[3.14] GH-133136: Revise QSBR to reduce excess memory held (gh-135473) (#135912)

The free threading build uses QSBR to delay the freeing of dictionary
keys and list arrays when the objects are accessed by multiple threads
in order to allow concurrent reads to proceed with holding the object
lock. The requests are processed in batches to reduce execution
overhead, but for large memory blocks this can lead to excess memory
usage.

Take into account the size of the memory block when deciding when to
process QSBR requests.

Also track the amount of memory being held by QSBR for mimalloc pages.  Advance the write sequence if this memory exceeds a limit.  Advancing the sequence will allow it to be freed more quickly.

Process the held QSBR items from the "eval breaker", rather than from `_PyMem_FreeDelayed()`.  This gives a higher chance that the global read sequence has advanced enough so that items can be freed.

(cherry picked from commit 113de8545f)

Co-authored-by: Neil Schemenauer <nas-github@arctrix.com>
Co-authored-by: Sam Gross <colesbury@gmail.com>

Objects/obmalloc.c

@@ -124,6 +124,33 @@ _PyMem_mi_page_is_safe_to_free(mi_page_t *page)
 
 }
 
+#ifdef Py_GIL_DISABLED
+
+// If we are deferring collection of more than this amount of memory for
+// mimalloc pages, advance the write sequence.  Advancing allows these
+// pages to be re-used in a different thread or for a different size class.
+#define QSBR_PAGE_MEM_LIMIT 4096*20
+
+// Return true if the global write sequence should be advanced for a mimalloc
+// page that is deferred from collection.
+static bool
+should_advance_qsbr_for_page(struct _qsbr_thread_state *qsbr, mi_page_t *page)
+{
+    size_t bsize = mi_page_block_size(page);
+    size_t page_size = page->capacity*bsize;
+    if (page_size > QSBR_PAGE_MEM_LIMIT) {
+        qsbr->deferred_page_memory = 0;
+        return true;
+    }
+    qsbr->deferred_page_memory += page_size;
+    if (qsbr->deferred_page_memory > QSBR_PAGE_MEM_LIMIT) {
+        qsbr->deferred_page_memory = 0;
+        return true;
+    }
+    return false;
+}
+#endif
+
 static bool
 _PyMem_mi_page_maybe_free(mi_page_t *page, mi_page_queue_t *pq, bool force)
 {
@@ -139,7 +166,14 @@ _PyMem_mi_page_maybe_free(mi_page_t *page, mi_page_queue_t *pq, bool force)
 
         _PyMem_mi_page_clear_qsbr(page);
         page->retire_expire = 0;
-        page->qsbr_goal = _Py_qsbr_deferred_advance(tstate->qsbr);
+
+        if (should_advance_qsbr_for_page(tstate->qsbr, page)) {
+            page->qsbr_goal = _Py_qsbr_advance(tstate->qsbr->shared);
+        }
+        else {
+            page->qsbr_goal = _Py_qsbr_shared_next(tstate->qsbr->shared);
+        }
+
         llist_insert_tail(&tstate->mimalloc.page_list, &page->qsbr_node);
         return false;
     }
@@ -1141,8 +1175,44 @@ free_work_item(uintptr_t ptr, delayed_dealloc_cb cb, void *state)
     }
 }
 
+
+#ifdef Py_GIL_DISABLED
+
+// For deferred advance on free: the number of deferred items before advancing
+// the write sequence.  This is based on WORK_ITEMS_PER_CHUNK.  We ideally
+// want to process a chunk before it overflows.
+#define QSBR_DEFERRED_LIMIT 127
+
+// If the deferred memory exceeds 1 MiB, advance the write sequence.  This
+// helps limit memory usage due to QSBR delaying frees too long.
+#define QSBR_FREE_MEM_LIMIT 1024*1024
+
+// Return true if the global write sequence should be advanced for a deferred
+// memory free.
+static bool
+should_advance_qsbr_for_free(struct _qsbr_thread_state *qsbr, size_t size)
+{
+    if (size > QSBR_FREE_MEM_LIMIT) {
+        qsbr->deferred_count = 0;
+        qsbr->deferred_memory = 0;
+        qsbr->should_process = true;
+        return true;
+    }
+    qsbr->deferred_count++;
+    qsbr->deferred_memory += size;
+    if (qsbr->deferred_count > QSBR_DEFERRED_LIMIT ||
+            qsbr->deferred_memory > QSBR_FREE_MEM_LIMIT) {
+        qsbr->deferred_count = 0;
+        qsbr->deferred_memory = 0;
+        qsbr->should_process = true;
+        return true;
+    }
+    return false;
+}
+#endif
+
 static void
-free_delayed(uintptr_t ptr)
+free_delayed(uintptr_t ptr, size_t size)
 {
 #ifndef Py_GIL_DISABLED
     free_work_item(ptr, NULL, NULL);
@@ -1200,23 +1270,32 @@ free_delayed(uintptr_t ptr)
     }
 
     assert(buf != NULL && buf->wr_idx < WORK_ITEMS_PER_CHUNK);
-    uint64_t seq = _Py_qsbr_deferred_advance(tstate->qsbr);
+    uint64_t seq;
+    if (should_advance_qsbr_for_free(tstate->qsbr, size)) {
+        seq = _Py_qsbr_advance(tstate->qsbr->shared);
+    }
+    else {
+        seq = _Py_qsbr_shared_next(tstate->qsbr->shared);
+    }
     buf->array[buf->wr_idx].ptr = ptr;
     buf->array[buf->wr_idx].qsbr_goal = seq;
     buf->wr_idx++;
 
     if (buf->wr_idx == WORK_ITEMS_PER_CHUNK) {
+        // Normally the processing of delayed items is done from the eval
+        // breaker.  Processing here is a safety measure to ensure too much
+        // work does not accumulate.
         _PyMem_ProcessDelayed((PyThreadState *)tstate);
     }
 #endif
 }
 
 void
-_PyMem_FreeDelayed(void *ptr)
+_PyMem_FreeDelayed(void *ptr, size_t size)
 {
     assert(!((uintptr_t)ptr & 0x01));
     if (ptr != NULL) {
-        free_delayed((uintptr_t)ptr);
+        free_delayed((uintptr_t)ptr, size);
     }
 }
 
@@ -1226,7 +1305,10 @@ _PyObject_XDecRefDelayed(PyObject *ptr)
 {
     assert(!((uintptr_t)ptr & 0x01));
     if (ptr != NULL) {
-        free_delayed(((uintptr_t)ptr)|0x01);
+        // We use 0 as the size since we don't have an easy way to know the
+        // actual size.  If we are freeing many objects, the write sequence
+        // will be advanced due to QSBR_DEFERRED_LIMIT.
+        free_delayed(((uintptr_t)ptr)|0x01, 0);
     }
 }
 #endif
@@ -1302,6 +1384,8 @@ _PyMem_ProcessDelayed(PyThreadState *tstate)
     PyInterpreterState *interp = tstate->interp;
     _PyThreadStateImpl *tstate_impl = (_PyThreadStateImpl *)tstate;
 
+    tstate_impl->qsbr->should_process = false;
+
     // Process thread-local work
     process_queue(&tstate_impl->mem_free_queue, tstate_impl, true, NULL, NULL);