rcu: Drive quiescent-state-forcing delay from HZ

Systems with HZ=100 can have slow bootup times due to the default
three-jiffy delays between quiescent-state forcing attempts.  This
commit therefore auto-tunes the RCU_JIFFIES_TILL_FORCE_QS value based
on the value of HZ.  However, this would break very large systems that
require more time between quiescent-state forcing attempts.  This
commit therefore also ups the default delay by one jiffy for each
256 CPUs that might be on the system (based off of nr_cpu_ids at
runtime, -not- NR_CPUS at build time).

Updated to collapse #ifdefs for RCU_JIFFIES_TILL_FORCE_QS into a
step-function definition as suggested by Josh Triplett.

Reported-by: Paul Mackerras <paulus@au1.ibm.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

1 files changed, 16 insertions, 2 deletions

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 1009c0ccd4b1..f344d3c824a4 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -218,8 +218,8 @@ module_param(blimit, long, 0444);
 module_param(qhimark, long, 0444);
 module_param(qlowmark, long, 0444);
 
-static ulong jiffies_till_first_fqs = RCU_JIFFIES_TILL_FORCE_QS;
-static ulong jiffies_till_next_fqs = RCU_JIFFIES_TILL_FORCE_QS;
+static ulong jiffies_till_first_fqs = ULONG_MAX;
+static ulong jiffies_till_next_fqs = ULONG_MAX;
 
 module_param(jiffies_till_first_fqs, ulong, 0644);
 module_param(jiffies_till_next_fqs, ulong, 0644);
@@ -3265,11 +3265,25 @@ static void __init rcu_init_one(struct rcu_state *rsp,
  */
 static void __init rcu_init_geometry(void)
 {
+        ulong d;
         int i;
         int j;
         int n = nr_cpu_ids;
         int rcu_capacity[MAX_RCU_LVLS + 1];
 
+        /*
+         * Initialize any unspecified boot parameters.
+         * The default values of jiffies_till_first_fqs and
+         * jiffies_till_next_fqs are set to the RCU_JIFFIES_TILL_FORCE_QS
+         * value, which is a function of HZ, then adding one for each
+         * RCU_JIFFIES_FQS_DIV CPUs that might be on the system.
+         */
+        d = RCU_JIFFIES_TILL_FORCE_QS + nr_cpu_ids / RCU_JIFFIES_FQS_DIV;
+        if (jiffies_till_first_fqs == ULONG_MAX)
+                jiffies_till_first_fqs = d;
+        if (jiffies_till_next_fqs == ULONG_MAX)
+                jiffies_till_next_fqs = d;
+
         /* If the compile-time values are accurate, just leave. */
         if (rcu_fanout_leaf == CONFIG_RCU_FANOUT_LEAF &&
             nr_cpu_ids == NR_CPUS)