rust_binder: use bitmap for allocation of handles

To find an unused Binder handle, Rust Binder currently iterates the
red/black tree from the beginning until it finds a gap in the keys. This
is extremely slow.

To improve the performance, add a bitmap that keeps track of which
indices are actually in use. This allows us to quickly find an unused
key in the red/black tree.

For a benchmark, please see the below numbers that were obtained from
modifying binderThroughputTest to send a node with each transaction and
stashing it in the server. This results in the number of nodes
increasing by one for every transaction sent. I got the following table
of roundtrip latencies (in µs):

Transaction Range │ Baseline (Rust) │ Bitmap (Rust) │ Comparison (C)
0 - 10,000        │          176.88 │         92.93 │          99.41
10,000 - 20,000   │          437.37 │         87.74 │          98.55
20,000 - 30,000   │          677.49 │         76.24 │          96.37
30,000 - 40,000   │          901.76 │         83.39 │          96.73
40,000 - 50,000   │         1126.62 │        100.44 │          94.57
50,000 - 60,000   │         1288.98 │         94.38 │          96.64
60,000 - 70,000   │         1588.74 │         88.27 │          96.36
70,000 - 80,000   │         1812.97 │         93.97 │          91.24
80,000 - 90,000   │         2062.95 │         92.22 │         102.01
90,000 - 100,000  │         2330.03 │         97.18 │         100.31

It should be clear that the current Rust code becomes linearly slower
per insertion as the number of calls to rb_next() per transaction
increases. After this change, the time to find an ID number appears
constant. (Technically it is not constant-time as both insertion and
removal scan the entire bitmap. However, quick napkin math shows that
scanning the entire bitmap with N=100k takes ~1.5µs, which is neglible
in a benchmark where the rountrip latency is 100µs.)

I've included a comparison to the C driver, which uses the same bitmap
algorithm as this patch since commit 15d9da3f81 ("binder: use bitmap
for faster descriptor lookup").

This currently checks if the bitmap should be shrunk after every
removal. One potential future change is introducing a shrinker to make
this operation O(1), but based on the benchmark above this does not seem
required at this time.

Reviewed-by: Burak Emir <bqe@google.com>
Reviewed-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>
Acked-by: Carlos Llamas <cmllamas@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Yury Norov (NVIDIA) <yury.norov@gmail.com>

drivers/android/binder/process.rs

@@ -19,6 +19,7 @@ use kernel::{
     cred::Credential,
     error::Error,
     fs::file::{self, File},
+    id_pool::IdPool,
     list::{List, ListArc, ListArcField, ListLinks},
     mm,
     prelude::*,
@@ -394,6 +395,8 @@ kernel::list::impl_list_item! {
 struct ProcessNodeRefs {
     /// Used to look up nodes using the 32-bit id that this process knows it by.
     by_handle: RBTree<u32, ListArc<NodeRefInfo, { NodeRefInfo::LIST_PROC }>>,
+    /// Used to quickly find unused ids in `by_handle`.
+    handle_is_present: IdPool,
     /// Used to look up nodes without knowing their local 32-bit id. The usize is the address of
     /// the underlying `Node` struct as returned by `Node::global_id`.
     by_node: RBTree<usize, u32>,
@@ -408,6 +411,7 @@ impl ProcessNodeRefs {
     fn new() -> Self {
         Self {
             by_handle: RBTree::new(),
+            handle_is_present: IdPool::new(),
             by_node: RBTree::new(),
             freeze_listeners: RBTree::new(),
         }
@@ -802,7 +806,7 @@ impl Process {
     pub(crate) fn insert_or_update_handle(
         self: ArcBorrow<'_, Process>,
         node_ref: NodeRef,
-        is_mananger: bool,
+        is_manager: bool,
     ) -> Result<u32> {
         {
             let mut refs = self.node_refs.lock();
@@ -821,7 +825,33 @@ impl Process {
         let reserve2 = RBTreeNodeReservation::new(GFP_KERNEL)?;
         let info = UniqueArc::new_uninit(GFP_KERNEL)?;
 
-        let mut refs = self.node_refs.lock();
+        let mut refs_lock = self.node_refs.lock();
+        let mut refs = &mut *refs_lock;
+
+        let (unused_id, by_handle_slot) = loop {
+            // ID 0 may only be used by the manager.
+            let start = if is_manager { 0 } else { 1 };
+
+            if let Some(res) = refs.handle_is_present.find_unused_id(start) {
+                match refs.by_handle.entry(res.as_u32()) {
+                    rbtree::Entry::Vacant(entry) => break (res, entry),
+                    rbtree::Entry::Occupied(_) => {
+                        pr_err!("Detected mismatch between handle_is_present and by_handle");
+                        res.acquire();
+                        kernel::warn_on!(true);
+                        return Err(EINVAL);
+                    }
+                }
+            }
+
+            let grow_request = refs.handle_is_present.grow_request().ok_or(ENOMEM)?;
+            drop(refs_lock);
+            let resizer = grow_request.realloc(GFP_KERNEL)?;
+            refs_lock = self.node_refs.lock();
+            refs = &mut *refs_lock;
+            refs.handle_is_present.grow(resizer);
+        };
+        let handle = unused_id.as_u32();
 
         // Do a lookup again as node may have been inserted before the lock was reacquired.
         if let Some(handle_ref) = refs.by_node.get(&node_ref.node.global_id()) {
@@ -831,20 +861,9 @@ impl Process {
             return Ok(handle);
         }
 
-        // Find id.
-        let mut target: u32 = if is_mananger { 0 } else { 1 };
-        for handle in refs.by_handle.keys() {
-            if *handle > target {
-                break;
-            }
-            if *handle == target {
-                target = target.checked_add(1).ok_or(ENOMEM)?;
-            }
-        }
-
         let gid = node_ref.node.global_id();
         let (info_proc, info_node) = {
-            let info_init = NodeRefInfo::new(node_ref, target, self.into());
+            let info_init = NodeRefInfo::new(node_ref, handle, self.into());
             match info.pin_init_with(info_init) {
                 Ok(info) => ListArc::pair_from_pin_unique(info),
                 // error is infallible
@@ -865,9 +884,10 @@ impl Process {
         // `info_node` into the right node's `refs` list.
         unsafe { info_proc.node_ref2().node.insert_node_info(info_node) };
 
-        refs.by_node.insert(reserve1.into_node(gid, target));
-        refs.by_handle.insert(reserve2.into_node(target, info_proc));
-        Ok(target)
+        refs.by_node.insert(reserve1.into_node(gid, handle));
+        by_handle_slot.insert(info_proc, reserve2);
+        unused_id.acquire();
+        Ok(handle)
     }
 
     pub(crate) fn get_transaction_node(&self, handle: u32) -> BinderResult<NodeRef> {
@@ -932,6 +952,16 @@ impl Process {
                 let id = info.node_ref().node.global_id();
                 refs.by_handle.remove(&handle);
                 refs.by_node.remove(&id);
+                refs.handle_is_present.release_id(handle as usize);
+
+                if let Some(shrink) = refs.handle_is_present.shrink_request() {
+                    drop(refs);
+                    // This intentionally ignores allocation failures.
+                    if let Ok(new_bitmap) = shrink.realloc(GFP_KERNEL) {
+                        refs = self.node_refs.lock();
+                        refs.handle_is_present.shrink(new_bitmap);
+                    }
+                }
             }
         } else {
             // All refs are cleared in process exit, so this warning is expected in that case.