Disable EPOLLOUT if triggered while txbuf is empty

Currently the vsock connection always triggers on EPOLLOUT. When a host
applicationlistens to the UDS, it's almost always writable, and thus
keeps triggering backend events with EPOLLOUT on the connection. As a
result, vhost-device-vsock daemon consumes 100% CPU.

To make it more CPU efficient, it can instead unsubscribe EPOLLOUT
whenever there is no more data to send. It can re-subscribe if the
connection cannot write out all bytes. This change causes the CPU util
to drop to negligible level.

Signed-off-by: Leon Yang <lnyng@meta.com>

vhost-device-vsock/CHANGELOG.md

@@ -9,6 +9,7 @@
 ### Changed
 
 ### Fixed
+- [#800](https://github.com/rust-vmm/vhost-device/pull/800) Disable EPOLLOUT if triggered while txbuf is empty
 
 ### Deprecated
 

vhost-device-vsock/src/vhu_vsock_thread.rs

@@ -18,9 +18,7 @@ use std::{
     thread,
 };
 
-#[cfg(feature = "backend_vsock")]
-use log::error;
-use log::warn;
+use log::{error, warn};
 use vhost_user_backend::{VringEpollHandler, VringRwLock, VringT};
 use virtio_queue::QueueOwnedT;
 use virtio_vsock::packet::{VsockPacket, PKT_HEADER_SIZE};
@@ -431,6 +429,9 @@ impl VhostUserVsockThread {
                 }
             } else {
                 // Previously connected connection
+
+                // Get epoll fd before getting conn as that takes self mut ref
+                let epoll_fd = self.get_epoll_fd();
                 let key = self.thread_backend.listener_map.get(&fd).unwrap();
                 let conn = self.thread_backend.conn_map.get_mut(key).unwrap();
 
@@ -441,6 +442,12 @@ impl VhostUserVsockThread {
                             if cnt > 0 {
                                 conn.fwd_cnt += Wrapping(cnt as u32);
                                 conn.rx_queue.enqueue(RxOps::CreditUpdate);
+                            } else {
+                                // If no remaining data to flush, try to disable EPOLLOUT
+                                if Self::epoll_modify(epoll_fd, fd, epoll::Events::EPOLLIN).is_err()
+                                {
+                                    error!("Failed to disable EPOLLOUT");
+                                }
                             }
                             self.thread_backend
                                 .backend_rxq

vhost-device-vsock/src/vsock_conn.rs

@@ -345,6 +345,16 @@ impl<S: AsRawFd + ReadVolatile + Write + WriteVolatile + IsHybridVsock> VsockCon
         }
 
         if written_count != buf.len() {
+            // Try to re-enable EPOLLOUT in case it is disabled when txbuf is empty.
+            if VhostUserVsockThread::epoll_modify(
+                self.epoll_fd,
+                self.stream.as_raw_fd(),
+                epoll::Events::EPOLLIN | epoll::Events::EPOLLOUT,
+            )
+            .is_err()
+            {
+                error!("Failed to re-enable EPOLLOUT");
+            }
             return self.tx_buf.push(&buf.offset(written_count).unwrap());
         }