Scheduler

• Linux: Kernel Scheduling

Preemption

• A kernel is preemptive if a process switch may occur while the replaced process is executing a kernel function, that is, while it runs in Kernel Mode.
• The main characteristic of a preemptive kernel is that a process running in Kernel Mode can be replaced by another process while in the middle of a kernel function.
• A preempted process is not suspended, because it remains in the TASK_RUNNING state.

• planned process switch
• forced process switch

• dispatch latency
  • the delay between the time they become runnable and the time they actually begin running.

• kernel preemption is disabled(preempt_count field is greater than zero)
  1. The kernel is executing an interrupt service routine.
     • Hardirq counter subfield
     • It specifies the number of nested interrupt handlers on the local CPU (the value is increased by irq_enter( ) and decreased by irq_exit( )).
  2. The deferrable functions are disabled (always true when the kernel is executing a softirq or tasklet).
     • Softirq counter subfield
     • It specifies how many levels deep the disabling of deferrable functions is (level 0 means that deferrable functions are enabled).
  3. The kernel preemption has been explicitly disabled by setting the preemption counter to a positive value.
     • Preemption counter subfield

• kernel can be preempted only when(see preempt_schedule)
  • preempt_count field is zero
    • It is executing an exception handler (in particular a system call)
    • the kernel preemption has not been explicitly disabled.
  • The local CPU must have local interrupts enabled

• the time preempty happen
  • preempt_enable, preempt_check_resched, preempt_schedule
  • preempt_schedule_irq