diff --git a/ch3/os/Makefile b/ch3/os/Makefile
index 0c03143..04340cd 100644
--- a/ch3/os/Makefile
+++ b/ch3/os/Makefile
@@ -5,7 +5,6 @@ KERNEL_ELF := target/$(TARGET)/$(MODE)/os
 KERNEL_BIN := $(KERNEL_ELF).bin
 SYMBOL_MAP := target/system.map
 QEMU_CMD_PATH := ../../tools/qemu-system-riscv64
-QEMU_PID = $(shell ps aux | grep "[q]emu-system' | awk '{print $$2}")
 OBJDUMP := rust-objdump --arch-name=riscv64
 OBJCOPY := rust-objcopy --binary-architecture=riscv64
 
@@ -47,5 +46,5 @@ clean:
 	rm -rf ./target* && rm -rf ./src/link_app.S
 
 kill:
-	-kill -9 $(QEMU_PID)
+	pkill -9 qemu