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Goncalves" , Andreas Ziegler , Felix Moessbauer , Florian Bezdeka References: <3f7b7ce1-6dd4-4a4e-9789-4c0cbde057bd@siemens.com> <20250409121314.GA632990@bytedance> <3d13e35a-51bb-4057-8923-ebb280793351@siemens.com> <20250414150513.klU46xkw@linutronix.de> <438a5250-30c0-4820-9084-cdef50f67865@siemens.com> <20250415062320.bMvqQsQ4@linutronix.de> <20250415080041.kePLxkOG@linutronix.de> 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: <20250415080041.kePLxkOG@linutronix.de> Content-Type: text/plain; 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So >>>>> every blocker on the lock is scheduled out until the reader is gone. On >>>>> top of that, the reader gets RCU boosted with FIFO-1 by default to get >>>>> out. >>>> >>>> There is no boosting of the active readers on RT as there is no >>>> information recorded about who is currently holding a read lock. This is >>>> the whole point why rwlocks are hairy with RT, I thought. >>> >>> Kind of, yes. PREEMPT_RT has by default RCU boosting enabled with >>> SCHED_FIFO 1. If you acquire a readlock you start a RCU section. If you >>> get stuck in a RCU section for too long then this boosting will take >>> effect by making the task, within the RCU section, the owner of the >>> boost-lock and the boosting task will try to acquire it. This is used to >>> get SCHED_OTHER tasks out of the RCU section. >>> But if a SCHED_FIFO task is on the CPU then this boosting will have to >>> no effect because the scheduler will not switch to a task with lower >>> priority. >> >> Does that boosting happen to need ktimersd or ksoftirqd (which both are >> stalling in our case)? I'm still looking for the reason why it does not >> help in the observed stall scenarios. > > Your problem is that you likely have many reader which need to get out > first. That spinlock replacement will help. I'm not sure about the CFS > patch referenced in the thread here. Nope, we only have two readers, one which is scheduled out by CFS and another one - in soft IRQ context - that is getting stuck after the writer promoted the held lock to a write lock. > > That boosting requires a RCU reader that starts the mechanism (on rcu > unlock). But I don't think that it will help. You would also need to > raise the priority above to the writer level (manually) and that will > likely break other things. It is meant to unstuck SCHED_OTHER tasks and > not boost stuck reader as a side effect. Also I am not sure how that > works with multiple tasks. Ok, that is likely why we don't see that coming in for helping us out. Jan -- Siemens AG, Foundational Technologies Linux Expert Center