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McKenney wrote: >>>> About the deferred-preemption, I believe Steven Rostedt at one point was looking >>>> at that for VMs, but that effort stalled as Peter is concerned about doing that >>>> would mess up the scheduler. The idea (AFAIU) is to use the rseq page to >>>> communicate locking information between vCPU threads and the host and then let >>>> the host avoid vCPU preemption - but the scheduler needs to do something with >>>> that information. Otherwise, it's no use. >>> Has deferred preemption for userspace locking also stalled? If not, >>> then the scheduler's support for userspace should apply directly to >>> guest OSes, right? >> No, the user space deferred preemption is still moving along nicely (I >> believe Thomas has completed most of it). The issue here is that the >> deferred happens before going back to user space. That's a different >> location than going back to the guest. The logic needs to be in that path >> too. > > OK, got it, thank you! There's also the challenge of sharing the locking information with the guest even when there is *no contention*. KVM being unaware of lock critical sections in the VM-exit path. Then after that wiring it up with the deffered preemption infra and moving beyond the 50 micro second limits. If we VM exited and then made a decision, I think we are easily going to blow past 50 micro seconds anyway. But again to clarify, I didn't mean to use vCPU preemption as the driving usecase for this.. but I ran into it when I wrote a benchmark to see how RCU behaves in a VM. - Joel