Commit ee968ac6 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu: Eliminate panic when silly boot-time fanout specified 



This commit loosens rcutree.rcu_fanout_leaf range checks
and replaces a panic() with a fallback to compile-time values.
This fallback is accompanied by a WARN_ON(), and both occur when the
rcutree.rcu_fanout_leaf value is too small to accommodate the number of
CPUs.  For example, given the current four-level limit for the rcu_node
tree, a system with more than 16 CPUs built with CONFIG_FANOUT=2 must
have rcutree.rcu_fanout_leaf larger than 2.

Reported-by: default avatarPeter Zijlstra <peterz@infradead.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: default avatarJosh Triplett <josh@joshtriplett.org>
parent bb73c52b

Documentation/kernel-parameters.txt

+6 −3
Original line number Diff line number Diff line
@@ -3074,9 +3074,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted. 
			cache-to-cache transfer latencies.



	rcutree.rcu_fanout_leaf= [KNL]

			Increase the number of CPUs assigned to each

			leaf rcu_node structure.  Useful for very large

			systems.

			Change the number of CPUs assigned to each

			leaf rcu_node structure.  Useful for very

			large systems, which will choose the value 64,

			and for NUMA systems with large remote-access

			latencies, which will choose a value aligned

			with the appropriate hardware boundaries.



	rcutree.jiffies_till_sched_qs= [KNL]

			Set required age in jiffies for a

kernel/rcu/tree.c

+11 −9
Original line number Diff line number Diff line
@@ -4230,13 +4230,12 @@ static void __init rcu_init_geometry(void) 
		rcu_fanout_leaf, nr_cpu_ids);



	/*

	 * The boot-time rcu_fanout_leaf parameter is only permitted

	 * to increase the leaf-level fanout, not decrease it.  Of course,

	 * the leaf-level fanout cannot exceed the number of bits in

	 * the rcu_node masks.  Complain and fall back to the compile-

	 * time values if these limits are exceeded.

	 * The boot-time rcu_fanout_leaf parameter must be at least two

	 * and cannot exceed the number of bits in the rcu_node masks.

	 * Complain and fall back to the compile-time values if this

	 * limit is exceeded.

	 */

	if (rcu_fanout_leaf < RCU_FANOUT_LEAF ||

	if (rcu_fanout_leaf < 2 ||

	    rcu_fanout_leaf > sizeof(unsigned long) * 8) {

		rcu_fanout_leaf = RCU_FANOUT_LEAF;

		WARN_ON(1);
@@ -4253,10 +4252,13 @@ static void __init rcu_init_geometry(void) 


	/*

	 * The tree must be able to accommodate the configured number of CPUs.

	 * If this limit is exceeded than we have a serious problem elsewhere.

	 * If this limit is exceeded, fall back to the compile-time values.

	 */

	if (nr_cpu_ids > rcu_capacity[RCU_NUM_LVLS - 1])

		panic("rcu_init_geometry: rcu_capacity[] is too small");

	if (nr_cpu_ids > rcu_capacity[RCU_NUM_LVLS - 1]) {

		rcu_fanout_leaf = RCU_FANOUT_LEAF;

		WARN_ON(1);

		return;

	}



	/* Calculate the number of levels in the tree. */

	for (i = 0; nr_cpu_ids > rcu_capacity[i]; i++) {

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