优化linux内核启动时间之loops_per_jiffy

参考原文http://blog.csdn.net/reille/article/details/5694155
原文 Linux加速启动，启动时间的极限优化

CELF(The Consumer Electronics Linux Forum)论坛

CELF已经提出了一整套针对消费类电子产品所使用的嵌入式Linux的启动优化方案
CELF- CE Linux Forum 在kernel提交了一个“Preset LPJ”的patch，这个patch可以通过预设LPJ来减少linux kernel启动过程中校正loops_per_jiffy所需花费的时间。CELF这个patch，其实就是增加了一个kernel 参数，使得用户可以在kernel启动时预设lpj，无须复杂的计算
正常启动后记录下内核信息中的“Calibrating Delay”数值后就可以在启动参数中以下面的形式强制指定LPJ值了：

lpj=9600700

CELF Developer Wiki

ShortIDEDelays - 缩短IDE探测时长（我的应用场景中不包含硬盘，所以用不上）
　　KernelXIP - 直接在ROM或Flash中运行内核（考虑到兼容性因素，未采用）
　　IDENoProbe - 跳过未连接设备的IDE口
　　OptimizeRCScripts - 优化initrd中的linuxrc脚本（我采用了BusyBox更简洁的linuxrc）

CELF Developer Wiki

CELF已有专项的优化方案：“RTCNoSync”和“PresetLPJ”。

　　前者通过屏蔽启动过程中所进行的RTC时钟同步或者将这一过程放到启动后进行（视具体应用对时钟精度的需求而定），实现起来比较容易，但需要为内核打补丁。似乎CELF目前的工作仅仅是去掉了该过程，而没有实现所提到的“延后”处理RTC时钟的同步。考虑到这个原因，我的方案中暂时没有引入这一优化（毕竟它所带来的时间漂移已经达到了“秒”级），继续关注中。

　　后者是通过在启动参数中强制指定LPJ值而跳过实际的计算过程，这是基于LPJ值在硬件条件不变的情况下不会变化的考虑。所以在正常启动后记录下内核信息中的“Calibrating Delay”数值后就可以在启动参数中以下面的形式强制指定LPJ值了：

lpj=9600700

　　上面分析结果中的 4、5 两项都是SMP初始化的一部分，因此不在CELF研究的范畴（或许将来会有采用多核的MP4出现？……），只能自力更生了。研究了一下SMP的初始化代码，发现“Migration Cost”其实也可以像“Calibrating Delay”采用预置的方式跳过校准时间。方法类似，最后在内核启动参数中增加：

migration_cost=4000,4000

//////////////////////////////////////////////////////////////////////////////////////

init/main.c/start_kernel函数会调用此函数
/*
 * This is the number of bits of precision for the loops_per_jiffy.  Each
 * bit takes on average 1.5/HZ seconds.  This (like the original) is a little
 * better than 1%
 * For the boot cpu we can skip the delay calibration and assign it a value
 * calculated based on the timer frequency.
 * For the rest of the CPUs we cannot assume that the timer frequency is same as
 * the cpu frequency, hence do the calibration for those.
 */
void __cpuinit calibrate_delay(void)
{
 unsigned long ticks, loopbit;
 int lps_precision = LPS_PREC;
 static bool printed;

 if (preset_lpj) {
  pr_info("##########calibrate.c calibrate_delay 00\n");
  loops_per_jiffy = preset_lpj;
  if (!printed)
   pr_info("Calibrating delay loop (skipped) "
    "preset value.. ");
 } else if ((!printed) && lpj_fine) {
 pr_info("##########calibrate.c calibrate_delay 11\n");
  loops_per_jiffy = lpj_fine;
  pr_info("Calibrating delay loop (skipped), "
   "value calculated using timer frequency.. ");
 } else if ((loops_per_jiffy = calibrate_delay_direct()) != 0) {
 pr_info("##########calibrate.c calibrate_delay 22");
  if (!printed)
   pr_info("Calibrating delay using timer "
    "specific routine.. ");
 } else {
 pr_info("##########calibrate.c calibrate_delay 33");
  loops_per_jiffy = (1<<12);

  if (!printed)
   pr_info("Calibrating delay loop... ");
  while ((loops_per_jiffy <<= 1) != 0) {
   /* wait for "start of" clock tick */
   ticks = jiffies;
   while (ticks == jiffies)
    /* nothing */;
   /* Go .. */
   ticks = jiffies;
   __delay(loops_per_jiffy);
   ticks = jiffies - ticks;
   if (ticks)
    break;
  }

