NMI reentrant RCU list for -rt kernels

[Mathieu Desnoyers <compudj@krystal.dyndns.org>]

Hi Paul,

Following your presentation on RCU lists for -rt kernel, discussing with Bill
Huey led me to the following idea that could solve the problem of NMI reentrancy
of RCU read side in the -rt kernels.

If we consider that the RCU list modification that makes the read side
lock preemptible is only needed for very long code paths, we could leave the
original RCU implementation along with the preemptible one, so that very short
and frequent code paths could benefit of using the very cheap preempt count
protection without having a too big impact on the scheduler latency.

For instance, my LTTng tracer disables the preemption for about 95 ns, which I
doubt would be a problem for real-time behavior. I could easily fix maximum
a maximum list size so it can be run in a constant time.

So, basically, the idea is to have two RCU API that could take names like :
atomic_rcu_* and rcu_*

Does this idea make sense ?

Mathieu


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