Lab: Multithreading

Uthread: switching between threads

实现一个用户级线程，跟内核线程的切换没什么区别。只要上课听懂了，复制内核代码就行了
只贴关键代码

线程上下文

struct context {
  uint64 ra;
  uint64 sp;

  // callee-saved
  uint64 s0;
  uint64 s1;
  uint64 s2;
  uint64 s3;
  uint64 s4;
  uint64 s5;
  uint64 s6;
  uint64 s7;
  uint64 s8;
  uint64 s9;
  uint64 s10;
  uint64 s11;
};

struct thread {
  char       stack[STACK_SIZE]; /* the thread's stack */
  int        state;             /* FREE, RUNNING, RUNNABLE */
  struct context context;
};

线程切换

if (current_thread != next_thread) {         /* switch threads?  */
    next_thread->state = RUNNING;
    t = current_thread;
    current_thread = next_thread;
    /* YOUR CODE HERE
    * Invoke thread_switch to switch from t to next_thread:
    * thread_switch(??, ??);
    */
    thread_switch((uint64)&t->context,(uint64)&current_thread->context);
}

创建线程

设置好开始执行的位置和栈指针

void 
thread_create(void (*func)())
{
  struct thread *t;

  for (t = all_thread; t < all_thread + MAX_THREAD; t++) {
    if (t->state == FREE) break;
  }
  t->state = RUNNABLE;
  // YOUR CODE HERE
  t->context.ra=(uint64)func;
  t->context.sp=(uint64)t->stack+STACK_SIZE;
}

上下文切换

汇编代码，与内核一致
保存ra,sp,callee saved registers

thread_switch:
/* YOUR CODE HERE */

sd ra, 0(a0)
sd sp, 8(a0)
sd s0, 16(a0)
sd s1, 24(a0)
sd s2, 32(a0)
sd s3, 40(a0)
sd s4, 48(a0)
sd s5, 56(a0)
sd s6, 64(a0)
sd s7, 72(a0)
sd s8, 80(a0)
sd s9, 88(a0)
sd s10, 96(a0)
sd s11, 104(a0)

ld ra, 0(a1)
ld sp, 8(a1)
ld s0, 16(a1)
ld s1, 24(a1)
ld s2, 32(a1)
ld s3, 40(a1)
ld s4, 48(a1)
ld s5, 56(a1)
ld s6, 64(a1)
ld s7, 72(a1)
ld s8, 80(a1)
ld s9, 88(a1)
ld s10, 96(a1)
ld s11, 104(a1)

ret    /* return to ra */

Using threads

确定哈希表的线程安全
其实跟kalloc.c/freeList的意思是一样，上课听懂了就能理解
大致情况如下

在插入的时候加锁就行
性能上可以优化一下，变成每个桶一个锁，测试里能快个两秒左右吧

if(e){
    // update the existing key.
    e->value = value;
} else {
    // the new is new.
    pthread_mutex_lock(&locks[i]);
    insert(key, value, &table[i], table[i]);
    pthread_mutex_unlock(&locks[i]);
}

Barrier

线程间的屏障，可以参考barrier
大概意思就先到达这个点的线程要等待后面的线程，等到大家都执行到这里了，再统一放行，继续执行

static void 
barrier()
{
  // YOUR CODE HERE
  //
  // Block until all threads have called barrier() and
  // then increment bstate.round.
  //
  pthread_mutex_lock(&bstate.barrier_mutex);
  bstate.nthread++;
  if(bstate.nthread==nthread)
  {
    bstate.nthread=0;
    bstate.round++;
    pthread_cond_broadcast(&bstate.barrier_cond);
  }
  else
  {
    int cur_round=bstate.round;
    while(bstate.round==cur_round)
      pthread_cond_wait(&bstate.barrier_cond , &bstate.barrier_mutex);
  }
  pthread_mutex_unlock(&bstate.barrier_mutex);
}

为了不被偷袭，防了一手虚假唤醒，其实不防也没事

test