Messages in this thread | | | Date | Tue, 5 May 2015 15:00:26 +0200 | From | Peter Zijlstra <> | Subject | Re: question about RCU dynticks_nesting |
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On Tue, May 05, 2015 at 05:34:46AM -0700, Paul E. McKenney wrote: > 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?
There's another layer of accessors that can in fact manipulate the preempt_count even for !PREEMPT_COUNT kernels. They are currently used by things like pagefault_disable().
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