Am 05.10.2010 15:50, Gilles Chanteperdrix wrote:
> Jan Kiszka wrote:
>> Am 05.10.2010 15:42, Gilles Chanteperdrix wrote:
>>> 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.
>> The stack pointer mask trick only works with fixed-sized stacks, not a
>> guaranteed property of in-kernel Xenomai threads.
> Precisely the reason why I propose to replace it with a global variable
> reference, or a per-cpu variable for SMP systems.

Then why is Linux not using this in favor of the stack pointer approach
on, say, ARM?

For sure, we can patch all Adeos-supported archs away from stack-based
to per-cpu current & thread_info, but I don't feel comfortable with this
in some way invasive approach as well. Well, maybe it's just my personal


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