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We >>>>> also >>>>> have task hung problem from time to time in our environment due to >>>>> cfs quota. >>>>> It is mostly visible with rwsem: when a reader is throttled, writer >>>>> comes in >>>>> and has to wait, the writer also makes all subsequent readers wait, >>>>> causing problems of priority inversion or even whole system hung. >>>> >>>> for testing purposes I backported this series to 6.14. We're currently >>>> hunting for a sporadic bug with PREEMPT_RT enabled. We see RCU stalls >>>> and complete system freezes after a couple of days with some container >>>> workload deployed. See [1]. >>> >>> I tried to make a setup last week to reproduce the RT/cfs throttle >>> deadlock issue Valentin described but haven't succeeded yet... >>> >> >> Attached the bits with which we succeeded, sometimes. Setup: Debian 12, >> RT kernel, 2-4 cores VM, 1-5 instances of the test, 2 min - 2 h >> patience. As we have to succeed with at least 3 race conditions in a >> row, that is still not bad... But maybe someone has an idea how to >> increase probabilities further. > > Looking at run.sh, there are only fair tasks with one of them being run > with cfs bandwidth constraints. Are you saying something goes wrong on > PREEMPT_RT as a result of using bandwidth control on fair tasks? Yes, exactly. Also our in-field workload that triggers (most likely) this issue is not using RT tasks itself. Only kernel threads are RT here. > > What exactly is the symptom you are observing? Does one of the assert() > trip during the run? Do you see a stall logged on dmesg? Can you provide > more information on what to expect in this 2min - 2hr window? I've just lost my traces from yesterday ("you have 0 minutes to find a power adapter"), but I got nice RCU stall warnings in the VM, including backtraces from the involved tasks (minus the read-lock holder IIRC). Maybe Florian can drop one of his dumps. > > Additionally, do you have RT throttling enabled in your setup? Can long > running RT tasks starve fair tasks on your setup? RT throttling is enabled (default settings) but was not kicking in - why should it in that scenario? The only RT thread, ktimers, ran into the held lock and stopped. Jan -- Siemens AG, Foundational Technologies Linux Expert Center