heap snapshot: add gc roots and gc finalist roots to fix unrooted nodes (#52618)

JianFangAtRai · KristofferC · commit ee45196fb046 · 2024-01-24T13:54:26.000+01:00
(cherry picked from commit fe0db7d)
diff --git a/src/gc-heap-snapshot.cpp b/src/gc-heap-snapshot.cpp
@@ -3,6 +3,7 @@
 #include "gc-heap-snapshot.h"
 
 #include "julia_internal.h"
+#include "julia_assert.h"
 #include "gc.h"
 
 #include "llvm/ADT/StringMap.h"
@@ -11,9 +12,12 @@
 #include <vector>
 #include <string>
 #include <sstream>
+#include <iostream>
+#include <set>
 
 using std::vector;
 using std::string;
+using std::set;
 using std::ostringstream;
 using std::pair;
 using std::make_pair;
@@ -115,6 +119,8 @@ struct HeapSnapshot {
     DenseMap<void *, size_t> node_ptr_to_index_map;
 
     size_t num_edges = 0; // For metadata, updated as you add each edge. Needed because edges owned by nodes.
+    size_t _gc_root_idx = 1; // node index of the GC roots node
+    size_t _gc_finlist_root_idx = 2; // node index of the GC finlist roots node
 };
 
 // global heap snapshot, mutated by garbage collector
@@ -127,13 +133,13 @@ void serialize_heap_snapshot(ios_t *stream, HeapSnapshot &snapshot, char all_one
 static inline void _record_gc_edge(const char *edge_type,
                                    jl_value_t *a, jl_value_t *b, size_t name_or_index) JL_NOTSAFEPOINT;
 void _record_gc_just_edge(const char *edge_type, Node &from_node, size_t to_idx, size_t name_or_idx) JL_NOTSAFEPOINT;
-void _add_internal_root(HeapSnapshot *snapshot);
+void _add_synthetic_root_entries(HeapSnapshot *snapshot);
 
 
 JL_DLLEXPORT void jl_gc_take_heap_snapshot(ios_t *stream, char all_one)
 {
     HeapSnapshot snapshot;
-    _add_internal_root(&snapshot);
+    _add_synthetic_root_entries(&snapshot);
 
     jl_mutex_lock(&heapsnapshot_lock);
 
@@ -155,10 +161,12 @@ JL_DLLEXPORT void jl_gc_take_heap_snapshot(ios_t *stream, char all_one)
     serialize_heap_snapshot((ios_t*)stream, snapshot, all_one);
 }
 
-// adds a node at id 0 which is the "uber root":
-// a synthetic node which points to all the GC roots.
-void _add_internal_root(HeapSnapshot *snapshot)
+// mimicking https://github.com/nodejs/node/blob/5fd7a72e1c4fbaf37d3723c4c81dce35c149dc84/deps/v8/src/profiler/heap-snapshot-generator.cc#L212
+// add synthetic nodes for the uber root, the GC roots, and the GC finalizer list roots
+void _add_synthetic_root_entries(HeapSnapshot *snapshot)
 {
+    // adds a node at id 0 which is the "uber root":
+    // a synthetic node which points to all the GC roots.
     Node internal_root{
         snapshot->node_types.find_or_create_string_id("synthetic"),
         snapshot->names.find_or_create_string_id(""), // name
@@ -169,6 +177,44 @@ void _add_internal_root(HeapSnapshot *snapshot)
         vector<Edge>() // outgoing edges
     };
     snapshot->nodes.push_back(internal_root);
+
+    // Add a node for the GC roots
+    snapshot->_gc_root_idx = snapshot->nodes.size();
+    Node gc_roots{
+        snapshot->node_types.find_or_create_string_id("synthetic"),
+        snapshot->names.find_or_create_string_id("GC roots"), // name
+        snapshot->_gc_root_idx, // id
+        0, // size
+        0, // size_t trace_node_id (unused)
+        0, // int detachedness;  // 0 - unknown,  1 - attached;  2 - detached
+        SmallVector<Edge, 0>() // outgoing edges
+    };
+    snapshot->nodes.push_back(gc_roots);
+    snapshot->nodes.front().edges.push_back(Edge{
+        snapshot->edge_types.find_or_create_string_id("internal"),
+        snapshot->names.find_or_create_string_id("GC roots"), // edge label
+        snapshot->_gc_root_idx // to
+    });
+    snapshot->num_edges += 1;
+
+    // add a node for the gc finalizer list roots
+    snapshot->_gc_finlist_root_idx = snapshot->nodes.size();
+    Node gc_finlist_roots{
+        snapshot->node_types.find_or_create_string_id("synthetic"),
+        snapshot->names.find_or_create_string_id("GC finalizer list roots"), // name
+        snapshot->_gc_finlist_root_idx, // id
+        0, // size
+        0, // size_t trace_node_id (unused)
+        0, // int detachedness;  // 0 - unknown,  1 - attached;  2 - detached
+        SmallVector<Edge, 0>() // outgoing edges
+    };
+    snapshot->nodes.push_back(gc_finlist_roots);
+    snapshot->nodes.front().edges.push_back(Edge{
+        snapshot->edge_types.find_or_create_string_id("internal"),
+        snapshot->names.find_or_create_string_id("GC finlist roots"), // edge label
+        snapshot->_gc_finlist_root_idx // to
+    });
+    snapshot->num_edges += 1;
 }
 
