Re: [PATCH RFC 00/14] rcu: Reduce rnp->lock contention with per-CPU blocked task lists

[Joel Fernandes]

On 1/6/2026 3:35 PM, Paul E. 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