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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