Re: question about RCU dynticks_nesting

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, May 05, 2015 at 03:00:26PM +0200, Peter Zijlstra wrote:
> 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().

OK, fair enough.

I am going to focus first on getting rid of (or at least greatly reducing)
RCU's interrupt disabling on the user-kernel entry/exit paths, since
that seems to be the biggest cost.

							Thanx, Paul