static long long kernel_ticks; /* # of timer ticks in kernel threads. */
static long long user_ticks; /* # of timer ticks in user programs. */
+/* Scheduling. */
+#define TIME_SLICE 4 /* # of timer ticks to give each thread. */
+static unsigned thread_ticks; /* # of timer ticks since last yield. */
+
static void kernel_thread (thread_func *, void *aux);
static void idle (void *aux UNUSED);
static struct thread *running_thread (void);
static struct thread *next_thread_to_run (void);
static void init_thread (struct thread *, const char *name, int priority);
-static bool is_thread (struct thread *);
+static bool is_thread (struct thread *) UNUSED;
static void *alloc_frame (struct thread *, size_t size);
static void schedule (void);
void schedule_tail (struct thread *prev);
{
ASSERT (intr_get_level () == INTR_OFF);
- lock_init (&tid_lock, "tid");
+ lock_init (&tid_lock);
list_init (&ready_list);
/* Set up a thread structure for the running thread. */
void
thread_start (void)
{
- thread_create ("idle", PRI_DEFAULT, idle, NULL);
+ thread_create ("idle", PRI_MAX, idle, NULL);
intr_enable ();
}
-/* Called by the timer interrupt handler at each timer tick to
- update statistics. */
+/* Called by the timer interrupt handler at each timer tick.
+ Thus, this function runs in an external interrupt context. */
void
thread_tick (void)
{
struct thread *t = thread_current ();
+
+ /* Update statistics. */
if (t == idle_thread)
idle_ticks++;
#ifdef USERPROG
#endif
else
kernel_ticks++;
+
+ /* Enforce preemption. */
+ if (++thread_ticks >= TIME_SLICE)
+ intr_yield_on_return ();
}
/* Prints thread statistics. */
/* Creates a new kernel thread named NAME with the given initial
PRIORITY, which executes FUNCTION passing AUX as the argument,
- and adds it to the ready queue. If thread_start() has been
- called, then the new thread may be scheduled before
- thread_create() returns. It could even exit before
- thread_create() returns. Use a semaphore or some other form
- of synchronization if you need to ensure ordering. Returns
- the thread identifier for the new thread, or TID_ERROR if
- creation fails.
+ and adds it to the ready queue. Returns the thread identifier
+ for the new thread, or TID_ERROR if creation fails.
+
+ If thread_start() has been called, then the new thread may be
+ scheduled before thread_create() returns. It could even exit
+ before thread_create() returns. Contrariwise, the original
+ thread may run for any amount of time before the new thread is
+ scheduled. Use a semaphore or some other form of
+ synchronization if you need to ensure ordering.
The code provided sets the new thread's `priority' member to
PRIORITY, but no actual priority scheduling is implemented.
schedule ();
intr_set_level (old_level);
}
+
+/* Sets the current thread's priority to NEW_PRIORITY. */
+void
+thread_set_priority (int new_priority)
+{
+ thread_current ()->priority = new_priority;
+}
+
+/* Returns the current thread's priority. */
+int
+thread_get_priority (void)
+{
+ return thread_current ()->priority;
+}
+
+/* Sets the current thread's nice value to NICE. */
+void
+thread_set_nice (int nice UNUSED)
+{
+ /* Not yet implemented. */
+}
+
+/* Returns the current thread's nice value. */
+int
+thread_get_nice (void)
+{
+ /* Not yet implemented. */
+ return 0;
+}
+
+/* Returns 100 times the system load average. */
+int
+thread_get_load_avg (void)
+{
+ /* Not yet implemented. */
+ return 0;
+}
+
+/* Returns 100 times the current thread's recent_cpu value. */
+int
+thread_get_recent_cpu (void)
+{
+ /* Not yet implemented. */
+ return 0;
+}
\f
-/* Idle thread. Executes when no other thread is ready to run. */
+/* Idle thread. Executes when no other thread is ready to run.
+
+ The idle thread is initially put on the ready list by
+ thread_start(). It will be scheduled once initially, at which
+ point it initializes idle_thread and immediately blocks.
+ After that, the idle thread never appears in the ready list.
+ It is returned by next_thread_to_run() as a special case when
+ the ready list is empty. */
static void
idle (void *aux UNUSED)
{
+ /* Initialize idle_thread.
+
+ Until we run for the first time, idle_thread remains a null
+ pointer. That's okay because we know that, at that point,
+ the ready list has at least one element (the idle thread),
+ so next_thread_to_run() will not attempt to return the idle
+ thread. */
idle_thread = thread_current ();
for (;;)
/* Let someone else run. */
intr_disable ();
thread_block ();
- intr_enable ();
- /* Use CPU `hlt' instruction to wait for interrupt.
- See [IA32-v2a] "HLT" and [IA32-v3] 7.7. */
- asm ("hlt");
+ /* Re-enable interrupts and wait for the next one.
+
+ The `sti' instruction disables interrupts until the
+ completion of the next instruction, so these two
+ instructions are executed atomically. This atomicity is
+ important; otherwise, an interrupt could be handled
+ between re-enabling interrupts and waiting for the next
+ one to occur, wasting as much as one clock tick worth of
+ time.
+
+ See [IA32-v2a] "HLT", [IA32-v2b] "STI", and [IA32-v3a]
+ 7.11.1 "HLT Instruction". */
+ asm ("sti; hlt");
}
}
down to the start of a page. Because `struct thread' is
always at the beginning of a page and the stack pointer is
somewhere in the middle, this locates the curent thread. */
- asm ("movl %%esp, %0\n" : "=g" (esp));
+ asm ("mov %%esp, %0" : "=g" (esp));
return pg_round_down (esp);
}
/* Returns true if T appears to point to a valid thread. */
static bool
-is_thread (struct thread *t)
+is_thread (struct thread *t)
{
return t != NULL && t->magic == THREAD_MAGIC;
}
/* Mark us as running. */
cur->status = THREAD_RUNNING;
+ /* Start new time slice. */
+ thread_ticks = 0;
+
#ifdef USERPROG
/* Activate the new address space. */
process_activate ();
/* If the thread we switched from is dying, destroy its struct
thread. This must happen late so that thread_exit() doesn't
- pull out the rug under itself. */
- if (prev != NULL && prev->status == THREAD_DYING)
+ pull out the rug under itself. (We don't free
+ initial_thread because its memory was not obtained via
+ palloc().) */
+ if (prev != NULL && prev->status == THREAD_DYING && prev != initial_thread)
{
ASSERT (prev != cur);
- if (prev != initial_thread)
- palloc_free_page (prev);
+ palloc_free_page (prev);
}
}