Re: NMI reentrant RCU list for -rt kernels

[Esben Nielsen <nielsen.esben@googlemail.com>]

On Sat, 22 Jul 2006, Mathieu Desnoyers wrote:

> 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 ?

No,

1) Can you readily identify the very short code pathes? What about future 
code added to the kernel?
2) Having two parellel systems is a bad idea.
3) I believe RCU can be made much cheaper than the 
current implementation which look horrible.

I remember once discussing RCU on the list. I came up with the idea 
rcu_read_lock()/unlock() to be implemented as a per-task counter just as
preempt_disable()/disable(). The run-queue then has a sum of all the 
counters of tasks on that cpu (minus the counter for the current task).
I even made some sample code...
The only reason this wasn't considered working was the migration from CPU 
to CPU. I frankly can't see why this couldn't be fixed.

So the answer to you is: No. Fix the preemptible RCU instead. You have an 
idea above.

Esben


> Mathieu
>
>
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