  /*
   * Do a binary approximation to get loops_per_jiffy set to
   * equal one clock (up to lps_precision bits)
   */
  loops_per_jiffy >>= 1;
  loopbit = loops_per_jiffy;
  while (lps_precision-- && (loopbit >>= 1)) {
   loops_per_jiffy |= loopbit;
   ticks = jiffies;
   while (ticks == jiffies)
    /* nothing */;
   ticks = jiffies;
   __delay(loops_per_jiffy);
   if (jiffies != ticks) /* longer than 1 tick */
    loops_per_jiffy &= ~loopbit;
  }
 }
 if (!printed)
  pr_cont("%lu.%02lu BogoMIPS (lpj=%lu)\n",
   loops_per_jiffy/(500000/HZ),
   (loops_per_jiffy/(5000/HZ)) % 100, loops_per_jiffy);

 
 pr_info("loops_per_jiffy = %d \n",loops_per_jiffy);

 printed = true;
}

/////////////////////////////////////////////////////////////////////////////////////////

//串口输出信息如下
Console: colour dummy device 80x30
##########calibrate.c calibrate_delay 33
Calibrating delay loop... 226.09 BogoMIPS (lpj=1130496)
loops_per_jiffy = 1130496
pid_max: default: 32768 minimum: 301
Security Framework initialized
Mount-cache hash table entries: 512
CPU: Testing write buffer coherency: ok
regulator: core version 0.5

///////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

static unsigned long __cpuinit calibrate_delay_direct(void)
{
 unsigned long pre_start, start, post_start;
 unsigned long pre_end, end, post_end;
 unsigned long start_jiffies;
 unsigned long timer_rate_min, timer_rate_max;
 unsigned long good_timer_sum = 0;
 unsigned long good_timer_count = 0;
 int i;

 if (read_current_timer(&pre_start) < 0 )
  return 0;

 /*
  * A simple loop like
  * while ( jiffies < start_jiffies+1)
  *  start = read_current_timer();
  * will not do. As we don't really know whether jiffy switch
  * happened first or timer_value was read first. And some asynchronous
  * event can happen between these two events introducing errors in lpj.
  *
  * So, we do
  * 1. pre_start <- When we are sure that jiffy switch hasn't happened
  * 2. check jiffy switch
  * 3. start <- timer value before or after jiffy switch
  * 4. post_start <- When we are sure that jiffy switch has happened
  *
  * Note, we don't know anything about order of 2 and 3.
  * Now, by looking at post_start and pre_start difference, we can
  * check whether any asynchronous event happened or not
  */

 for (i = 0; i < MAX_DIRECT_CALIBRATION_RETRIES; i++) {
  pre_start = 0;
  read_current_timer(&start);
  start_jiffies = jiffies;
  while (jiffies <= (start_jiffies + 1)) {
   pre_start = start;
   read_current_timer(&start);
  }
  read_current_timer(&post_start);

  pre_end = 0;
  end = post_start;
  while (jiffies <=
         (start_jiffies + 1 + DELAY_CALIBRATION_TICKS)) {
   pre_end = end;
   read_current_timer(&end);
  }
  read_current_timer(&post_end);

  timer_rate_max = (post_end - pre_start) /
     DELAY_CALIBRATION_TICKS;
  timer_rate_min = (pre_end - post_start) /
     DELAY_CALIBRATION_TICKS;

  /*
   * If the upper limit and lower limit of the timer_rate is
   * >= 12.5% apart, redo calibration.
   */
  if (pre_start != 0 && pre_end != 0 &&
      (timer_rate_max - timer_rate_min) < (timer_rate_max >> 3)) {
   good_timer_count++;
   good_timer_sum += timer_rate_max;
  }
 }

 if (good_timer_count)
  return (good_timer_sum/good_timer_count);

 printk(KERN_WARNING "calibrate_delay_direct() failed to get a good "
        "estimate for loops_per_jiffy.\nProbably due to long platform interrupts. Consider using \"lpj=\" boot option.\n");

// Consider using \"lpj=\" boot option.\n这个应该就是内核启动项命令行传参数
 
 return 0;
}
#else
static unsigned long __cpuinit calibrate_delay_direct(void) {return 0;}
#endif