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December 17, 2015 01:58
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hokay, so .... the kernel's job is just to return an activation to userspace at a well known entry point, so the userspace can context switch itself and then tell The Authorities that An Intarrupt Has Occurred. | |
sooooo, | |
first : we need to know if the thing we interrupted is a thread: | |
- modify get current thread to use a magic number to recognize if the suspended RSP is a valid stack | |
- kernel sets null rsp upon reentering userspace... no stack at all means no thread | |
- stackless context switcher code must be idempotent - if interrrupted, does the same thing the interrupted code was doing | |
- what about stackless thread scheduler? we may have dequeued a thread to run but not be on its stack yet :( | |
if it is a thread: | |
a) be lazy | |
1) set a bit to resume thread from activation, and note activation address in thread struct (instead of the normal thread entry with the now-stale rsp saved in the thread struct) | |
2) yield to parent environment (with a message indicating the interrupt as the reason for control transfer) | |
b) clean up activation | |
1) redzone size is capped, copy activation to stack below redzone | |
2) push a resume thread function to stack | |
3) modify thread rsp to resume as normal | |
4) requeue thread for later scheduling | |
5) mark activation as unused | |
6) yield to parent environment | |
if it isn't a thread: | |
a) treat as a critical section and resume doing what we were doing (with assuming some check at the end to alter control flow and yield to parent) | |
b) make a thread to suspend it to so we can resume it? (stackless thread create == :((((( ) | |
c) stacklesss thread scheduler also tracks activations to resume with a higher priority than threads? | |
option a) exploits the reason we wanted to do userspace controlled context switches to begin with | |
option b) is basically a nonstarter.... maybe with enough preallocated state we could make it work... until we have nested stackless resumes :( | |
option c) is maybe reasonable, and should at least be able to handle nesting without frothing at the mouth, assuming we don't resume in the middle of an activation chain | |
what are stackless things we could be doing? | |
- yield thread, enter thread scheduler | |
- yield CPU to parent -this is what we want to do, but perhaps with a different payload | |
- yield CPU to child - if we are doing this on a thread's behalf, it would be important to know it never actually happened | |
- handling an activation for a thread | |
- handling an activation for a not-thread | |
- recieving a yield from child or parent... yield from child may already be carrying an interrupt. which one wins? can we merge em? |
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