Jan Kiszka wrote: > Am 05.10.2010 15:15, Gilles Chanteperdrix wrote: >> Jan Kiszka wrote: >>> Hi, >>> >>> quite a few limitations and complications of using Linux services over >>> non-Linux domains relate to potentially invalid "current" and >>> "thread_info". The non-Linux domain could maintain their own kernel >>> stacks while Linux tend to derive current and thread_info from the stack >>> pointer. This is not an issue anymore on x86-64 (both states are stored >>> in per-cpu variables) but other archs (e.g. x86-32 or ARM) still use the >>> stack and may continue to do so. >>> >>> I just looked into this thing again as I'm evaluating ways to exploit >>> the kernel's tracing framework also under Xenomai. Unfortunately, it >>> does a lot of fiddling with preempt_count and need_resched, so patching >>> it for Xenomai use would become a maintenance nightmare. >>> >>> An alternative, also for other use cases like kgdb and probably perf, is >>> to get rid of our dependency on home-grown stacks. I think we are on >>> that way already as in-kernel skins have been deprecated. The only >>> remaining user after them will be RTDM driver tasks. But I think those >>> could simply become in-kernel shadows of kthreads which would bind their >>> stacks to what Linux provides. Moreover, Xenomai could start updating >>> "current" and "thread_info" on context switches (unless this already >>> happens implicitly). That would give us proper contexts for system-level >>> tracing and profiling. >>> >>> My key question is currently if and how much of this could be realized >>> in 2.6. Could we drop in-kernel skins in that version? If not, what >>> about disabling them by default, converting RTDM tasks to a >>> kthread-based approach, and enabling tracing etc. only in that case? >>> However, this might be a bit fragile unless we can establish >>> compile-time or run-time requirements negotiation between Adeos and its >>> users (Xenomai) about the stack model. >> A stupid question: why not make things the other way around: patch the >> current and current_thread_info functions to be made I-pipe aware and >> use an "ipipe_current" pointer to the current thread task_struct. Of >> course, there are places where the current or current_thread_info macros >> are implemented in assembly, so it may be not simple as it sounds, but >> it would allow to keep 128 Kb stacks if we want. This also means that we >> would have to put a task_struct at the bottom of every Xenomai task. > > First of all, overhead vs. maintenance. Either every access to > preempt_count() would require a check for the current domain and its > foreign stack flag, or I would have to patch dozens (if that is enough) > of code sites in the tracer framework.
No. I mean we would dereference a pointer named ipipe_current. That is all, no other check. This pointer would be maintained elsewhere. And we modify the "current" macro, like: #ifdef CONFIG_IPIPE extern struct task_struct *ipipe_current; #define current ipipe_current #endif Any calll site gets modified automatically. Or current_thread_info, if it is current_thread_info which is obtained using the stack pointer mask trick. > > And, second, this would prevent aligning current/thread_info with the > currently running shadow, the nice add-on I would like to gain with this > rework. We would put a task_struct at the bottom of every Xenomai kthread. So, yes, we align on the current/thread_info stuff. I am not convinced that the stack pointer trick is really that much a big performance gain. -- Gilles. _______________________________________________ Xenomai-core mailing list Xenomaiemail@example.com https://mail.gna.org/listinfo/xenomai-core