On Thu, Jul 30, 2015 at 05:32:51PM +0200, Peter Zijlstra wrote:
> On Thu, Jul 30, 2015 at 08:25:17AM -0700, Paul E. McKenney wrote:
> > On Thu, Jul 30, 2015 at 02:28:35PM +0200, Peter Zijlstra wrote:
> > > On Fri, Jul 17, 2015 at 03:30:53PM -0700, Paul E. McKenney wrote:
> > > > + if (n > rcu_capacity[MAX_RCU_LVLS])
> > > > + panic("rcu_init_geometry: rcu_capacity[] is too small");
> > >
> > > How can this ever happen? We _know_ NR_CPUS at compile time, there's no
> > > way we can get more CPUs than that -- even if the hardware has more,
> > > we'll stop enumerating.
> >
> > You can make this happen by building with CONFIG_RCU_FANOUT=2 and
> > CONFIG_RCU_FANOUT_LEAF=2, then running on a system with more than 16 CPUs.
> > The kernel boot parameter rcutree.rcu_fanout_leaf=2 can be substituted for
> > CONFIG_RCU_FANOUT_LEAF=2, hence the need for a runtime test. I do this
> > sort of thing for my rcutorture testing in order to test a four-level
> > rcu_node tree with only 16 CPUs.
>
> How about we make the build fail if NR_CPUS exceeds that maximum fanout?
Or better yet, auto-select these numbers based on NR_CPUS and remove the
configs.
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