From mboxrd@z Thu Jan 1 00:00:00 1970 References: <5734A9F8.10305@siemens.com> <20160512163142.GC18298@hermes.click-hack.org> <5734B43B.4040001@siemens.com> <20160512165904.GF18298@hermes.click-hack.org> <20160512171246.GG18298@hermes.click-hack.org> <5734BA9B.1030503@siemens.com> <20160512182049.GQ13285@hermes.click-hack.org> <5734CA76.5000606@siemens.com> <5734CCF8.5090508@xenomai.org> <5734CE80.60707@siemens.com> <5734D4B9.4070900@xenomai.org> <5734D92F.1000206@siemens.com> <57350331.1020302@xenomai.org> <57356C0E.6080205@siemens.com> <5735D8E0.3040202@xenomai.org> <5735F39A.8050204@siemens.com> From: Jan Kiszka Message-ID: <57601E35.3010101@siemens.com> Date: Tue, 14 Jun 2016 17:09:41 +0200 MIME-Version: 1.0 In-Reply-To: <5735F39A.8050204@siemens.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai] RTDM syscalls & switching List-Id: Discussions about the Xenomai project List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Philippe Gerum , Gilles Chanteperdrix Cc: Xenomai On 2016-05-13 17:32, Jan Kiszka wrote: > On 2016-05-13 15:38, Philippe Gerum wrote: >> On 05/13/2016 07:54 AM, Jan Kiszka wrote: >>> On 2016-05-13 00:26, Philippe Gerum wrote: >>>> On 05/12/2016 09:27 PM, Jan Kiszka wrote: >>>>> On 2016-05-12 21:08, Philippe Gerum wrote: >>>>>> On 05/12/2016 08:42 PM, Jan Kiszka wrote: >>>>>>> On 2016-05-12 20:35, Philippe Gerum wrote: >>>>>>>> On 05/12/2016 08:24 PM, Jan Kiszka wrote: >>>>>>>>> On 2016-05-12 20:20, Gilles Chanteperdrix wrote: >>>>>>>>>> On Thu, May 12, 2016 at 07:17:15PM +0200, Jan Kiszka wrote: >>>>>>>>>>> On 2016-05-12 19:12, Gilles Chanteperdrix wrote: >>>>>>>>>>>> On Thu, May 12, 2016 at 06:59:04PM +0200, Gilles Chanteperdrix wrote: >>>>>>>>>>>>> On Thu, May 12, 2016 at 06:50:03PM +0200, Jan Kiszka wrote: >>>>>>>>>>>>>> On 2016-05-12 18:31, Gilles Chanteperdrix wrote: >>>>>>>>>>>>>>> On Thu, May 12, 2016 at 06:06:16PM +0200, Jan Kiszka wrote: >>>>>>>>>>>>>>>> Gilles, >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> regarding commit bec5d0dd42 (rtdm: make syscalls conforming rather than >>>>>>>>>>>>>>>> current) - I remember a discussion on that topic, but I do not find its >>>>>>>>>>>>>>>> traces any more. Do you have a pointer >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> In any case, I'm confronted with a use case for the old (Xenomai 2), >>>>>>>>>>>>>>>> lazy switching behaviour: lightweight, performance sensitive IOCTL >>>>>>>>>>>>>>>> services that can (and should) be called without any switching from both >>>>>>>>>>>>>>>> domains. >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> Why not using a plain linux driver? ioctl_nrt callbacks are >>>>>>>>>>>>>>> redundant with plain linux drivers. >>>>>>>>>>>>>> >>>>>>>>>>>>>> Because that enforces the calling layer to either call the same service >>>>>>>>>>>>>> via a plain Linux device if the calling thread is currently relaxed or >>>>>>>>>>>>>> go for the RT device if the caller is in primary. Doable, but I would >>>>>>>>>>>>>> really like to avoid this pain for the users. >>>>>>>>>>>>>> >>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> What were the arguments in favour of migrating threads to real-time first? >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> I currently see the real need only for IOCTLs, but the question is then >>>>>>>>>>>>>>>> if we shouldn't go back to "__xn_exec_current" in all RTDM cases to >>>>>>>>>>>>>>>> avoid unwanted migration costs (which are significantly higher than >>>>>>>>>>>>>>>> syscall restarts). >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> I do not find commit bec5d0dd42 in xenomai-2.6 git tree, and I do >>>>>>>>>>>>>> >>>>>>>>>>>>>> Xenomai 2 is still following the lazy scheme - we reverted that commit >>>>>>>>>>>>>> later on in 7df0c1d96b. Xenomai 3 changed it again with the commit above. >>>>>>>>>>>>>> >>>>>>>>>>>>>>> not remember merging this. However I find commit >>>>>>>>>>>>>>> 13bfdd477ab880499d2e8f3b82c49ef4d2cccff0 from 2010 which seems to >>>>>>>>>>>>>>> explain the reason pretty clear. >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> At the time of the discussion we had concluded that it was the way >>>>>>>>>>>>>>> to go. With __xn_exec_current you may enter the ioctl_rt callback >>>>>>>>>>>>>>> from secondary domain, which is counter-intuitive, error-prone, and >>>>>>>>>>>>>>> forces you to cripple driver code for checks for the current domain. >>>>>>>>>>>>>> >>>>>>>>>>>>>> Nope, normal drivers are not affected as