 // mimicking https://github.com/nodejs/node/blob/5fd7a72e1c4fbaf37d3723c4c81dce35c149dc84/deps/v8/src/profiler/heap-snapshot-generator.cc#L597-L597
@@ -326,6 +372,26 @@ void _gc_heap_snapshot_record_root(jl_value_t *root, char *name) JL_NOTSAFEPOINT
     _record_gc_just_edge("internal", internal_root, to_node_idx, edge_label);
 }
 
+void _gc_heap_snapshot_record_gc_roots(jl_value_t *root, char *name) JL_NOTSAFEPOINT
+{
+    record_node_to_gc_snapshot(root);
+
+    auto from_node_idx = g_snapshot->_gc_root_idx;
+    auto to_node_idx = record_node_to_gc_snapshot(root);
+    auto edge_label = g_snapshot->names.find_or_create_string_id(name);
+    _record_gc_just_edge("internal", g_snapshot->nodes[from_node_idx], to_node_idx, edge_label);
+}
+
+void _gc_heap_snapshot_record_finlist(jl_value_t *obj, size_t index) JL_NOTSAFEPOINT
+{
+    auto from_node_idx = g_snapshot->_gc_finlist_root_idx;
+    auto to_node_idx = record_node_to_gc_snapshot(obj);
+    ostringstream ss;
+    ss << "finlist-" << index;
+    auto edge_label = g_snapshot->names.find_or_create_string_id(ss.str());
+    _record_gc_just_edge("internal", g_snapshot->nodes[from_node_idx], to_node_idx, edge_label);
+}
+
 // Add a node to the heap snapshot representing a Julia stack frame.
 // Each task points at a stack frame, which points at the stack frame of
 // the function it's currently calling, forming a linked list.
@@ -392,27 +458,19 @@ void _gc_heap_snapshot_record_object_edge(jl_value_t *from, jl_value_t *to, void
                     g_snapshot->names.find_or_create_string_id(path));
 }
 
