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

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Date:	Thu, 30 Jul 2015 08:25:17 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	jiangshanlai@...il.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, rostedt@...dmis.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com,
	Alexander Gordeev <agordeev@...hat.com>
Subject: Re: [PATCH tip/core/rcu 02/12] rcu: Panic if RCU tree can not
 accommodate all CPUs

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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