they just implement those >>>>>>>>>>>>>> services in the respective mode they want to support there and have a >>>>>>>>>>>>>> simple -ENOSYS for the rest (explicitly in IOCTLs or implicitly by >>>>>>>>>>>>>> leaving out the implementation of the counterpart handler). >>>>>>>>>>>>> >>>>>>>>>>>>> Yes, I got mixed up trying to remember. I think the crux of the >>>>>>>>>>>>> problem is that if a thread running in primary mode gets >>>>>>>>>>>>> (temporarily) switched to secondary mode by gdb, the ioctl_nrt >>>>>>>>>>>>> handler gets invoked, which is almost certainly the wrong thing to >>>>>>>>>>>>> do. You want the thread to migrate to primary mode to execute >>>>>>>>>>>>> ioctl_rt, which __xn_exec_conforming achieves. Otherwise running an >>>>>>>>>>>>> application in gdb causes the application to behave differently. >>>>>>>>>>>> >>>>>>>>>>>> And trying and avoiding this issue indeed cripple codes with checks >>>>>>>>>>>> for rtdm_in_rt_context: >>>>>>>>>>>> https://git.xenomai.org/xenomai-2.6.git/tree/ksrc/drivers/analogy/rtdm_interface.c#n194 >>>>>>>>>>>> >>>>>>>>>>> >>>>>>>>>>> I don't remember details here, but this is a special case: The driver >>>>>>>>>>> provides also read_nrt - is that really useful for Analogy? >>>>>>>>>>> >>>>>>>>>>> In most cases, you are fine with not providing the nrt (or rt) handler, >>>>>>>>>>> or with a simple >>>>>>>>>>> >>>>>>>>>>> default: >>>>>>>>>>> return -ENOSYS; >>>>>>>>>>> >>>>>>>>>>> in your ioctl dispatcher. >>>>>>>>>> >>>>>>>>>> You are missing the point: if you enter read_nrt, there are two >>>>>>>>>> cases: >>>>>>>>>> - either the thread is real-time capable and has been relaxed by gdb >>>>>>>>>> and you want to switch to read_rt for the reasons I already >>>>>>>>>> explained, in that case, you must return -ENOSYS; >>>>>>>>>> - or the thread is not real-time capable and the nrt handler >>>>>>>>>> applies. >>>>>>>>>> >>>>>>>>>> So, you need at least >>>>>>>>>> >>>>>>>>>> read_nrt() >>>>>>>>>> { >>>>>>>>>> if (rt_capable) >>>>>>>>>> return -ENOSYS; >>>>>>>>>> >>>>>>>>>> /* Do the normal case here */ >>>>>>>>>> } >>>>>>>>> >>>>>>>>> Now tell me how many drivers have read_nrt, write_nrt? 1 in-tree. >>>>>>>>> recvmsg_nrt, sendmsg_nrt? 0 in-tree. Analogy is special (still like to >>>>>>>>> understand why, though). And having some special code in the exceptional >>>>>>>>> case is probably better then the side effects we get from eagerly >>>>>>>>> switching now. >>>>>>>>> >>>>>>>> >>>>>>>> Sorry, that is exactly the opposite: your use case is exceptional and I >>>>>>>> believe is wrong. The normal use case is the one that does not ask the >>>>>>>> user to track the current mode for knowing what any random driver would >>>>>>>> eventually do depending on the calling context. >>>>>>> >>>>>>> You still miss the point that this is not required in 99% of the cases. >>>>>>> There is no such problem. There only Analogy. >>>>>>> >>>>>> >>>>>> I'm not discussing Analogy at all, those drivers are still biased by the >>>>>> legacy 2.x logic for dealing with modes and need fixing. I have never >>>>>> been convinced by the reasoning behind rtdm_in_rt_context(), which >>>>>> perfectly illustrates why messing with the call mode is not the >>>>>> application's business. >>>>> >>>>> You still need rtdm_in_rt_context() for the (rare) case of having the >>>>> same handler for both service_rt and service_nrt. That didn't change >>>>> with any switching strategy adjustment. It can't as long as there are >>>>> services behind a syscall that may handle any mode, thus that syscall is >>>>> unable to filter for the service in the background. We really need to >>>>> differentiate here. >>>>> >>>>>> >>>>>>> Every driver must ensure that a service is only exposed to users in the >>>>>>> right mode. That is a functional requirement, and drivers that fail to >>>>>>> do so only work by chance (thus with the restricted workload they are >>>>>>> tested against). If that is fulfilled, it doesn't matter to the driver >>>>>>> when the switch happens. It's pure optimization. >>>>>>> >>>>>> >>>>>> You don't seem to get my point either. Let's proceed differently, please >>>>>> sketch the application code that would require __xn_exec_current for >>>>>> RTDM calls. >>>>> >>>>> You cut the more interesting case (migration ping-pong when calling >>>>> non-RT drivers from relaxed threads), and I hope you will not forget to >>>>> answer this. >>>>> >>>> >>>> I'm not ignoring the question, I have been postponing