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: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 22a4b68..23ec968 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -3074,9 +3074,12 @@
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
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 93c0f23..713eb92 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4230,13 +4230,12 @@
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 @@
/*
* 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++) {
