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Do not move root objects #705

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0b6de0a
Making sure that root objects do not move (for immix)
udesou Nov 21, 2022
c7c0801
Making sure that root objects do not move (for immix)
udesou Nov 21, 2022
42b5064
Merge branch 'master' into fix/non-moving-roots
udesou Nov 25, 2022
28004d9
Merge branch 'master' into fix/non-moving-roots
qinsoon Jun 26, 2023
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130 changes: 115 additions & 15 deletions src/scheduler/gc_work.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -3,6 +3,7 @@ use super::*;
use crate::plan::GcStatus;
use crate::plan::ObjectsClosure;
use crate::plan::VectorObjectQueue;
use crate::policy::immix::TRACE_KIND_FAST;
use crate::util::*;
use crate::vm::edge_shape::Edge;
use crate::vm::*;
Expand Down Expand Up @@ -561,13 +562,8 @@ impl<E: ProcessEdgesWork> RootsWorkFactory<EdgeOf<E>> for ProcessEdgesWorkRootsW

fn create_process_node_roots_work(&mut self, nodes: Vec<ObjectReference>) {
// Note: Node roots cannot be moved. Currently, this implies that the plan must never
// move objects. However, in the future, if we start to support object pinning, then
// moving plans that support object pinning (such as Immix) can still use node roots.
assert!(
!self.mmtk.plan.constraints().moves_objects,
"Attempted to add node roots when using a plan that moves objects. Plan: {:?}",
*self.mmtk.options.plan
);
// move objects. For immix, this is guaranteed through <PlanProcessEdges<E::VM, P, TRACE_KIND_FAST>> in PlanScanObjects
// but objects could also be pinned prior to being queued to ensure they do not move.

// We want to use E::create_scan_work.
let process_edges_work = E::new(vec![], true, self.mmtk);
Expand Down Expand Up @@ -783,7 +779,7 @@ impl<VM: VMBinding, P: PlanTraceObject<VM> + Plan<VM = VM>, const KIND: TraceKin
for PlanProcessEdges<VM, P, KIND>
{
type VM = VM;
type ScanObjectsWorkType = PlanScanObjects<Self, P>;
type ScanObjectsWorkType = PlanScanObjects<Self, P, KIND>;

fn new(edges: Vec<EdgeOf<Self>>, roots: bool, mmtk: &'static MMTK<VM>) -> Self {
let base = ProcessEdgesBase::new(edges, roots, mmtk);
Expand All @@ -797,7 +793,7 @@ impl<VM: VMBinding, P: PlanTraceObject<VM> + Plan<VM = VM>, const KIND: TraceKin
nodes: Vec<ObjectReference>,
roots: bool,
) -> Self::ScanObjectsWorkType {
PlanScanObjects::<Self, P>::new(self.plan, nodes, false, roots)
PlanScanObjects::<Self, P, KIND>::new(self.plan, nodes, false, roots)
}

#[inline(always)]
Expand Down Expand Up @@ -843,7 +839,11 @@ impl<VM: VMBinding, P: PlanTraceObject<VM> + Plan<VM = VM>, const KIND: TraceKin

/// This is an alternative to `ScanObjects` that calls the `post_scan_object` of the policy
/// selected by the plan. It is applicable to plans that derive `PlanTraceObject`.
pub struct PlanScanObjects<E: ProcessEdgesWork, P: Plan<VM = E::VM> + PlanTraceObject<E::VM>> {
pub struct PlanScanObjects<
E: ProcessEdgesWork,
P: Plan<VM = E::VM> + PlanTraceObject<E::VM>,
const KIND: TraceKind,
> {
plan: &'static P,
buffer: Vec<ObjectReference>,
#[allow(dead_code)]
Expand All @@ -852,7 +852,9 @@ pub struct PlanScanObjects<E: ProcessEdgesWork, P: Plan<VM = E::VM> + PlanTraceO
phantom: PhantomData<E>,
}

impl<E: ProcessEdgesWork, P: Plan<VM = E::VM> + PlanTraceObject<E::VM>> PlanScanObjects<E, P> {
impl<E: ProcessEdgesWork, P: Plan<VM = E::VM> + PlanTraceObject<E::VM>, const KIND: TraceKind>
PlanScanObjects<E, P, KIND>
{
pub fn new(
plan: &'static P,
buffer: Vec<ObjectReference>,
Expand All @@ -869,15 +871,113 @@ impl<E: ProcessEdgesWork, P: Plan<VM = E::VM> + PlanTraceObject<E::VM>> PlanScan
}
}

impl<E: ProcessEdgesWork, P: Plan<VM = E::VM> + PlanTraceObject<E::VM>> ScanObjectsWork<E::VM>
for PlanScanObjects<E, P>
impl<E: ProcessEdgesWork, P: Plan<VM = E::VM> + PlanTraceObject<E::VM>, const KIND: TraceKind>
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@wks wks Nov 25, 2022

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It looks like this const KIND: TraceKind parameter is not used.

