Linux internals

Linux context switch from user mode to kernel mode

The only way an user space application can explicitly initiate a switch to kernel mode during normal operation is by making an system call such as open, read, write etc.

Whenever a user application calls these system call APIs with appropriate parameters, a software interrupt/exception(SWI) is triggered.

As a result of this SWI, the control of the code execution jumps from the user application to a predefined location in the Interrupt Vector Table [IVT] provided by the OS.

This IVT contains an adress for the SWI exception handler routine, which performs all the necessary steps required to switch the user application to kernel mode and start executing kernel instructions on behalf of user process.

Useful link: https://stackoverflow.com/questions/11905934/how-to-switch-from-user-mode-to-kernel-mode

Linux Scheduling method

It uses Red black tree.

Priority is based on nice, number of processes running in the machine

Nice -> A factor to decide the priority

vruntime is used to select the one which process should get next CPX. Once a process qualifies for running, it will get fixed timeslice defined by RR_TIMESLICE. --> https://stackoverflow.com/questions/16401294/how-to-know-linux-scheduler-time-slice

Change Nice -> https://www.nixtutor.com/linux/changing-priority-on-linux-processes/

sched_statistics

struct sched_entity - https://elixir.bootlin.com/linux/v4.4.57/source/include/linux/sched.h#L1246

Update_curr -> https://elixir.bootlin.com/linux/v4.4.57/source/kernel/sched/fair.c#L699

Reference:

https://elixir.bootlin.com/linux/v4.4.57/source/include/linux/sched.h#L1246

https://elixir.bootlin.com/linux/v4.4.57/source/kernel/sched/fair.c#L699

https://stackoverflow.com/questions/39725102/how-linux-process-scheduler-prevents-starvation-of-a-process