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> <57601E35.3010101@siemens.com> <5a98b862-5b1f-449c-8989-f7e3d4fe8255@xenomai.org> From: Jan Kiszka Message-ID: <5760227A.3010203@siemens.com> Date: Tue, 14 Jun 2016 17:27:54 +0200 MIME-Version: 1.0 In-Reply-To: <5a98b862-5b1f-449c-8989-f7e3d4fe8255@xenomai.org> 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 2016-06-14 17:23, Philippe Gerum wrote: > On 06/14/2016 05:09 PM, Jan Kiszka wrote: >> 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? >> > > Restoring the original behavior unconditionally would not be a fix but > only a work-around for your own issue. Finding a better way acceptable > to all parties is on my todo list for the upcoming 3.0.3. We don't have all the issues, as I pointed out. It is a significant deficit of current Xenomai that you now have to create non-Xenomai threads explicitly (__real_pthread_create) in order to use Linux I/O syscalls efficiently (because of the otherwise enforces migration ping-pong). We didn't have that problem with the original design. BTW, we is corectl a probing syscall? It doesn't make use of this feature at all. Thanks, Jan -- Siemens AG, Corporate Technology, CT RDA ITP SES-DE Corporate Competence Center Embedded Linux