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Date: Tue, 5 May 2015 15:00:26 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Rik van Riel <riel@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>,
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        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
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().