Groups > linux.kernel > #1196135

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

From	Peter Zijlstra <peterz@infradead.org>
Newsgroups	linux.kernel
Subject	Re: [PATCH tip/core/rcu 02/12] rcu: Panic if RCU tree can not accommodate all CPUs
Date	2015-07-30 17:40 +0200
Message-ID	<pRY65-2v3-7@gated-at.bofh.it> (permalink)
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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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Thread

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> - 2015-07-30 17:30 +0200
  Re: [PATCH tip/core/rcu 02/12] rcu: Panic if RCU tree can not  accommodate all CPUs Peter Zijlstra <peterz@infradead.org> - 2015-07-30 17:40 +0200
    Re: [PATCH tip/core/rcu 02/12] rcu: Panic if RCU tree can not  accommodate all CPUs Peter Zijlstra <peterz@infradead.org> - 2015-07-30 17:40 +0200
    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> - 2015-07-30 18:00 +0200
      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> - 2015-07-31 18:00 +0200

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