add unloading of tasks to GC (#8671)

### Description

Inactive tasks can be completely unloaded. This disconnects them from the graph and frees nearly all memory (except for the TaskId mapping).

When running GC for a task we check if it's inactive and unload it when it is.

But we also need to enqueue a task for GC again when it becomes inactive. So we maintain a queue of potentially inactive tasks and walk the children during GC.

crates/turbo-tasks-memory/src/aggregation/mod.rs

@@ -81,7 +81,7 @@ impl<I, A> AggregationNode<I, A> {
  AggregationNode::Leaf {
  aggregation_number, ..
  } => *aggregation_number as u32,
- AggregationNode::Aggegating(aggegating) => aggegating.aggregation_number,
+ AggregationNode::Aggegating(aggregating) => aggregating.aggregation_number,
  }
  }
 
@@ -92,21 +92,21 @@ impl<I, A> AggregationNode<I, A> {
  fn uppers(&self) -> &CountHashSet<I> {
  match self {
  AggregationNode::Leaf { uppers, .. } => uppers,
- AggregationNode::Aggegating(aggegating) => &aggegating.uppers,
+ AggregationNode::Aggegating(aggregating) => &aggregating.uppers,
  }
  }
 
  fn uppers_mut(&mut self) -> &mut CountHashSet<I> {
  match self {
  AggregationNode::Leaf { uppers, .. } => uppers,
- AggregationNode::Aggegating(aggegating) => &mut aggegating.uppers,
+ AggregationNode::Aggegating(aggregating) => &mut aggregating.uppers,
  }
  }
 
  fn followers(&self) -> Option<&CountHashSet<I>> {
  match self {
  AggregationNode::Leaf { .. } => None,
- AggregationNode::Aggegating(aggegating) => Some(&aggegating.followers),
+ AggregationNode::Aggegating(aggregating) => Some(&aggregating.followers),
  }
  }
 }

crates/turbo-tasks-memory/src/cell.rs

@@ -158,9 +158,14 @@ impl Cell {
  }
  }
 
+ /// Assigns a new content to the cell. Will notify dependent tasks if the
+ /// content has changed.
+ /// If clean = true, the task inputs weren't changes since the last
+ /// execution and can be assumed to produce the same content again.
  pub fn assign(
  &mut self,
  content: CellContent,
+ clean: bool,
  turbo_tasks: &dyn TurboTasksBackendApi<MemoryBackend>,
  ) {
  match self {
@@ -175,31 +180,46 @@ impl Cell {
  ref mut dependent_tasks,
  } => {
  event.notify(usize::MAX);
- // Assigning to a cell will invalidate all dependent tasks as the content might
- // have changed.
- if !dependent_tasks.is_empty() {
- turbo_tasks.schedule_notify_tasks_set(dependent_tasks);
+ if clean {
+ // We can assume that the task is deterministic and produces the same content
+ // again. No need to notify dependent tasks.
+ *self = Cell::Value {
+ content,
+ dependent_tasks: take(dependent_tasks),
+ };
+ } else {
+ // Assigning to a cell will invalidate all dependent tasks as the content might
+ // have changed.
+ if !dependent_tasks.is_empty() {
+ turbo_tasks.schedule_notify_tasks_set(dependent_tasks);
+ }
+ *self = Cell::Value {
+ content,
+ dependent_tasks: AutoSet::default(),
+ };
  }
- *self = Cell::Value {
- content,
- dependent_tasks: AutoSet::default(),
- };
  }
  &mut Cell::TrackedValueless {
  ref mut dependent_tasks,
  } => {
- // Assigning to a cell will invalidate all dependent tasks as the content might
- // have changed.
- // TODO this leads to flagging task unnecessarily dirty when a GC'ed task is
- // recomputed. We need to use the notification of changed cells for the current
- // task to check if it's valid to skip the invalidation here
- if !dependent_tasks.is_empty() {
- turbo_tasks.schedule_notify_tasks_set(dependent_tasks);
+ if clean {
+ // We can assume that the task is deterministic and produces the same content
+ // again. No need to notify dependent tasks.
+ *self = Cell::Value {
+ content,
+ dependent_tasks: take(dependent_tasks),
+ };
+ } else {
+ // Assigning to a cell will invalidate all dependent tasks as the content might
+ // have changed.
+ if !dependent_tasks.is_empty() {
+ turbo_tasks.schedule_notify_tasks_set(dependent_tasks);
+ }
+ *self = Cell::Value {
+ content,
+ dependent_tasks: AutoSet::default(),
+ };
  }
- *self = Cell::Value {
- content,
- dependent_tasks: AutoSet::default(),
- };
  }
  Cell::Value {
  content: ref mut cell_content,