From mboxrd@z Thu Jan 1 00:00:00 1970 From: Philippe Gerum In-Reply-To: <4BB50F7C.1020102@domain.hid> References: <4B97BA0C.9000702@domain.hid> <4B9AD0DE.4020103@domain.hid> <1268472523.27899.135.camel@domain.hid> <4B9BB9B1.5050003@domain.hid> <1268498034.27899.167.camel@domain.hid> <4B9C2100.6090806@domain.hid> <1268584465.27899.197.camel@domain.hid> <4BB4F857.5020906@domain.hid> <4BB50C8B.2000405@domain.hid> <1270156940.2418.403.camel@domain.hid> <4BB50F7C.1020102@domain.hid> Content-Type: text/plain; charset="UTF-8" Date: Thu, 01 Apr 2010 23:31:47 +0200 Message-ID: <1270157507.2418.409.camel@domain.hid> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai-help] Analogy cmd_write example explanation List-Id: Help regarding installation and common use of Xenomai List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jan Kiszka Cc: Alexis Berlemont , xenomai@xenomai.org On Thu, 2010-04-01 at 23:26 +0200, Jan Kiszka wrote: > Philippe Gerum wrote: > > On Thu, 2010-04-01 at 23:13 +0200, Jan Kiszka wrote: > >> Gilles Chanteperdrix wrote: > >>> Philippe Gerum wrote: > >>>> On Sun, 2010-03-14 at 00:34 +0100, Alexis Berlemont wrote: > >>>>> Philippe Gerum wrote: > >>>>>> On Sat, 2010-03-13 at 17:13 +0100, Alexis Berlemont wrote: > >>>>>>> Hi, > >>>>>>> > >>>>>>> Philippe Gerum wrote: > >>>>>>>> On Sat, 2010-03-13 at 00:40 +0100, Alexis Berlemont wrote: > >>>>>>>>> Hi, > >>>>>>>>> > >>>>>>>>> Sorry for answering so late. I took a few days off far from any internet > >>>>>>>>> connection. > >>>>>>>>> > >>>>>>>>> It seems you sent many mails related with Analogy. Many thanks for your > >>>>>>>>> interest. I have not read all of them yet. However, I am beginning by > >>>>>>>>> this one (which seems unanswered). The answer is quick and easy :) > >>>>>>>>> > >>>>>>>>> Daniele Nicolodi wrote: > >>>>>>>>>> Hello. I'm looking into the analogy cmd_write example. > >>>>>>>>>> > >>>>>>>>>> I'm not sure I understand the reason for the rt_task_set_mode() function > >>>>>>>>>> call into the data acquisition loop (lines 413 or 464 in the code > >>>>>>>>>> shipped with xenomai 2.5.1). > >>>>>>>>>> > >>>>>>>>>> I do not understand why we have to set the primary mode at every > >>>>>>>>>> iteration, when we set it before for the task (line 380). > >>>>>>>>>> > >>>>>>>>>> Is it because the dump_function() uses system calls that can make the > >>>>>>>>>> task to switch to secondary mode, or there is a deeper reason I'm missing? > >>>>>>>>>> > >>>>>>>>> You are right. The dumping routine triggers a switch to secondary mode. > >>>>>>>>> That is why, the program switches back to primary mode after. > >>>>>>>> This is wrong. The Xenomai core will switch your real-time thread to > >>>>>>>> primary mode automatically when running a4l_insn* calls that end up > >>>>>>>> invoking rt_dev_ioctl(), since you did declare a real-time entry point > >>>>>>>> for this one. > >>>>>>>> > >>>>>>> I don't understand. I thought that rt_dev_ioctl() triggered an > >>>>>>> __rtdm_ioctl syscall, which, according to the rtdm systab, is declared > >>>>>>> with the flags "__xn_exec_current | __xn_exec_adaptive". > >>>>>>> > >>>>>>> So as __rt_dev_ioctl (the kernel handler behind the ioctl syscall) will > >>>>>>> return -ENOSYS neither in RT nor in NRT mode (because analogy declares > >>>>>>> both RT and NRT fops entries), I thought there was no automatic > >>>>>>> mode-switching. > >>>>>> The point is that your ioctl_nrt handler should return -ENOSYS when it > >>>>>> detects that the current request should be processed by the converse > >>>>>> domain, to trigger the switch to primary mode. This is why the adaptive > >>>>>> tag is provided in the first place. > >>>>> The problem is that rtdm does not provide any function to know whether > >>>>> the thread is shadowed. We just have rtdm_in_rt_context() which tells us > >>>>> whether the thread is RT or not. If it is NRT, we cannot distinguish a > >>>>> Linux thread from a Xenomai one. > >>>>> > >>>>> I thought with a little patch like this in ksrc/skins/rtdm/core.c, we > >>>>> could force -ENOSYS if the calling thread was a Xenomai NRT thread: > >>>>> > >>>>> diff --git a/ksrc/skins/rtdm/core.c b/ksrc/skins/rtdm/core.c > >>>>> index 8677c47..cc0cfe9 100644 > >>>>> --- a/ksrc/skins/rtdm/core.c > >>>>> +++ b/ksrc/skins/rtdm/core.c > >>>>> @@ -423,6 +423,9 @@ do { \ > >>>>> \ > >>>>> if (rtdm_in_rt_context()) \ > >>>>> ret = ops->operation##_rt(context, user_info, args); \ > >>>>> + else if (xnshadow_thread(user_info) != NULL && \ > >>>>> + ops->operation##_rt != (void *)rtdm_no_support) \ > >>>>> + ret = -ENOSYS; \ > >>>>> else \ > >>>>> ret = ops->operation##_nrt(context, user_info, args); \ > >>>>> \ > >>>> No, this would be a half-working kludge. But I think you have pinpointed > >>>> a more general issue with RTDM: syscalls should be tagged as both > >>>> adaptive and conforming, instead of bearing the __xn_exec_current bit. > >>>> Actually, we do want the current domain to change when it is not the > >>>> most appropriate, which __xn_exec_current prevents so far. > >>>> > >>>> What we rather want is to have shadows migrating to primary mode when > >>>> running rtdm_ioctl, since this is the preferred mode of operation for > >>>> Xenomai threads, so that ioctl_rt is always invoked first when present, > >>>> giving an opportunity to forward the request to secondary mode by > >>>> returning -ENOSYS. Conforming calls always enforce the preferred runtime > >>>> mode, i.e. primary for Xenomai shadows, secondary for plain Linux tasks. > >>>> That logic applies to all RTDM syscalls actually. > >>>> > >>>> __xn_exec_current allows application code to infer that the RTDM driver > >>>> might behave differently depending on the current runtime mode of the > >>>> calling thread, which is very much error-prone, and likely not what was > >>>> envisioned initially. > >>> Argh.... The switchtest driver is relying on __xn_exec_current to have > >>> context switches occur precisely in the mode we want. __xn_exec_adaptive > >>> introduce more context switches around which we can not place separate > >>> checks for fpu context, so, in short, breaks it badly. Fixing this > >>> requires turning the switchtest driver into a skin with its own syscalls. > >>> > >>> Note the sequence which occurs when a shadowed thread running in > >>> secondary mode calls an ioctl for which only an nrt implementation occurs: > >>> the thread is hardened to handle the ioctl > >>> ioctl_rt is called which returns -ENOSYS > >>> the thread is relaxed > >>> ioctl_nrt is called > >>> > >>> It boils down to putting an rt_task_set_mode(PRIMARY) before each rtdm > >>> syscall made by a thread with a shadow, and in fact seems to result in > >>> as bad a result. Is it really what we want? The __xn_exec_current bit > >>> resulted in a more lazy behaviour. > >>> > >>> Also note that, at least when using the posix skin, almost all threads > >>> have shadows, and only the priority makes the difference between a > >>> really critical thread, and non critical threads with the null priority. > >>> So, this will happen all the time. > >> Right. Actually, we only need the conforming property for the (fairly > >> rare) case that a service is provided in both flavors. > > > > Wrong. You want conforming because real-time is the preferred mode of > > operations for real-time threads. > > Sorry, wrong as well. :) > > Setup/reconfiguration phases happening over already shadowed threads can > suffer significantly when playing ping-pong via mode switches. That's > why threads stay in secondary mode after their first Linux syscall and > are not immediately switched back. I decided to implement lazy switch in that case because it made sense not to add overhead when it is not needed. The situation you describe is a code that actually cares for an extra switch at the expense of a proper design, by imposing a lot of mode switches in tight loops. Please fix the app, not the core, really. > > Jan > > > > >> And if such a > >> service is hidden behind a single IOCTL command, it's even not possible > >> to detect this at RTDM level. We do need help from the driver or its > >> user space library, or we need to give them the tools to do the adaptive > >> switch themselves. > >> > >> So three options: > >> - Implement adaptive switching inside RTDM > >> o For all major functions != ioctl, we need to check for a shadow > >> thread and the availability of an RT handler => switch > >> o For IOCTLs we need an extra bit, likely in the IOC_TYPE namespace > >> to indicate the availability of an RT handler. This comes at the > >> risk of colliding with existing drivers that do not use the > >> standard IOCTL encoding scheme. > >> - Push adaptive switching into the driver > >> o That means providing the information if the caller could be > >> switched to RT context. > >> - Push adaptive switching into the helper library > >> o That means exporting a service that performs a switch if the > >> caller is switchable, but currently relaxed. > >> > >> The second option is the least invasive one, I think. The first one is > >> most transparent, but also more fragile and complex than the rest. Three > >> is likely not Philippe's favorite as it would introduce yet another > >> explicit mode switching mechanism. > >> > >> Opinions? Other ideas? > >> > >> Jan > >> > > > > > > -- Philippe.