系统调用会引起CPU上下文切换吗？ · Issue #5 · Talk-Go-CSAPP-05Discusion

系统调用会引起CPU上下文切换吗？ · Issue #5 · Talk-Go-CSAPP-05Discusion

要明确系统调用的目的/系统调用的函数的作用

解释: 调用系统调用就是为了切换进程,那自然就会引起context switch

例子1: 示例场景中有两个并发的进程,shell进程和hello进程,最开始只有shell进程在运行,即等待命令行上的输入. 当我们让它运行hello程序时,shell通过调用一个专门的函数,即系统调用,来执行我们的请求,系统调用会将控制权传递给操作系统, 操作系统保存shell进程的上下文,创建一个新的hell进程及其上下文,然后将控制权传给新的hello进程, hello进程终止后,操作系统恢复shell进程的上下文, 并将控制权传回给它.^csappP12

例子二: 一个用于获取系统时间的 system call 详见

system-call-and-context-switch

这篇文章有两个例子：

• 一个用于获取系统时间的 system call ， 是否会造成上下文切换

• 一个用于 IO 场景，会被阻塞的 system call， 是否造成上下文切换

• You need to understand that a thread/process context has multiple parts, one, directly associated with execution and is held in the CPU and certain system tables in memory that the CPU uses (e.g. page tables), and the other, which is needed for the OS, for bookkeeping (think of the various IDs, handles, special OS-specific permissions, network connections and such).

  A full context switch would involve swapping both of these, the old current thread/process goes away for a while and the new current thread/process comes in for a while. That's the essence of thread/process scheduling.

  Now, system calls are very different w.r.t. each other.

  Consider something simple, for example, the system call for requesting the current date and time. The CPU switches from the user to kernel mode, preserving the user-mode register values, executes some kernel code to get the necessary data, stores it either in the memory or registers that the caller can access, restores the user-mode register values and returns. There's not much of context switch in here, only what's needed for the transition between the modes, user and kernel.

  Consider now a system call that involves blocking of the caller until some event or availability of data. Manipulating mutexes and reading files would be examples of such system calls. In this case the kernel is forced to save the full context of the caller, mark it as blocked so the scheduler can't run it until that event or data arrives, and load the context of another ready thread/process, so it can run.

  That's how system calls are related to context switches.

  Kernel executing in the context of a user or a process means that whenever the kernel does work on behalf of a certain process or user it has to take into consideration that user's/process's context, e.g. the current process/thread/user ID, the current directory, locale, access permissions for various resources (e.g. files), all that stuff, that can be different between different processes/threads/users.

  If processes have individual address spaces, the address spaces is also part of the process context. So, when the kernel needs to access memory of a process (to read/write file data or network packets), it has to have access to the process' address space, IOW, it has to be in its context (it doesn't mean, however, that the kernel has to load the full context just to access memory in a specific address space).

  Is that helpful?

总结：这两者的上下文切换，是否都是同一流程？

主要注意的是：

进程上下文切换，是指从一个进程切换到另一个进程运行 而系统调用过程中一直是同一个进程在运行

系统调用过程通常称为特权模式切换，而不是上下文切换。 当进程调用系统调用或者发生中断时，CPU从用户模式（用户态）切换成内核模式（内核态），此时，无论是系统调用程序还是中断服务程序，都处于当前进程的上下文中，并没有发生进程上下文切换。

当系统调用或中断处理程序返回时，CPU要从内核模式切换回用户模式，此时会执行操作系统的调用程序。如果发现就需队列中有比当前进程更高的优先级的进程，则会发生进程切换： 当前进程信息被保存，切换到就绪队列中的那个高优先级进程；否则，直接返回当前进程的用户模式，不会发生上下文切换。