[GitHub] [incubator-nuttx] sebastianene07 edited a comment on pull request #1009: arch/sim: Add support for preemptive scheduling in the simulator build

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sebastianene07 edited a comment on pull request #1009:
URL: https://github.com/apache/incubator-nuttx/pull/1009#issuecomment-671011407


   > > > Yes, sigsetjmp/siglongjmp can't be used cross the thread. As you and @patacongo investigate ucontext support on all host which we care about, so I am thinking to implement the preemption by ucontext. But even we use ucontext API, the current implementation may still hang in SMP mode.
   > > 
   > > 
   > > > Anyway, I refine your patch more clean(only irq part), here:
   > > > https://github.com/xiaoxiang781216/incubator-nuttx/commits/preempt
   > > > We can try switch to ucontext API and see whether SMP can work correctly. If not, the context switch logic may have to redesign like other real hardware platform.
   > > 
   > > 
   > > Thanks for looking on it & cleaning it up ! The current PR solves the problem with the 'fake interrupts' generated from idle task and doesn't affect other configurations if the `SIM_PREEMPTIBLE` config is off. Adding back the `ucontext API ` involves more changes like:
   > > 
   > > 1. modify the way arguments are passed in a new task (we need to call `makecontext` function and provide the task arguments. I saw `up_initial_state` is called before `nxtask_setup_stackargs` so we don't have the arguments in the TCB to be able to call `makecontext` from there.)
   > > 2. Adjusting the stack size for tasks (for example on OSX the stack needs to be ~128K)
   > > 
   > > These changes are not present in my initial version of the patch and I think is better to have a separate PR for this as it will involve some big changes and it will be an improvement of my current work. What do you think ?
   > 
   > Since sigsetjmp/siglongjmp can't work with SMP, we have to use ucontext API anyway. If we implement two context switch solution, this will make the code logic much complex but without benefit. I prefer to provide a solution work in all configuration.
   
   I implemented a solution with `ucontext` here : https://github.com/sebastianene07/incubator-nuttx/tree/sene/simulator_support_preemptive_scheduling_with_ucontext but it needs some cleanup. With this solution I was able to run the `sim:ostest` config with `SMP` option on and `SIM_PREEMPTIBLE`.
   Some notes on this implementation:
   - I implemented the handler that verifies if we are running in interrupt context by creating an alternative stack for the signals
   - for all the tasks in `up_initial_state` I used stack memory allocated from the host heap to prevent re-adjusting of all the stacks
    
   


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