Threads

Struct thread

The main Pintos data structure for threads is struct thread, declared in threads/thread.h.

Structure: struct thread
- Represents a thread or a user process.
- In the projects, you will have to add your own members to struct thread. You may also change or delete the definitions of existing members.

Members of struct thread

tid_t tid
- The thread's thread identifier or tid. Every thread must have a tid that is unique over the entire lifetime of the kernel.
- By default, tid_t is a typedef for int and each new thread receives the numerically next higher tid, starting from 1 for the initial process. You can change the type and the numbering scheme if you like.
enum thread_status status
- The thread's state, one of the following:
- THREAD_RUNNING:
  - The thread is running. Exactly one thread is running at a given time. thread_current() returns the running thread.
- THREAD_READY:
  - The thread is ready to run, but it's not running right now.
  - The thread could be selected to run the next time the scheduler is invoked.
  - Ready threads are kept in a doubly linked list called ready_list.
- THREAD_BLOCKED:
  - The thread is waiting for something, e.g. a lock to become available, an interrupt to be invoked.
  - The thread won't be scheduled again until it transitions to the THREAD_READY state with a call to thread_unblock().
  - This is most conveniently done indirectly, using one of the Pintos synchronization primitives that block and unblock threads automatically (see section Synchronization).
  - There is no a priori way to tell what a blocked thread is waiting for, but a backtrace can help (see section Backtraces).
- THREAD_DYING:
  - The thread will be destroyed by the scheduler after switching to the next thread.
char name[16]
- The thread's name as a string, or at least the first few characters of it.
uint8_t *stack
- Every thread has its own stack to keep track of its state. When the thread is running, the CPU's stack pointer register tracks the top of the stack and this member is unused. But when the CPU switches to another thread, this member saves the thread's stack pointer. No other members are needed to save the thread's registers, because the other registers that must be saved are saved on the stack.
- When an interrupt occurs, whether in the kernel or a user program, an struct intr_frame is pushed onto the stack. When the interrupt occurs in a user program, the struct intr_frame is always at the very top of the page. See section Interrupt Handling, for more information.
int priority
- A thread priority, ranging from PRI_MIN (0) to PRI_MAX (63).
- Lower numbers correspond to lower priorities, so that priority 0 is the lowest priority and priority 63 is the highest.
- Pintos as provided ignores thread priorities, but you will implement priority scheduling in project 1.
struct list_elem allelem
- This "list element" is used to link the thread into the list of all threads. Each thread is inserted into this list when it is created and removed when it exits.
- The thread_foreach() function should be used to iterate over all threads.
struct list_elem elem
- A "list element" used to put the thread into doubly linked lists, either ready_list (the list of threads ready to run) or a list of threads waiting on a semaphore in sema_down(). It can do double duty because a thread waiting on a semaphore is not ready, and vice versa.
uint32_t *pagedir
- Only present in project 2 and later. See section Page Table.
unsigned magic
- Always set to THREAD_MAGIC, which is just an arbitrary number defined in threads/thread.c, and used to detect stack overflow.
- thread_current() checks that the magic member of the running thread's struct thread is set to THREAD_MAGIC.
- Stack overflow tends to change this value, triggering the assertion. For greatest benefit, as you add members to struct thread, leave magic at the end.

Notes

Every struct thread occupies the beginning of its own page of memory. The rest of the page is used for the thread's stack, which grows downward from the end of the page. It looks like this:

                  4 kB +---------------------------------+
                       |          kernel stack           |
                       |                |                |
                       |                |                |
                       |                V                |
                       |         grows downward          |
                       |                                 |
                       |                                 |
                       |                                 |
                       |                                 |
                       |                                 |
                       |                                 |
                       |                                 |
                       |                                 |
sizeof (struct thread) +---------------------------------+
                       |              magic              |
                       |                :                |
                       |                :                |
                       |              status             |
                       |               tid               |
                  0 kB +---------------------------------+

This has two consequences.
1. First, struct thread must not be allowed to grow too big. If it does, then there will not be enough room for the kernel stack. The base struct thread is only a few bytes in size. It probably should stay well under 1 kB.
2. Second, kernel stacks must not be allowed to grow too large. If a stack overflows, it will corrupt the thread state. Thus, kernel functions should not allocate large structures or arrays as non-static local variables. Use dynamic allocation with malloc() or palloc_get_page() instead (see section Memory Allocation).

Thread Functions

threads/thread.c implements several public functions for thread support. Let's take a look at the most useful:

