Generational plans set next_gc_full_heap in end_of_gc (#714)

This PR adds `Plan.end_of_gc()`. Our generational plans compute whether we should do full heap GC at the end of `Plan.release()` which is incorrect. There could be other parallel release work when we execute `Plan.release()`. This PR adds `Plan.end_of_gc()`, and we compute `next_gc_full_heap` in `end_of_gc()`.

Author: qinsoon

src/plan/generational/copying/global.rs

@@ -123,12 +123,9 @@ impl<VM: VMBinding> Plan for GenCopy<VM> {
         if full_heap {
             self.fromspace().release();
         }
+    }
 
-        // TODO: Refactor so that we set the next_gc_full_heap in gen.release(). Currently have to fight with Rust borrow checker
-        // NOTE: We have to take care that the `Gen::should_next_gc_be_full_heap()` function is
-        // called _after_ all spaces have been released (including ones in `gen`) as otherwise we
-        // may get incorrect results since the function uses values such as available pages that
-        // will change dependant on which spaces have been released
+    fn end_of_gc(&mut self, _tls: VMWorkerThread) {
         self.gen
             .set_next_gc_full_heap(Gen::should_next_gc_be_full_heap(self));
     }

src/plan/generational/immix/global.rs

@@ -165,12 +165,9 @@ impl<VM: VMBinding> Plan for GenImmix<VM> {
         }
         self.last_gc_was_full_heap
             .store(full_heap, Ordering::Relaxed);
+    }
 
-        // TODO: Refactor so that we set the next_gc_full_heap in gen.release(). Currently have to fight with Rust borrow checker
-        // NOTE: We have to take care that the `Gen::should_next_gc_be_full_heap()` function is
-        // called _after_ all spaces have been released (including ones in `gen`) as otherwise we
-        // may get incorrect results since the function uses values such as available pages that
-        // will change dependant on which spaces have been released
+    fn end_of_gc(&mut self, _tls: VMWorkerThread) {
         self.gen
             .set_next_gc_full_heap(Gen::should_next_gc_be_full_heap(self));
     }

src/plan/global.rs

@@ -221,17 +221,22 @@ pub trait Plan: 'static + Sync + Downcast {
     }
 
     /// Prepare the plan before a GC. This is invoked in an initial step in the GC.
-    /// This is invoked once per GC by one worker thread. 'tls' is the worker thread that executes this method.
+    /// This is invoked once per GC by one worker thread. `tls` is the worker thread that executes this method.
     fn prepare(&mut self, tls: VMWorkerThread);
 
     /// Prepare a worker for a GC. Each worker has its own prepare method. This hook is for plan-specific
     /// per-worker preparation. This method is invoked once per worker by the worker thread passed as the argument.
     fn prepare_worker(&self, _worker: &mut GCWorker<Self::VM>) {}
 
-    /// Release the plan after a GC. This is invoked at the end of a GC when most GC work is finished.
-    /// This is invoked once per GC by one worker thread. 'tls' is the worker thread that executes this method.
+    /// Release the plan after transitive closure. A plan can implement this method to call each policy's release,
+    /// or create any work packet that should be done in release.
+    /// This is invoked once per GC by one worker thread. `tls` is the worker thread that executes this method.
     fn release(&mut self, tls: VMWorkerThread);
 
+    /// Inform the plan about the end of a GC. It is guaranteed that there is no further work for this GC.
+    /// This is invoked once per GC by one worker thread. `tls` is the worker thread that executes this method.
+    fn end_of_gc(&mut self, _tls: VMWorkerThread) {}
+
     /// Ask the plan if they would trigger a GC. If MMTk is in charge of triggering GCs, this method is called
     /// periodically during allocation. However, MMTk may delegate the GC triggering decision to the runtime,
     /// in which case, this method may not be called. This method returns true to trigger a collection.

src/scheduler/gc_work.rs

@@ -223,6 +223,11 @@ impl<VM: VMBinding> GCWork<VM> for EndOfGC {
     fn do_work(&mut self, worker: &mut GCWorker<VM>, mmtk: &'static MMTK<VM>) {
         info!("End of GC");
 
+        // We assume this is the only running work packet that accesses plan at the point of execution
+        #[allow(clippy::cast_ref_to_mut)]
+        let plan_mut: &mut dyn Plan<VM = VM> = unsafe { &mut *(&*mmtk.plan as *const _ as *mut _) };
+        plan_mut.end_of_gc(worker.tls);
+
         #[cfg(feature = "extreme_assertions")]
         if crate::util::edge_logger::should_check_duplicate_edges(&*mmtk.plan) {
             // reset the logging info at the end of each GC