diff --git a/crate/process/src/process_manager.rs b/crate/process/src/process_manager.rs
index beca345..b03bff1 100644
--- a/crate/process/src/process_manager.rs
+++ b/crate/process/src/process_manager.rs
@@ -39,23 +39,17 @@ pub struct ProcessManager {
     scheduler: Mutex<Box<Scheduler>>,
     wait_queue: Vec<Mutex<Vec<Pid>>>,
     event_hub: Mutex<EventHub<Event>>,
+    exit_handler: fn(Pid),
 }
 
 impl ProcessManager {
-    pub fn new(scheduler: Box<Scheduler>, max_proc_num: usize) -> Self {
+    pub fn new(scheduler: Box<Scheduler>, max_proc_num: usize, exit_handler: fn(Pid)) -> Self {
         ProcessManager {
-            procs: {
-                let mut vec = Vec::new();
-                vec.resize_default(max_proc_num);
-                vec
-            },
+            procs: new_vec_default(max_proc_num),
             scheduler: Mutex::new(scheduler),
-            wait_queue: {
-                let mut vec = Vec::new();
-                vec.resize_default(max_proc_num);
-                vec
-            },
+            wait_queue: new_vec_default(max_proc_num),
             event_hub: Mutex::new(EventHub::new()),
+            exit_handler,
         }
     }
 
@@ -124,7 +118,7 @@ impl ProcessManager {
         proc.context = Some(context);
         match proc.status {
             Status::Ready => self.scheduler.lock().insert(pid),
-            Status::Exited(_) => proc.context = None,
+            Status::Exited(_) => self.exit_handler(pid, proc),
             _ => {}
         }
     }
@@ -152,7 +146,7 @@ impl ProcessManager {
             _ => proc.status = status,
         }
         match proc.status {
-            Status::Exited(_) => proc.context = None,
+            Status::Exited(_) => self.exit_handler(pid, proc),
             _ => {}
         }
     }
@@ -189,8 +183,20 @@ impl ProcessManager {
 
     pub fn exit(&self, pid: Pid, code: ExitCode) {
         self.set_status(pid, Status::Exited(code));
+    }
+
+    /// Called when a process exit
+    fn exit_handler(&self, pid: Pid, proc: &mut Process) {
         for waiter in self.wait_queue[pid].lock().drain(..) {
             self.wakeup(waiter);
         }
+        proc.context = None;
+        (self.exit_handler)(pid);
     }
 }
+
+fn new_vec_default<T: Default>(size: usize) -> Vec<T> {
+    let mut vec = Vec::new();
+    vec.resize_default(size);
+    vec
+}
diff --git a/kernel/src/arch/x86_64/interrupt/handler.rs b/kernel/src/arch/x86_64/interrupt/handler.rs
index 2b8b188..a9e9ecf 100644
--- a/kernel/src/arch/x86_64/interrupt/handler.rs
+++ b/kernel/src/arch/x86_64/interrupt/handler.rs
@@ -80,6 +80,7 @@ pub extern fn rust_trap(tf: &mut TrapFrame) {
         T_PGFLT => page_fault(tf),
         T_IRQ0...63 => {
             let irq = tf.trap_num as u8 - T_IRQ0;
+            super::ack(irq); // must ack before switching
             match irq {
                 IRQ_TIMER => ::trap::timer(),
                 IRQ_KBD => keyboard(),
@@ -88,7 +89,6 @@ pub extern fn rust_trap(tf: &mut TrapFrame) {
                 IRQ_IDE => ide(),
                 _ => panic!("Invalid IRQ number: {}", irq),
             }
-            super::ack(irq);
         }
         T_SWITCH_TOK => to_kernel(tf),
         T_SWITCH_TOU => to_user(tf),
diff --git a/kernel/src/process/mod.rs b/kernel/src/process/mod.rs
index 8531278..c5b3413 100644
--- a/kernel/src/process/mod.rs
+++ b/kernel/src/process/mod.rs
@@ -12,7 +12,7 @@ mod context;
 pub fn init() {
     // NOTE: max_time_slice <= 5 to ensure 'priority' test pass
     let scheduler = Box::new(scheduler::RRScheduler::new(5));
-    let manager = Arc::new(ProcessManager::new(scheduler, MAX_PROCESS_NUM));
+    let manager = Arc::new(ProcessManager::new(scheduler, MAX_PROCESS_NUM, Process::proc_exit));
 
     extern fn idle(_arg: usize) -> ! {
         loop { cpu::halt(); }
diff --git a/kernel/src/syscall.rs b/kernel/src/syscall.rs
index 79c4ea4..80037c8 100644
--- a/kernel/src/syscall.rs
+++ b/kernel/src/syscall.rs
@@ -91,6 +91,7 @@ fn sys_wait(pid: usize, code: *mut i32) -> i32 {
                 _ => {}
             }
         }
+        info!("wait: {} -> {}, sleep", thread::current().id(), pid);
         if pid == 0 {
             Process::wait_child();
         } else {
@@ -107,8 +108,8 @@ fn sys_yield() -> i32 {
 
 /// Kill the process
 fn sys_kill(pid: usize) -> i32 {
+    info!("kill: {}", pid);
     processor().manager().exit(pid, 0x100);
-    Process::proc_exit(pid);
     if pid == thread::current().id() {
         processor().yield_now();
     }
@@ -123,8 +124,8 @@ fn sys_getpid() -> i32 {
 /// Exit the current process
 fn sys_exit(exit_code: usize) -> i32 {
     let pid = thread::current().id();
+    info!("exit: {}", pid);
     processor().manager().exit(pid, exit_code);
-    Process::proc_exit(pid);
     processor().yield_now();
     unreachable!();
 }