Functions of struct thread

Function: void thread_init (void)
- Called by pintos_init() to initialize the thread system. Its main purpose is to create a struct thread for Pintos's initial thread. This is possible because the Pintos loader puts the initial thread's stack at the top of a page, in the same position as any other Pintos thread.
- Before thread_init() runs, thread_current() will fail because the running thread's magic value is incorrect. Lots of functions call thread_current() directly or indirectly, including lock_acquire() for locking a lock, so thread_init() is called early in Pintos initialization.
Function: void thread_start (void)
- Called by pintos_init() to start the scheduler. Creates the idle thread, that is, the thread that is scheduled when no other thread is ready.
- Then enables interrupts, which as a side effect enables the scheduler because the scheduler runs on return from the timer interrupt, using intr_yield_on_return().
Function: void thread_tick (void)
- Called by the timer interrupt at each timer tick.
- It keeps track of thread statistics and triggers the scheduler when a time slice expires.
Function: void thread_print_stats (void)
- Called during Pintos shutdown to print thread statistics.
Function: tid_t thread_create (const char *name, int priority, thread_func *func, void *aux)
- Creates and starts a new thread named name with the given priority, returning the new thread's tid. The thread executes func, passing aux as the function's single argument.
- thread_create() allocates a page for the thread's struct thread and stack and initializes its members, then it sets up a set of fake stack frames for it (see section Thread Switching).
- The thread is initialized in the blocked state, then unblocked just before returning, which allows the new thread to be scheduled (see Thread States).
Type: void thread_func (void *aux)
- This is the type of the function passed to thread_create(), whose aux argument is passed along as the function's argument.
Function: void thread_block (void)
- Transitions the running thread from the running state to the blocked state (see Thread States). The thread will not run again until thread_unblock() is called on it, so you'd better have some way arranged for that to happen.
- Because thread_block() is so low-level, you should prefer to use one of the synchronization primitives instead (see section Synchronization).
Function: void thread_unblock (struct thread *thread)
- Transitions thread, which must be in the blocked state, to the ready state, allowing it to resume running (see Thread States).
- This is called when the event that the thread is waiting for occurs, e.g. when the lock that the thread is waiting on becomes available.
Function: struct thread *thread_current (void)
- Returns the running thread.
Function: tid_t thread_tid (void)
- Returns the running thread's thread id.
- Equivalent to thread_current ()->tid.
Function: const char *thread_name (void)
- Returns the running thread's name.
- Equivalent to thread_current ()->name.
Function: void thread_exit (void) NO_RETURN
- Causes the current thread to exit.
- Never returns, hence NO_RETURN (see section Function and Parameter Attributes).
Function: void thread_yield (void)
- Yields the CPU to the scheduler, which picks a new thread to run.
- The new thread might be the current thread, so you can't depend on this function to keep this thread from running for any particular length of time.
Function: void thread_foreach (thread_action_func *action, void *aux)
- Iterates over all threads t and invokes action(t, aux) on each.
- action must refer to a function that matches the signature given by thread_action_func():
Type: void thread_action_func (struct thread *thread, void *aux)
- Performs some action on a thread, given aux.

Function: int thread_get_priority (void)
Function: void thread_set_priority (int new_priority)
- Stub to set and get thread priority. Used in project 1.

Function: int thread_get_nice (void)
Function: void thread_set_nice (int new_nice)
Function: int thread_get_recent_cpu (void)
Function: int thread_get_load_avg (void)
- Stubs for the advanced scheduler. See section 4.4BSD Scheduler.

Thread Switching

schedule() is responsible for switching threads.

It is internal to threads/thread.c and called only by the three public thread functions that need to switch threads: thread_block(), thread_exit(), and thread_yield().
Before any of these functions call schedule(), they disable interrupts (or ensure that they are already disabled) and then change the running thread's state to something other than running.

Schedule()

schedule() is short but tricky.

It records the current thread in local variable cur,
determines the next thread to run as local variable next (by calling next_thread_to_run()),
and then calls switch_threads() to do the actual thread switch.
- The thread we switched to was also running inside switch_threads(), as are all the threads not currently running, so the new thread now returns out of switch_threads(), returning the previously running thread.
- switch_threads() is an assembly language routine in threads/switch.S.
  - It saves registers on the stack, saves the CPU's current stack pointer in the current struct thread's stack member.
  - It restores the new thread's stack into the CPU's stack pointer, restores registers from the stack, and returns.
The rest of the scheduler is implemented in thread_schedule_tail().
- It marks the new thread as running.
- If the thread we just switched from is in the dying state, then it also frees the page that contained the dying thread's struct thread and stack.
- These couldn't be freed prior to the thread switch because the switch needed to use it.

Run a Thread for the First Time

Running a thread for the first time is a special case.

When thread_create() creates a new thread, it goes through a fair amount of trouble to get it started properly. In particular, the new thread hasn't started running yet, so there's no way for it to be running inside switch_threads() as the scheduler expects. To solve the problem, thread_create() creates some _fake stack frames_ in the new thread's stack:

The topmost fake stack frame is for switch_threads(), represented by struct switch_threads_frame. The important part of this frame is its eip member, the return address. We point eip to switch_entry(), indicating it to be the function that called switch_entry().
The next fake stack frame is for switch_entry(), an assembly language routine in threads/switch.S that adjusts the stack pointer, calls thread_schedule_tail() (this special case is why thread_schedule_tail() is separate from schedule()), and returns. We fill in its stack frame so that it returns into kernel_thread(), a function in threads/thread.c.
The final stack frame is for kernel_thread(), which enables interrupts and calls the thread's function (the function passed to thread_create()). If the thread's function returns, it calls thread_exit() to terminate the thread.

The following is the stack page layout of a thread created by thread_create() and scheduled for the first time. If you find some problems, feel free to contact us.

                  4 kB +---------------------------------+
                       |               aux               |
                       |            function             |
                       |            eip (NULL)           | kernel_thread_frame
                       +---------------------------------+
                       |      eip (to kernel_thread)     | switch_entry_frame
                       +---------------------------------+
                       |               next              | 
                       |               cur               |
                       |      eip (to switch_entry)      | 
                       |               ebx               | 
                       |               ebp               | 
                       |               esi               | 
                       |               edi               | switch_threads_frame
                       +---------------------------------+
                       |          kernel stack           |
                       |                |                |
                       |                |                |
                       |                V                |
                       |         grows downward          | 
sizeof (struct thread) +---------------------------------+
                       |              magic              |
                       |                :                |
                       |                :                |
                       |               name              |
                       |              status             |
                  0 kB +---------------------------------+

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