Re: [PATCH] rust_binder: add TF_DEFER_COMPLETE flag for avoiding userspace roundtrip

On Thu, Jul 16, 2026 at 11:20:56AM +0000, Alice Ryhl wrote:
> Outgoing transactions are able to send a message and wait for its reply
> in a single ioctl. Why not avoid a userspace roundtrip by applying the
> same logic for replying to incoming messages and waiting for the next
> incoming message?
> 
> Generally, when you send a reply using BC_REPLY, the kernel sends
> BR_TRANSACTION_COMPLETE as a reply to BC_REPLY right away. The
> BR_TRANSACTION_COMPLETE command indicates that it's safe for userspace
> to free any resources associated with this message (such as embedded fds
> or Binder nodes). However, the BR_TRANSACTION_COMPLETE message is
> problematic because after BC_REPLY is issued, there will be a pending
> message for userspace. The kernel will refuse to sleep for incoming
> messages in this scenario.
> 
> The way this is handled for outgoing transaction is through a mechanism
> known as deferred delivery of BR_TRANSACTION_COMPLETE. The idea is that
> when you send an outgoing transaction, then we do not return to
> userspace right away if BR_TRANSACTION_COMPLETE is the only pending
> message. This patch adds a new flag called TF_DEFER_COMPLETE that lets
> userspace opt-in to the same deferred delivery mechanism for
> BR_TRANSACTION_COMPLETE when using BC_REPLY.
> 
> Given this new uapi, we can adjust sendReply in userspace libbinder
> so that it writes the BC_REPLY command into mOut but does not flush the
> buffer to the kernel. Then, userspace simply continues running until it
> returns all the way out to the top-level joinThreadPool() loop, which
> calls into the kernel to get the next incoming transaction. At this
> point, mOut is flushed, sending the reply. The same ioctl then proceeds
> to sleep for an incoming message.
> 
> Userspace only actually specifies TF_DEFER_COMPLETE when the Parcel does
> not contain fds or refcounts on binder objects. This is because
> otherwise said fd or binder node will not be freed until the binder
> thread receives another incoming transaction, which could be a long
> time. In the case of fds, this is especially important because delaying
> fclose() can result in processes hanging because they read from a pipe
> that isn't being closed due to fclose() not getting called. Note that
> even if TF_DEFER_COMPLETE is not specified for this transaction, it can
> still be useful to defer the BC_REPLY command, as it can still avoid a
> userspace roundtrip when a new incoming transaction is available right
> away.
> 
> Observing the cuttlefish logs while booting with this change shows that
> there were 4297 opportunities for this optimization to kick in (that is,
> boot invoked BC_REPLY 4297 times). Out of those, 3441 binder ioctls sent
> and received a transaction in the same ioctl. This indicates that we
> successfully eliminated a syscall on the server side for 80% of incoming
> transactions. Generally, this means that a server is now able to handle
> incoming messages using one syscall per incoming message (for each
> incoming transaction, the syscall handles one BC_FREE_BUFFER and
> BC_REPLY command, and then waits for the next incoming transaction).
> 
> Signed-off-by: Alice Ryhl <[email protected]>

Does not apply against the char-misc-testing branch :(