3.2.2.2. Identifying the current process

The close association between the thread_info structure and the Kernel Mode stack just described offers a key benefit in terms of efficiency: the kernel can easily obtain the address of the thread_info structure of the process currently running on a CPU from the value of the esp register. In fact, if the thread_union structure is 8 KB ($2^{13}$ bytes) long, the kernel masks out the 13 least significant bits of esp to obtain the base address of the thread_info structure; on the other hand, if the thread_union structure is 4 KB long, the kernel masks out the 12 least significant bits of esp . This is done by the current_thread_info( ) function, which produces assembly language instructions like the following:

movl $0xffffe000,%ecx /* or 0xfffff000 for 4KB stacks */
andl %esp,%ecx
movl %ecx,p

After executing these three instructions, p contains the thread_info structure pointer of the process running on the CPU that executes the instruction.

Most often the kernel needs the address of the process descriptor rather than the address of the thread_info structure. To get the process descriptor pointer of the process currently running on a CPU, the kernel makes use of the current macro, which is essentially equivalent to current_thread_info( )->task and produces assembly language instructions like the following:

movl $0xffffe000,%ecx /* or 0xfffff000 for 4KB stacks */
andl %esp,%ecx
movl (%ecx),p

Because the task field is at offset 0 in the thread_info structure, after executing these three instructions p contains the process descriptor pointer of the process running on the CPU.

The current macro often appears in kernel code as a prefix to fields of the process descriptor. For example, current->pid returns the process ID of the process currently running on the CPU.

Another advantage of storing the process descriptor with the stack emerges on multiprocessor systems: the correct current process for each hardware processor can be derived just by checking the stack, as shown previously. Earlier versions of Linux did not store the kernel stack and the process descriptor together. Instead, they were forced to introduce a global static variable called current to identify the process descriptor of the running process. On multiprocessor systems, it was necessary to define current as an array one element for each available CPU.

NOTE: : 所有的**process descriptor** 和 kernel stack 都是位于kernel中；由kernel来执行调度；当CPU需要执行某个**process descriptor**的时候，它需要读取这个**process descriptor**的一些数据，比如之前保存的register数据等以便resume；从上面的描述可以看出，CPU是根据esp的值来获得**process descriptor**的地址，并且，从前面的描述来看，每个thread_union都有一个自己的 kernel stack ，而从上面的描述来看，是根据esp的值来获得**process descriptor**的地址，所以CPU是在某个`thread_union的 kernel stack**中执行，然后得到对应的**process descriptor；

NOTE: : 因为scheduler在调度一个task开始运行之前会将这个task的所有的register都恢复到CPU中，所以必然会包含esp，所以它就可以根据esp快速地定位到process descriptor；