-void _gc_heap_snapshot_record_module_to_binding(jl_module_t *module, jl_binding_t *binding) JL_NOTSAFEPOINT
+void _gc_heap_snapshot_record_module_to_binding(jl_module_t *module, jl_value_t *bindings, jl_value_t *bindingkeyset) JL_NOTSAFEPOINT
 {
-    jl_globalref_t *globalref = binding->globalref;
-    jl_sym_t *name = globalref->name;
     auto from_node_idx = record_node_to_gc_snapshot((jl_value_t*)module);
-    auto to_node_idx = record_pointer_to_gc_snapshot(binding, sizeof(jl_binding_t), jl_symbol_name(name));
-
-    jl_value_t *value = jl_atomic_load_relaxed(&binding->value);
-    auto value_idx = value ? record_node_to_gc_snapshot(value) : 0;
-    jl_value_t *ty = jl_atomic_load_relaxed(&binding->ty);
-    auto ty_idx = ty ? record_node_to_gc_snapshot(ty) : 0;
-    auto globalref_idx = record_node_to_gc_snapshot((jl_value_t*)globalref);
-
+    auto to_bindings_idx = record_node_to_gc_snapshot(bindings);
+    auto to_bindingkeyset_idx = record_node_to_gc_snapshot(bindingkeyset);
     auto &from_node = g_snapshot->nodes[from_node_idx];
-    auto &to_node = g_snapshot->nodes[to_node_idx];
-
-    _record_gc_just_edge("property", from_node, to_node_idx, g_snapshot->names.find_or_create_string_id("<native>"));
-    if (value_idx)     _record_gc_just_edge("internal", to_node, value_idx, g_snapshot->names.find_or_create_string_id("value"));
-    if (ty_idx)        _record_gc_just_edge("internal", to_node, ty_idx, g_snapshot->names.find_or_create_string_id("ty"));
-    if (globalref_idx) _record_gc_just_edge("internal", to_node, globalref_idx, g_snapshot->names.find_or_create_string_id("globalref"));
-}
+    if (to_bindings_idx > 0) {
+        _record_gc_just_edge("internal", from_node, to_bindings_idx, g_snapshot->names.find_or_create_string_id("bindings"));
+    }
+    if (to_bindingkeyset_idx > 0) {
+        _record_gc_just_edge("internal", from_node, to_bindingkeyset_idx, g_snapshot->names.find_or_create_string_id("bindingkeyset"));
+    }
+ }
 
 void _gc_heap_snapshot_record_internal_array_edge(jl_value_t *from, jl_value_t *to) JL_NOTSAFEPOINT
 {
@@ -491,6 +549,8 @@ void serialize_heap_snapshot(ios_t *stream, HeapSnapshot &snapshot, char all_one
 
     ios_printf(stream, "\"nodes\":[");
     bool first_node = true;
+    // use a set to track the nodes that do not have parents
+    set<size_t> orphans;
     for (const auto &from_node : snapshot.nodes) {
         if (first_node) {
             first_node = false;
@@ -507,6 +567,14 @@ void serialize_heap_snapshot(ios_t *stream, HeapSnapshot &snapshot, char all_one
                             from_node.edges.size(),
                             from_node.trace_node_id,
                             from_node.detachedness);
+        if (from_node.id != snapshot._gc_root_idx && from_node.id != snapshot._gc_finlist_root_idx) {
+            // find the node index from the node object pointer
+            void * ptr = (void*)from_node.id;
+            size_t n_id = snapshot.node_ptr_to_index_map[ptr];
+            orphans.insert(n_id);
+        } else {
+            orphans.insert(from_node.id);
+        }
     }
     ios_printf(stream, "],\n");
 
@@ -524,6 +592,12 @@ void serialize_heap_snapshot(ios_t *stream, HeapSnapshot &snapshot, char all_one
                                 edge.type,
                                 edge.name_or_index,
                                 edge.to_node * k_node_number_of_fields);
+            auto n_id = edge.to_node;
+            auto it = orphans.find(n_id);
+            if (it != orphans.end()) {
+                // remove the node from the orphans if it has at least one incoming edge
+                orphans.erase(it);
+            }
         }
     }
     ios_printf(stream, "],\n"); // end "edges"
@@ -533,4 +607,8 @@ void serialize_heap_snapshot(ios_t *stream, HeapSnapshot &snapshot, char all_one
     snapshot.names.print_json_array(stream, true);
 
