binder: use bitmap for faster descriptor lookup

When creating new binder references, the driver assigns a descriptor id
that is shared with userspace. Regrettably, the driver needs to keep the
descriptors small enough to accommodate userspace potentially using them
as Vector indexes. Currently, the driver performs a linear search on the
rb-tree of references to find the smallest available descriptor id. This
approach, however, scales poorly as the number of references grows.

This patch introduces the usage of bitmaps to boost the performance of
descriptor assignments. This optimization results in notable performance
gains, particularly in processes with a large number of references. The
following benchmark with 100,000 references showcases the difference in
latency between the dbitmap implementation and the legacy approach:

  [  587.145098] get_ref_desc_olocked: 15us (dbitmap on)
  [  602.788623] get_ref_desc_olocked: 47343us (dbitmap off)

Note the bitmap size is dynamically adjusted in line with the number of
references, ensuring efficient memory usage. In cases where growing the
bitmap is not possible, the driver falls back to the slow legacy method.

A previous attempt to solve this issue was proposed in [1]. However,
such method involved adding new ioctls which isn't great, plus older
userspace code would not have benefited from the optimizations either.

Link: https://lore.kernel.org/all/20240417191418.1341988-1-cmllamas@google.com/ [1]
Cc: Tim Murray <timmurray@google.com>
Cc: Arve Hjønnevåg <arve@android.com>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Martijn Coenen <maco@android.com>
Cc: Todd Kjos <tkjos@android.com>
Cc: John Stultz <jstultz@google.com>
Cc: Steven Moreland <smoreland@google.com>
Suggested-by: Nick Chen <chenjia3@oppo.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Link: https://lore.kernel.org/r/20240612042535.1556708-1-cmllamas@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 99 insertions, 13 deletions

diff --git a/drivers/android/binder.c b/drivers/android/binder.c
index b21a7b246a0d..0c2161b1f057 100644
--- a/drivers/android/binder.c
+++ b/drivers/android/binder.c
@@ -1045,6 +1045,66 @@ static struct binder_ref *binder_get_ref_olocked(struct binder_proc *proc,
 	return NULL;
 }
 
+/* Find the smallest unused descriptor the "slow way" */
+static u32 slow_desc_lookup_olocked(struct binder_proc *proc)
+{
+	struct binder_ref *ref;
+	struct rb_node *n;
+	u32 desc;
+
+	desc = 1;
+	for (n = rb_first(&proc->refs_by_desc); n; n = rb_next(n)) {
+		ref = rb_entry(n, struct binder_ref, rb_node_desc);
+		if (ref->data.desc > desc)
+			break;
+		desc = ref->data.desc + 1;
+	}
+
+	return desc;
+}
+
+/*
+ * Find an available reference descriptor ID. The proc->outer_lock might
+ * be released in the process, in which case -EAGAIN is returned and the
+ * @desc should be considered invalid.
+ */
+static int get_ref_desc_olocked(struct binder_proc *proc,
+				struct binder_node *node,
+				u32 *desc)
+{
+	struct dbitmap *dmap = &proc->dmap;
+	unsigned long *new, bit;
+	unsigned int nbits;
+
+	/* 0 is reserved for the context manager */
+	if (node == proc->context->binder_context_mgr_node) {
+		*desc = 0;
+		return 0;
+	}
+
+	if (!dbitmap_enabled(dmap)) {
+		*desc = slow_desc_lookup_olocked(proc);
+		return 0;
+	}
+
+	if (dbitmap_acquire_first_zero_bit(dmap, &bit) == 0) {
+		*desc = bit;
+		return 0;
+	}
+
+	/*
+	 * The dbitmap is full and needs to grow. The proc->outer_lock
+	 * is briefly released to allocate the new bitmap safely.
+	 */
+	nbits = dbitmap_grow_nbits(dmap);
+	binder_proc_unlock(proc);
+	new = bitmap_zalloc(nbits, GFP_KERNEL);
+	binder_proc_lock(proc);
+	dbitmap_grow(dmap, new, nbits);
+
+	return -EAGAIN;
+}
+
 /**
  * binder_get_ref_for_node_olocked() - get the ref associated with given node
  * @proc:	binder_proc that owns the ref
@@ -1068,12 +1128,14 @@ static struct binder_ref *binder_get_ref_for_node_olocked(
 					struct binder_node *node,
 					struct binder_ref *new_ref)
 {
-	struct binder_context *context = proc->context;
-	struct rb_node **p = &proc->refs_by_node.rb_node;
-	struct rb_node *parent = NULL;
 	struct binder_ref *ref;
-	struct rb_node *n;
+	struct rb_node *parent;
+	struct rb_node **p;
+	u32 desc;
 
