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Task Switches in Cooperative Scheduling

During cooperative scheduling, each task must explicitly allow the kernel to perform task switches. If a task fails to do so, it runs exclusively and other tasks will not be able to run.

If a task switch has become necessary during cooperative scheduling by a hardware interrupt, the pending task switch is done later during a kernel call of the currently running task or a task running on a different CPU. All kernel functions can be classified in three types: calls that will always perform pending task switches, calls that will only perform task switches if they have caused a task state change, and calls that will never perform a pending task switch.

The following table lists all kernel functions of the first two types:

Other kernel calls do not lead to task switches during cooperative scheduling.

Naturally, all operations containing one of the above calls can lead to a task switch indirectly. In particular, some of these are:

• Screen I/O using module RTTextIO
• COMSendChar, COMSendCharPolled, and COMReceiveCharPolled in module RTCom
• KBKeyPressed and KBGetCh in module RTKeybrd

During cooperative scheduling, you should make sure that each endless loop in a program contains at least one call to the kernel to allow it to perform pending task switches caused by interrupts.

Advanced Topics

OpenMP

Writing Custom Kernel Drivers