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McKenney" , Anna-Maria Behnsen , Davidlohr Bueso , Frederic Weisbecker , Ingo Molnar , Josh Triplett , Thomas Gleixner , Florian Bezdeka , Pavel Machek References: <20241106150419.2593080-1-bigeasy@linutronix.de> <20241106150419.2593080-4-bigeasy@linutronix.de> <0d66a966-0b89-416a-8712-6a6131af355e@siemens.com> From: Jan Kiszka Content-Language: en-US Autocrypt: addr=jan.kiszka@siemens.com; keydata= xsFNBGZY+hkBEACkdtFD81AUVtTVX+UEiUFs7ZQPQsdFpzVmr6R3D059f+lzr4Mlg6KKAcNZ uNUqthIkgLGWzKugodvkcCK8Wbyw+1vxcl4Lw56WezLsOTfu7oi7Z0vp1XkrLcM0tofTbClW xMA964mgUlBT2m/J/ybZd945D0wU57k/smGzDAxkpJgHBrYE/iJWcu46jkGZaLjK4xcMoBWB I6hW9Njxx3Ek0fpLO3876bszc8KjcHOulKreK+ezyJ01Hvbx85s68XWN6N2ulLGtk7E/sXlb 79hylHy5QuU9mZdsRjjRGJb0H9Buzfuz0XrcwOTMJq7e7fbN0QakjivAXsmXim+s5dlKlZjr L3ILWte4ah7cGgqc06nFb5jOhnGnZwnKJlpuod3pc/BFaFGtVHvyoRgxJ9tmDZnjzMfu8YrA +MVv6muwbHnEAeh/f8e9O+oeouqTBzgcaWTq81IyS56/UD6U5GHet9Pz1MB15nnzVcyZXIoC roIhgCUkcl+5m2Z9G56bkiUcFq0IcACzjcRPWvwA09ZbRHXAK/ao/+vPAIMnU6OTx3ejsbHn oh6VpHD3tucIt+xA4/l3LlkZMt5FZjFdkZUuAVU6kBAwElNBCYcrrLYZBRkSGPGDGYZmXAW/ VkNUVTJkRg6MGIeqZmpeoaV2xaIGHBSTDX8+b0c0hT/Bgzjv8QARAQABzSNKYW4gS2lzemth IDxqYW4ua2lzemthQHNpZW1lbnMuY29tPsLBlAQTAQoAPhYhBABMZH11cs99cr20+2mdhQqf QXvYBQJmWPvXAhsDBQkFo5qABQsJCAcCBhUKCQgLAgQWAgMBAh4BAheAAAoJEGmdhQqfQXvY zPAP/jGiVJ2VgPcRWt2P8FbByfrJJAPCsos+SZpncRi7tl9yTEpS+t57h7myEKPdB3L+kxzg K3dt1UhYp4FeIHA3jpJYaFvD7kNZJZ1cU55QXrJI3xu/xfB6VhCs+VAUlt7XhOsOmTQqCpH7 pRcZ5juxZCOxXG2fTQTQo0gfF5+PQwQYUp0NdTbVox5PTx5RK3KfPqmAJsBKdwEaIkuY9FbM 9lGg8XBNzD2R/13cCd4hRrZDtyegrtocpBAruVqOZhsMb/h7Wd0TGoJ/zJr3w3WnDM08c+RA 5LHMbiA29MXq1KxlnsYDfWB8ts3HIJ3ROBvagA20mbOm26ddeFjLdGcBTrzbHbzCReEtN++s gZneKsYiueFDTxXjUOJgp8JDdVPM+++axSMo2js8TwVefTfCYt0oWMEqlQqSqgQwIuzpRO6I ik7HAFq8fssy2cY8Imofbj77uKz0BNZC/1nGG1OI9cU2jHrqsn1i95KaS6fPu4EN6XP/Gi/O 0DxND+HEyzVqhUJkvXUhTsOzgzWAvW9BlkKRiVizKM6PLsVm/XmeapGs4ir/U8OzKI+SM3R8 VMW8eovWgXNUQ9F2vS1dHO8eRn2UqDKBZSo+qCRWLRtsqNzmU4N0zuGqZSaDCvkMwF6kIRkD ZkDjjYQtoftPGchLBTUzeUa2gfOr1T4xSQUHhPL8zsFNBGZY+hkBEADb5quW4M0eaWPIjqY6 aC/vHCmpELmS/HMa5zlA0dWlxCPEjkchN8W4PB+NMOXFEJuKLLFs6+s5/KlNok/kGKg4fITf Vcd+BQd/YRks3qFifckU+kxoXpTc2bksTtLuiPkcyFmjBph/BGms35mvOA0OaEO6fQbauiHa QnYrgUQM+YD4uFoQOLnWTPmBjccoPuiJDafzLxwj4r+JH4fA/4zzDa5OFbfVq3ieYGqiBrtj tBFv5epVvGK1zoQ+Rc+h5+dCWPwC2i3cXTUVf0woepF8mUXFcNhY+Eh8vvh1lxfD35z2CJeY txMcA44Lp06kArpWDjGJddd+OTmUkFWeYtAdaCpj/GItuJcQZkaaTeiHqPPrbvXM361rtvaw XFUzUlvoW1Sb7/SeE/BtWoxkeZOgsqouXPTjlFLapvLu5g9MPNimjkYqukASq/+e8MMKP+EE v3BAFVFGvNE3UlNRh+ppBqBUZiqkzg4q2hfeTjnivgChzXlvfTx9M6BJmuDnYAho4BA6vRh4 Dr7LYTLIwGjguIuuQcP2ENN+l32nidy154zCEp5/Rv4K8SYdVegrQ7rWiULgDz9VQWo2zAjo TgFKg3AE3ujDy4V2VndtkMRYpwwuilCDQ+Bpb5ixfbFyZ4oVGs6F3jhtWN5Uu43FhHSCqUv8 