Re: question about RCU dynticks_nesting

[Paul E. McKenney]

On Tue, May 05, 2015 at 12:53:46PM +0200, Peter Zijlstra wrote:
> 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.

We have the field, you mean?  I might be missing something, but it still
appears to me thta preempt_disable() does nothing for PREEMPT=n kernels.
So what am I missing?

							Thanx, Paul

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