Re: question about RCU dynticks_nesting

[Peter Zijlstra]

On Mon, May 04, 2015 at 12:39:23PM -0700, Paul E. McKenney wrote:
> But in non-preemptible RCU, we have PREEMPT=n, so there is no preempt
> counter in production kernels.  Even if there was, we have to sample this
> on other CPUs, so the overhead of preempt_disable() and preempt_enable()
> would be where kernel entry/exit is, so I expect that this would be a
> net loss in overall performance.

We unconditionally have the preempt_count, its just not used much for
PREEMPT_COUNT=n kernels.
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