diff --git a/lab4/.gdbinit b/lab4/.gdbinit
index 52dd98f..6608d0e 100644
--- a/lab4/.gdbinit
+++ b/lab4/.gdbinit
@@ -13,4 +13,4 @@ define add-symbol-file-auto
 end
 
 add-symbol-file-auto ./build/kernel.img
-add-symbol-file-auto ./user/build/ramdisk/yield_single.bin
+add-symbol-file-auto ./user/build/ramdisk/yield_spin.bin
diff --git a/lab4/kernel/exception/exception.c b/lab4/kernel/exception/exception.c
index 932a151..be3af16 100644
--- a/lab4/kernel/exception/exception.c
+++ b/lab4/kernel/exception/exception.c
@@ -32,7 +32,7 @@ void exception_init_per_cpu(void)
 	 * Uncomment the timer_init() when you are handling preemptive
 	 * shceduling
 	 */
-	// timer_init();
+	timer_init();
 
 	/**
 	 * Lab3: Your code here
diff --git a/lab4/kernel/exception/irq.c b/lab4/kernel/exception/irq.c
index 9a71412..802037a 100644
--- a/lab4/kernel/exception/irq.c
+++ b/lab4/kernel/exception/irq.c
@@ -50,6 +50,8 @@ void handle_irq(int type)
 	 * Lab4
 	 * Do you miss something?
 	 */
+	sched();
+    eret_to_thread(switch_context());
 }
 
 void plat_handle_irq(void)
diff --git a/lab4/kernel/sched/policy_rr.c b/lab4/kernel/sched/policy_rr.c
index 5ef239d..a0658be 100644
--- a/lab4/kernel/sched/policy_rr.c
+++ b/lab4/kernel/sched/policy_rr.c
@@ -106,6 +106,7 @@ int rr_sched_dequeue(struct thread *thread)
     // 出队
     list_del(&thread->ready_queue_node);
     thread->thread_ctx->state = TS_INTER;
+    thread->thread_ctx->sc->budget = DEFAULT_BUDGET;
 
 	return 0;
 }
@@ -164,11 +165,18 @@ int rr_sched(void)
 {
     struct thread *dest_thread;
 
-    // 若当前有运行进程，放入队列
-    if (current_thread != NULL && current_thread->thread_ctx != NULL
-            && current_thread->thread_ctx->type != TYPE_IDLE) {
-        rr_sched_enqueue(current_thread);
-        kdebug("[RR] Cpu #%d yield thread %lx to queue\n", smp_get_cpu_id(), current_thread->thread_ctx);
+    // 若当前有运行进程
+    if (current_thread != NULL && current_thread->thread_ctx != NULL) {
+        // 预算非 0
+        if (current_thread->thread_ctx->sc->budget != 0) {
+            return 0;
+        }
+
+        // 若非空闲线程，放入队列
+        if (current_thread->thread_ctx->type != TYPE_IDLE) {
+            rr_sched_enqueue(current_thread);
+            kdebug("[RR] Cpu #%d yield thread %lx to queue\n", smp_get_cpu_id(), current_thread->thread_ctx);
+        }
     }
 
     // 选择新线程
@@ -221,6 +229,12 @@ int rr_sched_init(void)
  */
 void rr_sched_handle_timer_irq(void)
 {
+    // 预算
+    if (current_thread != NULL && current_thread->thread_ctx != NULL) {
+        if (current_thread->thread_ctx->sc->budget > 0) {
+            current_thread->thread_ctx->sc->budget--;
+        }
+    }
 }
 
 struct sched_ops rr = {
diff --git a/lab4/kernel/sched/sched.c b/lab4/kernel/sched/sched.c
index d09defa..582898e 100644
--- a/lab4/kernel/sched/sched.c
+++ b/lab4/kernel/sched/sched.c
@@ -140,6 +140,10 @@ u64 switch_context(void)
  */
 void sys_yield(void)
 {
+    // 重置预算
+    current_thread->thread_ctx->sc->budget = 0;
+
+    // 调度
     sched();
     eret_to_thread(switch_context());
 }