the answer until I >>>> understand why the application could be put in a situation making this >>>> migration a problem, and whether another approach would exist for >>>> solving that problem within the current scheme. >>> >>> These two scenarios are unrelated: this migration issue would still be >>> there even if we solved the one below via a different application/driver >>> design. >>> >> >> Which starts to be an issue only because the caller is a Cobalt shadow >> undergoing the SCHED_WEAK policy, calling a RTDM driver for a non-rt >> operation very frequently. For this reason, those two scenarii are very >> much related. > > Not SCHED_WEAK, but being a shadow in the first place. Unless you > enforce non-shadow thread creation, all are shadowed in a Xenomai > application, thus are affected. However, asking our users to user > __real_pthread_create extensively may not lead to the desired portable > designs. > >> >>>> >>>>> But let's go to our case: >>>>> >>>>> We have a non-blocking service in the driver, the classic case of >>>>> accessing a privileged resource that userspace can't or shouldn't touch >>>>> directly. Think of some kind of register access that requires low-level >>>>> synchronization with other threads and interrupt handlers. That service >>>>> is called by both RT and non-RT threads (SCHED_WEAK) at higher frequency >>>>> (some thousand times per second). The RT threads are obviously on the >>>>> time critical path, must not migrate, and that can be achieved perfectly >>>>> already by providing that service under ioctl_rt. The non-RT threads >>>>> could be migrated to RT, but then they would pay an unneeded price, >>>>> contributing to a higher system load, in the worst case overload. >>>>> Therefore, the very same service shall be provided under ioctl_nrt as >>>>> well. Makes sense? >>>>> >>>> >>>> I understand the conflict with the "rt-always-has-precedence" rule >>>> implemented by the conforming state, then I have another question: >>>> >>>> assuming the nrt thread undergoes the SCHED_WEAK policy because it is >>>> mainly operating from the Linux space but still needs to synchronize >>>> with the rt side at some point, which kind of high frequency interaction >>>> with the rt side is this? >>>> >>>> Sharing some resource requiring mutual exclusion via a Cobalt synchro, >>>> waiting for rt events, something else? >>>> >>> >>> There synchronization need is first of all only on the hardware access >>> (thus inside the driver), not necessarily at application level. In fact, >>> there are even scenarios where you only want to exploit the driver as >>> permission checker on privileged resource accesses (userspace shall only >>> access certain MMIO registers in a page, thus the driver acts as >>> gatekeeper). Then there could be no synchronization at all but still the >>> need to provide migration-free accesses. >>> >> >> I get the idea of the resource gatekeeper, which does make a lot of sense. >> >> However I still don't get which benefit your caller has in undergoing >> the SCHED_WEAK policy - which implies that it has to share >> synchronization points with Cobalt - compared to running as a regular >> (glibc) thread, under whichever policy that could fit? > > See above: it's additional, non-portable instrumentation of your code to > tag non-shadowed threads. And then you may easily run into troubles in > larger, layered application designs that a non-shadowed thread will > still need a blocking Xenomai service, e.g. via some hidden dependency. > >> >> Leaving the non-RT ioctl call aside, which are those Cobalt calls the >> SCHED_WEAK thread needs to invoke for synchronizing with rt threads? > > I don't have these details at hand, but let's consider a large layered > application that also does significant work against Linux APIs during > runtime. You can't always enforce the complete separation. Because if > you can, you could also move the non-RT part into a separate process > that has nothing to do with Xenomai. > > We promote the transparency of the Xenomai POSIX interface, and that > should not make the usage of non-Xenomai services needlessly expensive > or require extensive non-portable tagging via __real_ prefixes. > Ping on this still open topic (will now have to introduce a local patch that restores the original behaviour). Can we resolve the issue upstream as well? Thanks, Jan -- Siemens AG, Corporate Technology, CT RDA ITP SES-DE Corporate Competence Center Embedded Linux