进程描述符
每个进程描述符都包括几个与调度相关的字段，如下代码所示：
//include/asm-arm/thread_info.h
/** low level task data that entry.S needs immediate access to.* __switch_to() assumes cpu_context follows immediately after cpu_domain.*/
struct thread_info {
//存放TIF NEED RESCHED标志，如果必须调用调度程序，则设置该标志 unsigned long flags; /* low level flags */
..
：深入理解继承、多态和抽象)
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