give each cpu its own clock, so that preemption works on cpu 1

main.c

@@ -28,6 +28,8 @@ main()
     acquire_spinlock(&kernel_lock);
     idtinit(); // CPU's idt
     lapic_init(cpu());
+    lapic_timerinit();
+    lapic_enableintr();
     scheduler();
   }
   acpu = 1;

usertests.c

@@ -93,8 +93,8 @@ preempt()
 main()
 {
   puts("usertests starting\n");
-  pipe1();
-  //preempt();
+  //pipe1();
+  preempt();
 
   while(1)
     ;