     ios_printf(stream, "}");
+
+    // remove the uber node from the orphans
+    orphans.erase(0);
+    assert(orphans.size() == 0 && "all nodes except the uber node should have at least one incoming edge");
 }
diff --git a/src/gc-heap-snapshot.h b/src/gc-heap-snapshot.h
@@ -20,15 +20,18 @@ void _gc_heap_snapshot_record_task_to_frame_edge(jl_task_t *from, void *to) JL_N
 void _gc_heap_snapshot_record_frame_to_frame_edge(jl_gcframe_t *from, jl_gcframe_t *to) JL_NOTSAFEPOINT;
 void _gc_heap_snapshot_record_array_edge(jl_value_t *from, jl_value_t *to, size_t index) JL_NOTSAFEPOINT;
 void _gc_heap_snapshot_record_object_edge(jl_value_t *from, jl_value_t *to, void* slot) JL_NOTSAFEPOINT;
-void _gc_heap_snapshot_record_module_to_binding(jl_module_t* module, jl_binding_t* binding) JL_NOTSAFEPOINT;
+void _gc_heap_snapshot_record_module_to_binding(jl_module_t* module, jl_value_t *bindings, jl_value_t *bindingkeyset) JL_NOTSAFEPOINT;
 // Used for objects managed by GC, but which aren't exposed in the julia object, so have no
 // field or index.  i.e. they're not reachable from julia code, but we _will_ hit them in
 // the GC mark phase (so we can check their type tag to get the size).
 void _gc_heap_snapshot_record_internal_array_edge(jl_value_t *from, jl_value_t *to) JL_NOTSAFEPOINT;
 // Used for objects manually allocated in C (outside julia GC), to still tell the heap snapshot about the
 // size of the object, even though we're never going to mark that object.
 void _gc_heap_snapshot_record_hidden_edge(jl_value_t *from, void* to, size_t bytes, uint16_t alloc_type) JL_NOTSAFEPOINT;
-
+// Used for objects that are reachable from the GC roots
+void _gc_heap_snapshot_record_gc_roots(jl_value_t *root, char *name) JL_NOTSAFEPOINT;
+// Used for objects that are reachable from the finalizer list
+void _gc_heap_snapshot_record_finlist(jl_value_t *finlist, size_t index) JL_NOTSAFEPOINT;
 
 extern int gc_heap_snapshot_enabled;
 extern int prev_sweep_full;
@@ -60,6 +63,12 @@ static inline void gc_heap_snapshot_record_root(jl_value_t *root, char *name) JL
         _gc_heap_snapshot_record_root(root, name);
     }
 }
+static inline void gc_heap_snapshot_record_array_edge_index(jl_value_t *from, jl_value_t *to, size_t index) JL_NOTSAFEPOINT
+{
+    if (__unlikely(gc_heap_snapshot_enabled && prev_sweep_full && from != NULL && to != NULL)) {
+        _gc_heap_snapshot_record_array_edge(from, to, index);
+    }
+}
 static inline void gc_heap_snapshot_record_array_edge(jl_value_t *from, jl_value_t **to) JL_NOTSAFEPOINT
 {
     if (__unlikely(gc_heap_snapshot_enabled && prev_sweep_full)) {
@@ -73,10 +82,10 @@ static inline void gc_heap_snapshot_record_object_edge(jl_value_t *from, jl_valu
     }
 }
 
-static inline void gc_heap_snapshot_record_module_to_binding(jl_module_t* module, jl_binding_t* binding) JL_NOTSAFEPOINT
+static inline void gc_heap_snapshot_record_module_to_binding(jl_module_t* module, jl_value_t *bindings, jl_value_t *bindingkeyset) JL_NOTSAFEPOINT
 {
-    if (__unlikely(gc_heap_snapshot_enabled && prev_sweep_full)) {
-        _gc_heap_snapshot_record_module_to_binding(module, binding);
+    if (__unlikely(gc_heap_snapshot_enabled && prev_sweep_full) && bindings != NULL && bindingkeyset != NULL) {
+        _gc_heap_snapshot_record_module_to_binding(module, bindings, bindingkeyset);
     }
 }
 
@@ -94,6 +103,20 @@ static inline void gc_heap_snapshot_record_hidden_edge(jl_value_t *from, void* t
     }
 }
 
