Linux process scheduling in kernel mode

Question

Here is some description quoted from Wiki The Linux kernel provides preemptive scheduling under certain conditions. Until kernel version 2.4, only processes were preemptive, i.e. in addition to time quantum expiration, an execution of current process in user mode would be interrupted if higher dynamic priority processes entered TASK_RUNNING state. Towards Linux 2.6, an ability to interrupt a task executing kernel code was added, although with that not all sections of the kernel code can be preempted. Then it also says this, Preemption improves latency, increases responsiveness, and makes Linux more suitable for desktop and real-time applications. Older versions of the kernel had a so-called big kernel lock for synchronization across the entire kernel. This was finally removed by Arnd Bergmann in 2011 So does the above statement hold true for the current linux kernel that kernel preemption is conditional? e.g. if a process is trapped into kernel mode by making a system call, this proce…

Accepted Answer

CPU scheduling decisions may take place when a process:

```
1. Switches from running to waiting state (e.g. I/O request)
2. Switches from running to ready state (e.g. Interrupt)
3. Switches from waiting to ready (e.g. I/O completion)
4. Terminates
```

Scheduling under 1 and 4 are non-preemptive and all other scheduling is preemptive, have to deal with possibility that operations (system calls) may be incomplete.

Yes Linux provides preemptive scheduling under certain conditions unlike some Unix variants where kernel schedules until completion without preemption. In Linux 2.6, kernel was made preemptive a task running as long it is not holding a lock and safe to rescheduling.

```
Older versions of the kernel had a so-called big kernel lock for synchronization 
across the entire kernel.
```

refers to each user-level thread maps only to one kernel thread.

Linux process scheduling in kernel mode

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