ScanObjectsWork<E::VM> for PlanScanObjects<E, P, KIND>
{
type E = E;

fn roots(&self) -> bool {
self.roots
}

fn do_work_common(
&self,
buffer: &[ObjectReference],
worker: &mut GCWorker<<Self::E as ProcessEdgesWork>::VM>,
mmtk: &'static MMTK<<Self::E as ProcessEdgesWork>::VM>,
) {
let tls = worker.tls;

// If this is a root packet, the objects in this packet will have not been traced, yet.
//
// This step conceptually traces the edges from root slots to the objects they point to.
// However, VMs that deliver root objects instead of root edges are incapable of updating
// root slots. Like processing an edge, we call `trace_object` on those objects, and
// assert the GC doesn't move those objects because we cannot store back to the slots.
//
// If this is a root packet, the `scanned_root_objects` variable will hold those root
// objects which are traced for the first time.
let scanned_root_objects = self.roots().then(|| {
// We create an instance of E to use its `trace_object` method and its object queue.
let mut process_edges_work =
<PlanProcessEdges<E::VM, P, TRACE_KIND_FAST>>::new(vec![], false, mmtk);
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@wks wks Nov 25, 2022

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Here we hard-coded TRACE_KIND_FAST instead of using the KIND type parameter in PlanScanObjects<E, P, KIND>. It is correct for Immix, but it needs to be extended to other plans, though.

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@qinsoon qinsoon Nov 25, 2022

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This work packet is general for all the plans. TRACE_KIND_FAST is immix specific, and should not appear in general code.


for object in buffer.iter().copied() {
let new_object = process_edges_work.trace_object(object);
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Here trace_object is a method of process_edges_work. Generally speaking the ProcessEdgesWork (along with its TraceKind if relevant) is provided by the Plan (at schedule_collection) But I think whether we pin the object is a different dimension. These pinning roots needs to pinned regardless whether it is a normal collection or a defrag collection, or whether it is a marking trace or forwarding trace. Probably it is better to add one more argument to trace_object itself, or add a different trace_object_pin method to SFT and PlanTraceObject.

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In other words, TraceKind is local to a trace (should be part of TraceLocal which disappeared when we ported MMTk to Rust), while whether to pin or not is specific to this trace_object call.

debug_assert_eq!(
object, new_object,
"Object moved while tracing root unmovable root object: {} -> {}",
object, new_object
);
}

// This contains root objects that are visited the first time.
// It is sufficient to only scan these objects.
process_edges_work.nodes.take()
});

// If it is a root packet, scan the nodes that are first scanned;
// otherwise, scan the nodes in the buffer.
let objects_to_scan = scanned_root_objects.as_deref().unwrap_or(buffer);

// Then scan those objects for edges.
let mut scan_later = vec![];
{
let mut closure = ObjectsClosure::<Self::E>::new(worker);
for object in objects_to_scan.iter().copied() {
if <E::VM as VMBinding>::VMScanning::support_edge_enqueuing(tls, object) {
// If an object supports edge-enqueuing, we enqueue its edges.
<E::VM as VMBinding>::VMScanning::scan_object(tls, object, &mut closure);
self.post_scan_object(object);
} else {
// If an object does not support edge-enqueuing, we have to use
// `Scanning::scan_object_and_trace_edges` and offload the job of updating the
// reference field to the VM.
//
// However, at this point, `closure` is borrowing `worker`.
// So we postpone the processing of objects that needs object enqueuing
scan_later.push(object);
}
}
}

// If any object does not support edge-enqueuing, we process them now.
if !scan_later.is_empty() {
// We create an instance of E to use its `trace_object` method and its object queue.
let mut process_edges_work = Self::E::new(vec![], false, mmtk);
let mut closure = |object| process_edges_work.trace_object(object);

// Scan objects and trace their edges at the same time.
for object in scan_later.iter().copied() {
<E::VM as VMBinding>::VMScanning::scan_object_and_trace_edges(
tls,
object,
&mut closure,
);
self.post_scan_object(object);
}

// Create work packets to scan adjacent objects. We skip ProcessEdgesWork and create
// object-scanning packets directly, because the edges are already traced.
if !process_edges_work.nodes.is_empty() {
let next_nodes = process_edges_work.nodes.take();
let make_packet = |nodes| {
let work_packet = self.make_another(nodes);
memory_manager::add_work_packet(mmtk, WorkBucketStage::Closure, work_packet);
};

// Divide the resulting nodes into appropriately sized packets.
if next_nodes.len() <= Self::E::CAPACITY {
make_packet(next_nodes);
} else {
for chunk in next_nodes.chunks(Self::E::CAPACITY) {
make_packet(chunk.into());
}
}
}
}
}

#[inline(always)]
fn post_scan_object(&self, object: ObjectReference) {
self.plan.post_scan_object(object);
Expand All @@ -888,8 +988,8 @@ impl<E: ProcessEdgesWork, P: Plan<VM = E::VM> + PlanTraceObject<E::VM>> ScanObje
}
}

impl<E: ProcessEdgesWork, P: Plan<VM = E::VM> + PlanTraceObject<E::VM>> GCWork<E::VM>
for PlanScanObjects<E, P>
impl<E: ProcessEdgesWork, P: Plan<VM = E::VM> + PlanTraceObject<E::VM>, const KIND: TraceKind>
GCWork<E::VM> for PlanScanObjects<E, P, KIND>
{
fn do_work(&mut self, worker: &mut GCWorker<E::VM>, mmtk: &'static MMTK<E::VM>) {
trace!("PlanScanObjects");
Expand Down