;
-}
-
 void BLI_task_pool_cancel(TaskPool *pool)
 {
 	pool->do_cancel = true;