From mboxrd@z Thu Jan 1 00:00:00 1970 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> <20160614153852.GC23680@hermes.click-hack.org> From: Jan Kiszka Message-ID: <57602631.10900@siemens.com> Date: Tue, 14 Jun 2016 17:43:45 +0200 MIME-Version: 1.0 In-Reply-To: <20160614153852.GC23680@hermes.click-hack.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: Gilles Chanteperdrix Cc: Xenomai On 2016-06-14 17:38, Gilles Chanteperdrix wrote: > 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. Sorry, there *is*: Shadowed thread (anything created by wrapped pthread_create) calls, say, read() on some Linux file descriptor, read() is wrapped, first probes the call on RTDM, which means migration to RT (for currently relaxed threads, like SCHED_WEAK), no RTDM match in the kernel, and finally the migration to NRT in order to do the Linux read() syscall. That didn't happen with the original design. Jan