Jan Kiszka wrote:
Philippe Gerum wrote:Philippe Gerum wrote:Jan Kiszka wrote:Gilles Chanteperdrix wrote:Jeroen Van den Keybus wrote:Hello,I'm currently not at a level to participate in yourdiscussion. Although I'mwilling to supply you with stresstests, I would nevertheless liketo learnmore from task migration as this debugging session proceeds. Inorder to doso, please confirm the following statements or indicate where Iwent wrong.I hope others may learn from this as well.xn_shadow_harden(): This is called whenever a Xenomai threadperforms aLinux (root domain) system call (notified by Adeos ?).xnshadow_harden() is called whenever a thread running in secondary mode (that is, running as a regular Linux thread, handled by Linux scheduler) is switching to primary mode (where it will run as a Xenomai thread, handled by Xenomai scheduler). Migrations occur for some system calls. More precisely, Xenomai skin system calls tables associates a few flags with each system call, and some of these flags cause migration of the caller when it issues the system call. Each Xenomai user-space thread has two contexts, a regular Linux thread context, and a Xenomai thread called "shadow" thread. Both contexts share the same stack and program counter, so that at any time, at least one of the two contexts is seen as suspended by the scheduler which handles it. Before xnshadow_harden is called, the Linux thread is running, and its shadow is seen in suspended state with XNRELAX bit by Xenomai scheduler. After xnshadow_harden, the Linux context is seen suspended with INTERRUPTIBLE state by Linux scheduler, and its shadow is seen as running by Xenomai scheduler. The migrating thread(nRT) is marked INTERRUPTIBLE and run by the Linux kernel wake_up_interruptible_sync() call. Is this thread actually run ordoes itmerely put the thread in some Linux to-do list (I assumed thefirst case) ? Here, I am not sure, but it seems that when calling wake_up_interruptible_sync the woken up task is put in the current CPU runqueue, and this task (i.e. the gatekeeper), will not run until the current thread (i.e. the thread running xnshadow_harden) marks itself as suspended and calls schedule(). Maybe, marking the running thread asDepends on CONFIG_PREEMPT. If set, we get a preempt_schedule already here - and a switch if the prio of the woken up task is higher. BTW, an easy way to enforce the current trouble is to remove the "_sync" from wake_up_interruptible. As I understand it this _sync is just an optimisation hint for Linux to avoid needless scheduler runs.You could not guarantee the following execution sequence doing so either, i.e. 1- current wakes up the gatekeeper 2- current goes sleeping to exit the Linux runqueue in schedule() 3- the gatekeeper resumes the shadow-side of the old current The point is all about making 100% sure that current is going to be unlinked from the Linux runqueue before the gatekeeper processes the resumption request, whatever event the kernel is processing asynchronously in the meantime. This is the reason why, as you already noticed, preempt_schedule_irq() nicely breaks our toy by stealing the CPU from the hardening thread whilst keeping it linked to the runqueue: upon return from such preemption, the gatekeeper might have run already, hence the newly hardened thread ends up being seen as runnable by both the Linux and Xeno schedulers. Rainy day indeed. We could rely on giving "current" the highest SCHED_FIFO priority in xnshadow_harden() before waking up the gk, until the gk eventually promotes it to the Xenomai scheduling mode and downgrades this priority back to normal, but we would pay additional latencies induced by each aborted rescheduling attempt that may occur during the atomic path we want to enforce. The other way is to make sure that no in-kernel preemption of the hardening task could occur after step 1) and until step 2) is performed, given that we cannot currently call schedule() with interrupts or preemption off. I'm on it.Could anyone interested in this issue test the following couple of patches? atomic-switch-state.patch is to be applied against Adeos-1.1-03/x86 for 2.6.15 atomic-wakeup-and-schedule.patch is to be applied against Xeno 2.1-rc2 Both patches are needed to fix the issue. TIA,Looks good. I tried Jeroen's test-case and I was not able to reproduce the crash anymore. I think it's time for a new ipipe-release. ;)
Looks like, indeed.
At this chance: any comments on the panic-freeze extension for the tracer? I need to rework the Xenomai patch, but the ipipe side should be ready for merge.
No issue with the ipipe side since it only touches the tracer support code. No issue either at first sight with the Xeno side, aside of the trace being frozen twice in do_schedule_event? (once in this routine, twice in xnpod_fatal); but maybe it's wanted to freeze the situation before the stack is dumped; is it?
I'm queuing the ipipe side patch for 1.2, which will also provide the support we need for atomic scheduling in order to solve the migration bug.