Messages in this thread | | | Date | Thu, 30 Jul 2015 17:32:51 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH tip/core/rcu 02/12] rcu: Panic if RCU tree can not accommodate all CPUs |
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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?
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