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 22a4b687ea5b4b3cb9d576bfeffaed813256a795..23ec96877311e49fc93360a717ceab84d1f64e81 100644 (file)
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -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

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 93c0f23c3e45d42fccb9fa946767450ea836ac5a..713eb92314b43ec2e98c888223bd80a60fef5fe4 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -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++) {