+static inline void gc_heap_snapshot_record_gc_roots(jl_value_t *root, char *name) JL_NOTSAFEPOINT
+{
+    if (__unlikely(gc_heap_snapshot_enabled && prev_sweep_full && root != NULL)) {
+        _gc_heap_snapshot_record_gc_roots(root, name);
+    }
+}
+
+static inline void gc_heap_snapshot_record_finlist(jl_value_t *finlist, size_t index) JL_NOTSAFEPOINT
+{
+    if (__unlikely(gc_heap_snapshot_enabled && prev_sweep_full && finlist != NULL)) {
+        _gc_heap_snapshot_record_finlist(finlist, index);
+    }
+}
+
 // ---------------------------------------------------------------------
 // Functions to call from Julia to take heap snapshot
 // ---------------------------------------------------------------------
diff --git a/src/gc.c b/src/gc.c
@@ -2190,9 +2190,10 @@ STATIC_INLINE void gc_mark_chunk(jl_ptls_t ptls, jl_gc_markqueue_t *mq, jl_gc_ch
             break;
         }
         case GC_finlist_chunk: {
+            jl_value_t *fl_parent = c->parent;
             jl_value_t **fl_begin = c->begin;
             jl_value_t **fl_end = c->end;
-            gc_mark_finlist_(mq, fl_begin, fl_end);
+            gc_mark_finlist_(mq, fl_parent, fl_begin, fl_end);
             break;
         }
         default: {
@@ -2290,6 +2291,7 @@ STATIC_INLINE void gc_mark_module_binding(jl_ptls_t ptls, jl_module_t *mb_parent
     jl_value_t *bindingkeyset = (jl_value_t *)jl_atomic_load_relaxed(&mb_parent->bindingkeyset);
     gc_assert_parent_validity((jl_value_t *)mb_parent, bindingkeyset);
     gc_try_claim_and_push(mq, bindingkeyset, &nptr);
+    gc_heap_snapshot_record_module_to_binding(mb_parent, bindings, bindingkeyset);
     gc_assert_parent_validity((jl_value_t *)mb_parent, (jl_value_t *)mb_parent->parent);
     gc_try_claim_and_push(mq, (jl_value_t *)mb_parent->parent, &nptr);
     size_t nusings = mb_parent->usings.len;
@@ -2308,7 +2310,7 @@ STATIC_INLINE void gc_mark_module_binding(jl_ptls_t ptls, jl_module_t *mb_parent
     }
 }
 
-void gc_mark_finlist_(jl_gc_markqueue_t *mq, jl_value_t **fl_begin, jl_value_t **fl_end)
+void gc_mark_finlist_(jl_gc_markqueue_t *mq, jl_value_t *fl_parent, jl_value_t **fl_begin, jl_value_t **fl_end)
 {
     jl_value_t *new_obj;
     // Decide whether need to chunk finlist
@@ -2318,8 +2320,10 @@ void gc_mark_finlist_(jl_gc_markqueue_t *mq, jl_value_t **fl_begin, jl_value_t *
         gc_chunkqueue_push(mq, &c);
         fl_end = fl_begin + GC_CHUNK_BATCH_SIZE;
     }
+    size_t i = 0;
     for (; fl_begin < fl_end; fl_begin++) {
-        new_obj = *fl_begin;
+        jl_value_t **slot = fl_begin;
+        new_obj = *slot;
         if (__unlikely(new_obj == NULL))
             continue;
         if (gc_ptr_tag(new_obj, 1)) {
@@ -2330,6 +2334,13 @@ void gc_mark_finlist_(jl_gc_markqueue_t *mq, jl_value_t **fl_begin, jl_value_t *
         if (gc_ptr_tag(new_obj, 2))
             continue;
         gc_try_claim_and_push(mq, new_obj, NULL);
+        if (fl_parent != NULL) {
+            gc_heap_snapshot_record_array_edge(fl_parent, slot);
+        } else {
+            // This is a list of objects following the same format as a finlist
+            // if `fl_parent` is NULL
+            gc_heap_snapshot_record_finlist(new_obj, ++i);
+        }
     }
 }
 
@@ -2341,7 +2352,7 @@ void gc_mark_finlist(jl_gc_markqueue_t *mq, arraylist_t *list, size_t start)
         return;
     jl_value_t **fl_begin = (jl_value_t **)list->items + start;
     jl_value_t **fl_end = (jl_value_t **)list->items + len;
-    gc_mark_finlist_(mq, fl_begin, fl_end);
+    gc_mark_finlist_(mq, NULL, fl_begin, fl_end);
 }
 
