Re: [PATCH tip/core/rcu 02/12] rcu: Panic if RCU tree can not accommodate all CPUs

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

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?

Good point, and it already does, and I clearly was confused, apologies.

So the real way to make this happen is (for example) to build
with CONFIG_RCU_FANOUT=2 and CONFIG_RCU_FANOUT_LEAF=16 (the
default), which could accommodate up to 128 CPUs.  Then boot with
rcutree.rcu_fanout_leaf=2 on a system with more than 16 CPUs, with
rcutree.rcu_fanout_leaf=3 on a system with more than 24 CPUs, and so on.

Of course, the truly macho way to get this error message is to build
with CONFIG_RCU_FANOUT=64 and CONFIG_RCU_FANOUT_LEAF=64, then boot with
rcutree.rcu_fanout_leaf=63 on a system with more than 16,515,072 CPUs.
Of course, you get serious style points if the system manages to stay
up for more than 24 hours without a hardware failure.  ;-)

							Thanx, Paul