AVR thread yield #5745
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Platform: AVR
Platform: This PR/issue effects AVR-based platforms
Type: bug
The issue reports a bug / The PR fixes a bug (including spelling errors)
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LudwigKnuepfer
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Type: bug
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Wow, this might explain a lot of trouble we've been having with AVR's interrupts behaving ... weird. |
Quick googling suggests that this might be the only way to have proper ISR handling. So a PR would be very welcome. |
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I agree with Kaspar, this could be the origin of several trouble we have especially with timers, maybe this could fix it. @jthacker feel free to open a PR from your branch. Notice that not all external interrupts are implemented for this MCU family, so maybe we can complete the support and use a free pin for context switching. |
jthacker
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Aug 20, 2016
Fixes RIOT-OS#5745 For AVR based boards, three defines must be defined AVR_CONTEXT_SWAP_INIT, AVR_CONTEXT_SWAP_INTERRUPT_VECT, and AVR_CONTEXT_SWAP_TRIGGER. These defines are used to trigger a software interrupt used for context switching. When AVR_CONTEXT_SWAP_INTERRUPT_VECT is handled, the scheduler is run and a context swap will happen if necessary, with the resulting thread starting following the reti instruction. This results in threads running at normal priority instead of at interrupt priority. Atmega devices do provide a pure software interrupt. The method used here for waspmote-pro and arduino-mega2560 is to use pin change interrupts, set the pin to act as an output, and toggle the value to simulate a software interrupt. The main limitation here is that a physical pin is now occupied and must be defined for each board supported by RIOT. On the plus side, it provides an easy method for detecting context swaps with an oscilloscope.
jthacker
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Sep 1, 2016
Fixes RIOT-OS#5745 For AVR based boards, three defines must be defined AVR_CONTEXT_SWAP_INIT, AVR_CONTEXT_SWAP_INTERRUPT_VECT, and AVR_CONTEXT_SWAP_TRIGGER. These defines are used to trigger a software interrupt used for context switching. When AVR_CONTEXT_SWAP_INTERRUPT_VECT is handled, the scheduler is run and a context swap will happen if necessary, with the resulting thread starting following the reti instruction. This results in threads running at normal priority instead of at interrupt priority. Atmega devices do provide a pure software interrupt. The method used here for waspmote-pro and arduino-mega2560 is to use pin change interrupts, set the pin to act as an output, and toggle the value to simulate a software interrupt. The main limitation here is that a physical pin is now occupied and must be defined for each board supported by RIOT. On the plus side, it provides an easy method for detecting context swaps with an oscilloscope.
jthacker
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to jthacker/RIOT
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Sep 7, 2016
Fixes RIOT-OS#5745 For AVR based boards, three defines must be defined AVR_CONTEXT_SWAP_INIT, AVR_CONTEXT_SWAP_INTERRUPT_VECT, and AVR_CONTEXT_SWAP_TRIGGER. These defines are used to trigger a software interrupt used for context switching. When AVR_CONTEXT_SWAP_INTERRUPT_VECT is handled, the scheduler is run and a context swap will happen if necessary, with the resulting thread starting following the reti instruction. This results in threads running at normal priority instead of at interrupt priority. Atmega devices do provide a pure software interrupt. The method used here for waspmote-pro and arduino-mega2560 is to use pin change interrupts, set the pin to act as an output, and toggle the value to simulate a software interrupt. The main limitation here is that a physical pin is now occupied and must be defined for each board supported by RIOT. On the plus side, it provides an easy method for detecting context swaps with an oscilloscope.
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For me this breaks the default example I get a kernel panic because the msg_queue is not initialized |
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I believe the AVR thread yield is not implemented correctly.
thread_arch_yield(akathread_yield_higher) is currently implemented as a naked function that will begin execution of the new thread at the same level as the calling point. In the atmega version oftimer.c,thread_yield_higheris called in an ISR, which can result in long running functions at this level possible halting the execution of all other threads. https://github.com/RIOT-OS/RIOT/blob/master/cpu/atmega_common/periph/timer.c#L193I discovered this when working with xtimer. It would consistently run the spin loop version instead of sleeping the thread due to running at interrupt level. https://github.com/RIOT-OS/RIOT/blob/master/sys/xtimer/xtimer.c#L45
A possible method for eliminating this issue is to use an interrupt driven thread swap like on the other platforms. For example, on the cortex platform, an interrupt is triggered and will be serviced as normal, the resulting context swap will place the new thread at the normal execution level since it will only start execution after returning the thread swapping ISR returns. Unlike the cortex platform, AVRs do not have a dedicated software driven interrupt for context swaps.
I have implemented a method for doing this by using one of the external interrupt pins, while keeping it as an output and driving it through software. master...jthacker:avr_thread_arch_isr_stack_usage
CONTEXT_SWAP_INIT,CONTEXT_SWAP_INT_VECTandCONTEXT_SWAP_TRIGGERwould need to implemented by each board since this requires sacrificing a physical pin on each board, and may be dependent on the layout. Obviously this is not ideal. For this reason I have opened this as an issue and not a pull request to see if firstly there is agreement with the issue at hand and to discuss possible alternative solutions.The text was updated successfully, but these errors were encountered: