From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4CAB2C4B.6090005@domain.hid> Date: Tue, 05 Oct 2010 15:46:51 +0200 From: Jan Kiszka MIME-Version: 1.0 References: <4CAAF0CA.7060603@domain.hid> <4CAB24D9.5000308@domain.hid> <4CAB2994.40505@domain.hid> <4CAB2B37.7010104@domain.hid> In-Reply-To: <4CAB2B37.7010104@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: Gilles Chanteperdrix Cc: Xenomai core 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. Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux