关于printk

2008.06.12.
  printk可以在任何上下文中调用。如果拿不到console_sem，就写入buffer后返回。推迟输出。推迟可能导致log level的变化。见代码注释。
/*
 * This is printk.  It can be called from any context.  We want it to work.
 *
 * We try to grab the console_sem.  If we succeed, it's easy - we log the output and
 * call the console drivers.  If we fail to get the semaphore we place the output
 * into the log buffer and return.  The current holder of the console_sem will
 * notice the new output in release_console_sem() and will send it to the
 * consoles before releasing the semaphore.
 *
 * One effect of this deferred printing is that code which calls printk() and
 * then changes console_loglevel may break. This is because console_loglevel
 * is inspected when the actual printing occurs.
 */

2008.07.10.慎用printk...
    写代码的时候在收到某种数据包的时候printk了一句话，结果发现故意触发这种情况会导致性能严重下降。所以，应该使用一些宏来打印debug信息，方便在性能测试时关闭打印。可怜的处理器不应该消耗在样的工作上。