diff --git a/Makefile b/Makefile
index 81a90e65..7c240815 100644
--- a/Makefile
+++ b/Makefile
@@ -44,6 +44,7 @@ TEST_DIR := ./testcase/$(TEST)
 export STRACE := 
 export SMP :=
 export PREEMPT :=
+export DEBUG :=
 
 # Args
 DISASM_ARGS = -d
diff --git a/arch/src/riscv64/sstatus.rs b/arch/src/riscv64/sstatus.rs
index 9941347c..c20553cd 100644
--- a/arch/src/riscv64/sstatus.rs
+++ b/arch/src/riscv64/sstatus.rs
@@ -24,6 +24,10 @@ impl Sstatus {
         }
     }
 
+    pub fn sie(&mut self) -> bool {
+        self.bits.get_bit(1)
+    }
+
     pub fn set_spie(&mut self, val: bool) {
         self.bits.set_bit(5, val);
     }
diff --git a/kernel/Makefile b/kernel/Makefile
index 366b7ce6..b24892d5 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -9,6 +9,9 @@ endif
 ifneq ($(PREEMPT), )
 	FEATURES += preempt
 endif
+ifneq ($(DEBUG), )
+	FEATURES += debug
+endif
 
 CARGO_BUILD_ARGS :=
 ifeq ($(MODE), release)
diff --git a/kernel/src/syscall/time.rs b/kernel/src/syscall/time.rs
index a6e9f36f..f3873c7e 100644
--- a/kernel/src/syscall/time.rs
+++ b/kernel/src/syscall/time.rs
@@ -173,7 +173,7 @@ impl Syscall<'_> {
                         return Ok(0);
                     }
                     let sleep = req - current;
-                    task.suspend_timeout(req).await
+                    task.suspend_timeout(sleep).await
                 } else {
                     task.suspend_timeout(req).await
                 };
diff --git a/kernel/src/task/schedule.rs b/kernel/src/task/schedule.rs
index 04df8cb8..30f10b4a 100644
--- a/kernel/src/task/schedule.rs
+++ b/kernel/src/task/schedule.rs
@@ -79,6 +79,12 @@ impl<F: Future<Output = ()> + Send + 'static> Future for KernelTaskFuture<F> {
 pub async fn task_loop(task: Arc<Task>) {
     *task.waker() = Some(get_waker().await);
     loop {
+        match task.state() {
+            Zombie => break,
+            Stopped => suspend_now().await,
+            _ => {}
+        }
+
         trap::user_trap::trap_return(&task);
 
         // task may be set to zombie by other task, e.g. execve will kill other tasks in
diff --git a/kernel/src/trap/user_trap.rs b/kernel/src/trap/user_trap.rs
index f2558bac..f9a2092a 100644
--- a/kernel/src/trap/user_trap.rs
+++ b/kernel/src/trap/user_trap.rs
@@ -162,6 +162,8 @@ pub fn trap_return(task: &Arc<Task>) {
     // 2. This task encounter a signal handler
     task.trap_context_mut().user_fx.restore();
     task.trap_context_mut().sstatus.set_fs(FS::Clean);
+    assert!(!task.trap_context_mut().sstatus.sie());
+    assert!(!task.is_zombie());
     unsafe {
         __return_to_user(task.trap_context_mut());
         // NOTE: next time when user traps into kernel, it will come back here