From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Tue, 14 Jun 2016 17:38:52 +0200 From: Gilles Chanteperdrix Message-ID: <20160614153852.GC23680@hermes.click-hack.org> References: <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> <5760227A.3010203@siemens.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5760227A.3010203@siemens.com> Subject: Re: [Xenomai] RTDM syscalls & switching List-Id: Discussions about the Xenomai project List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jan Kiszka Cc: Xenomai On Tue, Jun 14, 2016 at 05:27:54PM +0200, Jan Kiszka wrote: > 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. > > 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). Please don't spread misconceptions, there is no ping-pong when using Linux I/O syscalls. There is ping-pong when using RTDM I/O with _nrt handlers. -- Gilles. https://click-hack.org