+retry:
+	p = &proc->refs_by_node.rb_node;
+	parent = NULL;
 	while (*p) {
 		parent = *p;
 		ref = rb_entry(parent, struct binder_ref, rb_node_node);
@@ -1088,6 +1150,10 @@ static struct binder_ref *binder_get_ref_for_node_olocked(
 	if (!new_ref)
 		return NULL;
 
+	/* might release the proc->outer_lock */
+	if (get_ref_desc_olocked(proc, node, &desc) == -EAGAIN)
+		goto retry;
+
 	binder_stats_created(BINDER_STAT_REF);
 	new_ref->data.debug_id = atomic_inc_return(&binder_last_id);
 	new_ref->proc = proc;
@@ -1095,14 +1161,7 @@ static struct binder_ref *binder_get_ref_for_node_olocked(
 	rb_link_node(&new_ref->rb_node_node, parent, p);
 	rb_insert_color(&new_ref->rb_node_node, &proc->refs_by_node);
 
-	new_ref->data.desc = (node == context->binder_context_mgr_node) ? 0 : 1;
-	for (n = rb_first(&proc->refs_by_desc); n != NULL; n = rb_next(n)) {
-		ref = rb_entry(n, struct binder_ref, rb_node_desc);
-		if (ref->data.desc > new_ref->data.desc)
-			break;
-		new_ref->data.desc = ref->data.desc + 1;
-	}
-
+	new_ref->data.desc = desc;
 	p = &proc->refs_by_desc.rb_node;
 	while (*p) {
 		parent = *p;
@@ -1131,6 +1190,7 @@ static struct binder_ref *binder_get_ref_for_node_olocked(
 
 static void binder_cleanup_ref_olocked(struct binder_ref *ref)
 {
+	struct dbitmap *dmap = &ref->proc->dmap;
 	bool delete_node = false;
 
 	binder_debug(BINDER_DEBUG_INTERNAL_REFS,
@@ -1138,6 +1198,8 @@ static void binder_cleanup_ref_olocked(struct binder_ref *ref)
 		      ref->proc->pid, ref->data.debug_id, ref->data.desc,
 		      ref->node->debug_id);
 
+	if (dbitmap_enabled(dmap))
+		dbitmap_clear_bit(dmap, ref->data.desc);
 	rb_erase(&ref->rb_node_desc, &ref->proc->refs_by_desc);
 	rb_erase(&ref->rb_node_node, &ref->proc->refs_by_node);
 
@@ -1298,6 +1360,25 @@ static void binder_free_ref(struct binder_ref *ref)
 	kfree(ref);
 }
 
+/* shrink descriptor bitmap if needed */
+static void try_shrink_dmap(struct binder_proc *proc)
+{
+	unsigned long *new;
+	int nbits;
+
+	binder_proc_lock(proc);
+	nbits = dbitmap_shrink_nbits(&proc->dmap);
+	binder_proc_unlock(proc);
+
+	if (!nbits)
+		return;
+
+	new = bitmap_zalloc(nbits, GFP_KERNEL);
+	binder_proc_lock(proc);
+	dbitmap_shrink(&proc->dmap, new, nbits);
+	binder_proc_unlock(proc);
+}
+
 /**
  * binder_update_ref_for_handle() - inc/dec the ref for given handle
  * @proc:	proc containing the ref
@@ -1334,8 +1415,10 @@ static int binder_update_ref_for_handle(struct binder_proc *proc,
 		*rdata = ref->data;
 	binder_proc_unlock(proc);
 
-	if (delete_ref)
+	if (delete_ref) {
 		binder_free_ref(ref);
+		try_shrink_dmap(proc);
+	}
 	return ret;
 
 err_no_ref:
@@ -4931,6 +5014,7 @@ static void binder_free_proc(struct binder_proc *proc)
 	put_task_struct(proc->tsk);
 	put_cred(proc->cred);
 	binder_stats_deleted(BINDER_STAT_PROC);
+	dbitmap_free(&proc->dmap);
 	kfree(proc);
 }
 
@@ -5634,6 +5718,8 @@ static int binder_open(struct inode *nodp, struct file *filp)
 	proc = kzalloc(sizeof(*proc), GFP_KERNEL);
 	if (proc == NULL)
 		return -ENOMEM;
+
+	dbitmap_init(&proc->dmap);
 	spin_lock_init(&proc->inner_lock);
 	spin_lock_init(&proc->outer_lock);
 	get_task_struct(current->group_leader);