 JL_DLLEXPORT int jl_gc_mark_queue_obj(jl_ptls_t ptls, jl_value_t *obj)
@@ -3003,27 +3014,36 @@ static void gc_mark_roots(jl_gc_markqueue_t *mq)
 {
     // modules
     gc_try_claim_and_push(mq, jl_main_module, NULL);
-    gc_heap_snapshot_record_root((jl_value_t*)jl_main_module, "main_module");
+    gc_heap_snapshot_record_gc_roots((jl_value_t*)jl_main_module, "main_module");
     // invisible builtin values
     gc_try_claim_and_push(mq, jl_an_empty_vec_any, NULL);
+    gc_heap_snapshot_record_gc_roots((jl_value_t*)jl_an_empty_vec_any, "an_empty_vec_any");
     gc_try_claim_and_push(mq, jl_module_init_order, NULL);
+    gc_heap_snapshot_record_gc_roots((jl_value_t*)jl_module_init_order, "module_init_order");
     for (size_t i = 0; i < jl_current_modules.size; i += 2) {
         if (jl_current_modules.table[i + 1] != HT_NOTFOUND) {
             gc_try_claim_and_push(mq, jl_current_modules.table[i], NULL);
-            gc_heap_snapshot_record_root((jl_value_t*)jl_current_modules.table[i], "top level module");
+            gc_heap_snapshot_record_gc_roots((jl_value_t*)jl_current_modules.table[i], "top level module");
         }
     }
     gc_try_claim_and_push(mq, jl_anytuple_type_type, NULL);
+    gc_heap_snapshot_record_gc_roots((jl_value_t*)jl_anytuple_type_type, "anytuple_type_type");
     for (size_t i = 0; i < N_CALL_CACHE; i++) {
         jl_typemap_entry_t *v = jl_atomic_load_relaxed(&call_cache[i]);
         gc_try_claim_and_push(mq, v, NULL);
+        gc_heap_snapshot_record_array_edge_index((jl_value_t*)jl_anytuple_type_type, (jl_value_t*)v, i);
     }
     gc_try_claim_and_push(mq, jl_all_methods, NULL);
+    gc_heap_snapshot_record_gc_roots((jl_value_t*)jl_all_methods, "all_methods");
     gc_try_claim_and_push(mq, _jl_debug_method_invalidation, NULL);
+    gc_heap_snapshot_record_gc_roots((jl_value_t*)_jl_debug_method_invalidation, "debug_method_invalidation");
     // constants
     gc_try_claim_and_push(mq, jl_emptytuple_type, NULL);
+    gc_heap_snapshot_record_gc_roots((jl_value_t*)jl_emptytuple_type, "emptytuple_type");
     gc_try_claim_and_push(mq, cmpswap_names, NULL);
+    gc_heap_snapshot_record_gc_roots((jl_value_t*)cmpswap_names, "cmpswap_names");
     gc_try_claim_and_push(mq, jl_global_roots_table, NULL);
+    gc_heap_snapshot_record_gc_roots((jl_value_t*)jl_global_roots_table, "global_roots_table");
 }
 
 // find unmarked objects that need to be finalized from the finalizer list "list".
diff --git a/src/gc.h b/src/gc.h
@@ -427,7 +427,7 @@ extern uv_cond_t gc_threads_cond;
 extern uv_sem_t gc_sweep_assists_needed;
 extern _Atomic(int) gc_n_threads_marking;
 void gc_mark_queue_all_roots(jl_ptls_t ptls, jl_gc_markqueue_t *mq);
-void gc_mark_finlist_(jl_gc_markqueue_t *mq, jl_value_t **fl_begin, jl_value_t **fl_end) JL_NOTSAFEPOINT;
+void gc_mark_finlist_(jl_gc_markqueue_t *mq, jl_value_t *fl_parent, jl_value_t **fl_begin, jl_value_t **fl_end) JL_NOTSAFEPOINT;
 void gc_mark_finlist(jl_gc_markqueue_t *mq, arraylist_t *list, size_t start) JL_NOTSAFEPOINT;
 void gc_mark_loop_serial_(jl_ptls_t ptls, jl_gc_markqueue_t *mq);
 void gc_mark_loop_serial(jl_ptls_t ptls);
