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.
Thanx, Paul
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