Hello Mathieu, Norbert and Jan, Thank you for all of your explainations and the overview of the system. No I didn't change the ipipe patch for the vDSO, I may try this. If I have correctly understood, this patch prevents Cobalt from entering in a deadlock when the kernel is using the vDSO and the program interrupts the kernel at the same time. On which kernel does it word (aroubd 4.19) ? I currently try to avoid kernel 5.4 because I remember I faced some boot issues (but it is on another topic). Here the issues i faced (drawn on TraceCompass). Are these the deadlocks we are talking about ? https://postimg.cc/BP4G3bF0 (on 11:02:56:380) https://postimg.cc/q6wHvrcC Regards, ________________________________ De : Norbert Lange Envoyé : jeudi 20 mai 2021 17:39 À : Mathieu Desnoyers Cc : MONTET Julien ; lttng-dev ; Jan Kiszka ; Xenomai Objet : Re: [lttng-dev] LTTng - Xenomai : different results between timestamp-lttng and rt_time_read() Am Do., 20. Mai 2021 um 17:09 Uhr schrieb Mathieu Desnoyers : > > ----- On May 20, 2021, at 9:56 AM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote: > > > ----- On May 20, 2021, at 9:54 AM, lttng-dev lttng-dev@lists.lttng.org wrote: > > > >> ----- On May 20, 2021, at 5:11 AM, lttng-dev lttng-dev@lists.lttng.org wrote: > >> > >>> Am Do., 20. Mai 2021 um 10:28 Uhr schrieb MONTET Julien > >>> : > >>>> > >>>> Hi Norbert, > >>>> > >>>> Thank you for your answer ! > >>>> > >>>> Yes, I am using a Xenomai cobalt - xenomai is 3.1 > >>>> cat /proc/xenomai/version => 3.1 > >>>> > >>>> After the installation, I tested "test tools" in /proc/xenomai/ and it worked > >>>> nice. > >>> > >>> Just asked to make sure, thought the scripts usual add some -xeno tag > >>> to the kernel version. > >>> > >>>> What do you mean by "it might deadlock really good" ? > >>> > >>> clock_gettime will either use a syscall (kills realtime always) or is > >>> optimized via VDSO (which very likely is your case). > >>> > >>> What happens is that the kernel will take a spinlock, then write new > >>> values, then releases the spinlock. > >>> your program will aswell spin (but just to see if the spinlock is > >>> free), read the values and interpolates them. > >>> > >>> But if your program interrupts the kernel while the kernel holds the > >>> lock (all on the same cpu core), then it will spin forever and the > >>> kernel will never execute. > >> > >> Just one clarification: the specific locking strategy used by the > >> Linux kernel monotonic clock vDSO is a "seqlock", where the kernel > >> sets a bit which keeps concurrent readers looping until they observe > > > > When I say "sets a bit", I actually mean "increment a sequence counter", > > and readers observe either odd or even state, thus knowing whether > > they need to retry, and whether the value read before/after reading > > the data structure changed. > > Looking again at the Linux kernel's kernel/time/vsyscall.c implementation > of vdso_update_{begin,end}, I notice that interrupts are disabled across > the entire update. So I understand that the Interrupt pipeline (I-pipe) > interrupt gets delivered even when the kernel disables interrupts. Did > you consider modifying the I-pipe kernel patch to change the vdso update so > it updates the vdso from within an I-pipe virq handler ? Yes, I did use an non-upstreamed patch for a while to get things in order: https://eur02.safelinks.protection.outlook.com/?url=https%3A%2F%2Fwww.xenomai.org%2Fpipermail%2Fxenomai%2F2018-December%2F040134.html&data=04%7C01%7Cjulien.montet%40reseau.eseo.fr%7Cef0b71ac314f4ab2321f08d91ba57c9d%7C4d7ad1591265437ab9f62946247d5bf9%7C0%7C0%7C637571219835495365%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=dOiRFzeKFQA%2B25R6aqrjL2ZJMkV5c782DSBGiHHoYZc%3D&reserved=0 I would prefer just a NMI safe source that might jump back a bit, no matter how. > AFAIU this would allow Xenomai userspace to use the Linux kernel vDSO > clock sources. The Xenomai folks are trying to get their next-gen abstraction "dovetail" closer coupled to the kernel, AFAIR their will be VDSO support and unification of the clock sources. Still need to get stuff running today =) Norbert