Messages in this thread | | | Date | Mon, 11 Jul 2022 16:02:21 -0400 | Subject | Re: [PATCH v3] locking/rtmutex: Limit # of lock stealing for non-RT waiters | From | Waiman Long <> |
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On 7/11/22 05:34, Peter Zijlstra wrote: > On Wed, Jul 06, 2022 at 09:59:16AM -0400, Waiman Long wrote: >> Commit 48eb3f4fcfd3 ("locking/rtmutex: Implement equal priority lock >> stealing") allows unlimited number of lock stealing's for non-RT >> tasks. That can lead to lock starvation of non-RT top waiter tasks if >> there is a constant incoming stream of non-RT lockers. This can cause >> rcu_preempt self-detected stall or even task lockup in PREEMPT_RT kernel. >> For example, >> >> [77107.424943] rcu: INFO: rcu_preempt self-detected stall on CPU >> [ 1249.921363] INFO: task systemd:2178 blocked for more than 622 seconds. >> >> Avoiding this problem and ensuring forward progress by limiting the >> number of times that a lock can be stolen from each waiter. This patch >> sets a threshold of 32. That number is arbitrary and can be changed >> if needed. >> > Why not do the same thing we do for regular mutexes? > The mutex way is another possible alternative. So we can set a flag to disable lock stealing if the current top waiter wake up and the rtmutex has been stolen. I will need to run some tests to find out how many time lock stealing can happen before it is blocked. I would like to allow sufficient number of lock stealing to minimize the performance impact of this change.
Cheers, Longman
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