FCzl44AyGulVYU7hTQARAQABwsF8BBgBCgAmFiEEAExkfXVyz31yvbT7aZ2FCp9Be9gFAmZY +hkCGwwFCQWjmoAACgkQaZ2FCp9Be9hN3g/8CdNqlOfBZGCFNZ8Kf4tpRpeN3TGmekGRpohU bBMvHYiWW8SvmCgEuBokS+Lx3pyPJQCYZDXLCq47gsLdnhVcQ2ZKNCrr9yhrj6kHxe1Sqv1S MhxD8dBqW6CFe/mbiK9wEMDIqys7L0Xy/lgCFxZswlBW3eU2Zacdo0fDzLiJm9I0C9iPZzkJ gITjoqsiIi/5c3eCY2s2OENL9VPXiH1GPQfHZ23ouiMf+ojVZ7kycLjz+nFr5A14w/B7uHjz uL6tnA+AtGCredDne66LSK3HD0vC7569sZ/j8kGKjlUtC+zm0j03iPI6gi8YeCn9b4F8sLpB lBdlqo9BB+uqoM6F8zMfIfDsqjB0r/q7WeJaI8NKfFwNOGPuo93N+WUyBi2yYCXMOgBUifm0 T6Hbf3SHQpbA56wcKPWJqAC2iFaxNDowcJij9LtEqOlToCMtDBekDwchRvqrWN1mDXLg+av8 qH4kDzsqKX8zzTzfAWFxrkXA/kFpR3JsMzNmvextkN2kOLCCHkym0zz5Y3vxaYtbXG2wTrqJ 8WpkWIE8STUhQa9AkezgucXN7r6uSrzW8IQXxBInZwFIyBgM0f/fzyNqzThFT15QMrYUqhhW ZffO4PeNJOUYfXdH13A6rbU0y6xE7Okuoa01EqNi9yqyLA8gPgg/DhOpGtK8KokCsdYsTbk= In-Reply-To: Content-Type: text/plain; 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Goncalves wrote: > On Mon, Dec 01, 2025 at 10:51:50PM +0100, Jan Kiszka wrote: >> On 06.11.24 15:51, Sebastian Andrzej Siewior wrote: >>> A timer/ hrtimer softirq is raised in-IRQ context. With threaded >>> interrupts enabled or on PREEMPT_RT this leads to waking the ksoftirqd >>> for the processing of the softirq. ksoftirqd runs as SCHED_OTHER which >>> means it will compete with other tasks for CPU ressources. >>> This can introduce long delays for timer processing on heavy loaded >>> systems and is not desired. >>> >>> Split the TIMER_SOFTIRQ and HRTIMER_SOFTIRQ processing into a dedicated >>> timers thread and let it run at the lowest SCHED_FIFO priority. >>> Wake-ups for RT tasks happen from hardirq context so only timer_list timers >>> and hrtimers for "regular" tasks are processed here. The higher priority >>> ensures that wakeups are performed before scheduling SCHED_OTHER tasks. >>> >>> Using a dedicated variable to store the pending softirq bits values >>> ensure that the timer are not accidentally picked up by ksoftirqd and >>> other threaded interrupts. >>> It shouldn't be picked up by ksoftirqd since it runs at lower priority. >>> However if ksoftirqd is already running while a timer fires, then >>> ksoftird will be PI-boosted due to the BH-lock to ktimer's priority. >>> Ideally we try to avoid having ksoftirqd running. >>> >>> The timer thread can pick up pending softirqs from ksoftirqd but only >>> if the softirq load is high. It is not be desired that the picked up >>> softirqs are processed at SCHED_FIFO priority under high softirq load >>> but this can already happen by a PI-boost by a force-threaded interrupt. >>> >>> [ frederic@kernel.org: rcutorture.c fixes, storm fix by introduction of >>> local_timers_pending() for