add preemption timer #102
Merged
add preemption timer #102
Conversation
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This is the first step for preemptive scheduling. Some processes will never invoke any syscalls on their own and so the scheduler never has a chance to run another thread. Similarly, we never have a chance to check if a thread is still supposed to be running if the thread never yields to the kernel. To solve this, we now expect the supervisors to periodically inject timer interrupts and force a transition out of userspace into kernelspace.
When the CPU jumps to the IRQ handler, it clears the IF flag. We shouldn't set it again when entering to kernel space, because we want the IF flag to continue to be clear up until the IRQ has been handled.
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
The supervisors now try to inject a timer interrupt into each kernel thread. Assuming a well-behaved hypervisor, the timer frequency will be 100 Hz, though if the hypervisor is malicious it could skew the frequency. The main use case for this is to give the scheduler a chance to preempt user threads that never voluntarily yield to the kernel. This also gives as an opportunity to evaluate the live-ness of threads that don't yield to the kernel.