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.

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