From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4CAB2D1C.70907@domain.hid> Date: Tue, 05 Oct 2010 15:50:20 +0200 From: Gilles Chanteperdrix MIME-Version: 1.0 References: <4CAAF0CA.7060603@domain.hid> <4CAB24D9.5000308@domain.hid> <4CAB2994.40505@domain.hid> <4CAB2B37.7010104@domain.hid> <4CAB2C4B.6090005@domain.hid> In-Reply-To: <4CAB2C4B.6090005@domain.hid> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai-core] Overcoming the "foreign" stack List-Id: Xenomai life and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jan Kiszka Cc: Xenomai core 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. -- Gilles.