tick_nohz_next_event() ] >>> >>> [ junxiao.chang@intel.com: Ensure ktimersd gets woken up even if a >>> softirq is currently served. ] >>> >>> Reviewed-by: Paul E. McKenney [rcutorture] >>> Reviewed-by: Frederic Weisbecker >>> Signed-off-by: Sebastian Andrzej Siewior >> >> This went into 6.13 and was never backported to 6.12-lts. And that is >> why you can easily stall the latter with a workload like this and >> CONFIG_PREEMPT_RT enabled: >> >> echo "+cpu" >> /sys/fs/cgroup/cgroup.subtree_control >> echo "+cpuset" >> /sys/fs/cgroup/cgroup.subtree_control >> >> mkdir /sys/fs/cgroup/stalltest.sub1 >> mkdir /sys/fs/cgroup/stalltest.sub2 >> sleep 10000000 & >> pid=$! >> >> systemd-run --slice "stalltest.slice" taskset -c 0 sh -c " \ >> while true; do >> echo $pid > /sys/fs/cgroup/stalltest.sub1/cgroup.procs; >> echo $pid > /sys/fs/cgroup/stalltest.sub2/cgroup.procs; >> done" >> >> echo "1000 20000" > /sys/fs/cgroup/stalltest.slice/cpu.max >> >> This triggers a lock-up if a holder of cgroup_file_kn_lock with >> SCHED_OTHER is scheduled out after using up its timeslice and then >> cgroup_file_notify_timer fires over a SCHED_OTHER context as well, >> trying to get this lock, failing and then never being able to reactivate >> the lock holder again as well. >> >> I've nicely reproduced this with upstream 6.12.58 while Debian's lastest >> 6.12-rt does not trigger because it additionally has the downstream -rt >> patches on board. >> >> How should we handle this? Consider 6.12 mainline with -rt and cgroups >> as potentially broken, asking people to user 6.12-rt? Or port this back? >> >> BTW, the original report of this issue came from an older >> 5.10.194-cip39-rt16 kernel (based on rt94 for 5.10). When was this >> feature introduced to the -rt patches? Was it ever backported to 5.10-rt >> or other -rt versions? > > Hi Jan! > > I failed to locate the original discussion (from v5.10-rt) as the V1 of this > patchset is a new thread. Anyway, you are correct, the commit below (and the > other two changes from the series) are not present in v5.10-rt. > > AFAICT commit 49a17639508c ("softirq: Use a dedicated thread for timer wakeups > on PREEMPT_RT.") was merged initially to v6.13-rc1, it was never exclusive > to the RT tree. So, we have this right now in the 6.12-rt and 6.1-rt trees. The patch (b12e35832805) that enabled the lockup above was added to 4.18. In 4.19-rt (still under maintenance via 4.19-cip-rt), we had ktimersoftd - was that addressing the issue as well? Or could timers have expired back then also outside of that thread, thus potentially without SCHED_FIFO prio? Thanks, Jan -- Siemens AG, Foundational Technologies Linux Expert Center