sync from my github:https://donald-zhuang.github.io/2018/12/15/SysrqInDebug/ 这阵子一直在处理各种死机和STR开关机问题，虽说稍微有点虐，不过技术面的扩展和更多新技巧的学习了解，也让我挺enjoy的。kernel世界对我来说，一直是一个black box，很迷。随着上阵子切bionic、看完程序员的自我修养以及这阵子为处理一个语音唤醒问题学习AN8.0的DC待机流程，发现这个是通过reboot函数做syscall进kernel待机，感觉打开一个新世界。虽然之前也是知道这条通路，但当问题和整个代码流程浮现在你面前时，一切依旧像盒子中的巧克力让你兴奋不已。
5. 通过重复sysrq dump all CPU status，我们可以看到Exception Stack一直不变,因此查看CPU2上处于running的task确认为PID 2548占着没退出。而这个task的调用栈如下，我们可以看到异常时卡在media_intf_read这只函数上，该函数采用自旋锁，不能进行处理器抢占，其他进程无法获取CPU，因此需要请对应的kernel driver owner定位下media_intf_read卡住的原因。
从Linux Programmer’s Manual的SCHED一节可以看到如下说明，SCHED_FIFO一旦占用cpu便会一直运行(CHED_FIFO is a simple scheduling algorithm without time slicing.)，直到有IO请求、更高优先级任务到达或主动退出(sched_yield)才释放CPU，他和RR都是有很高的调度优先级的。因此代码上对线程调度控制的修改也要十分谨慎，不然很容易出现问题。
SCHED_FIFO: First in-first out schedulingSCHED_FIFO can be used only with static priorities higher than 0,which means that when a SCHED_FIFO threads becomes runnable, it willalways immediately preempt any currently running SCHED_OTHER,SCHED_BATCH, or SCHED_IDLE thread. SCHED_FIFO is a simple schedulingalgorithm without time slicing. For threads scheduled under theSCHED_FIFO policy, the following rules apply:1) A running SCHED_FIFO thread that has been preempted by anotherthread of higher priority will stay at the head of the list forits priority and will resume execution as soon as all threads ofhigher priority are blocked again.2) When a blocked SCHED_FIFO thread becomes runnable, it will beinserted at the end of the list for its priority.3) If a call to sched_setscheduler(2), sched_setparam(2),sched_setattr(2), pthread_setschedparam(3), orpthread_setschedprio(3) changes the priority of the running orrunnable SCHED_FIFO thread identified by pid the effect on thethread&＃39;s position in the list depends on the direction of thechange to threads priority:· If the thread&＃39;s priority is raised, it is placed at the end ofthe list for its new priority. As a consequence, it maypreempt a currently running thread with the same priority.· If the thread&＃39;s priority is unchanged, its position in the runlist is unchanged.· If the thread&＃39;s priority is lowered, it is placed at the frontof the list for its new priority.According to POSIX.1-2008, changes to a thread&＃39;s priority (orpolicy) using any mechanism other than pthread_setschedprio(3)should result in the thread being placed at the end of the listfor its priority.4) A thread calling sched_yield(2) will be put at the end of thelist.No other events will move a thread scheduled under the SCHED_FIFOpolicy in the wait list of runnable threads with equal staticpriority.A SCHED_FIFO thread runs until either it is blocked by an I/Orequest, it is preempted by a higher priority thread, or it callssched_yield(2).
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