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, 10 insertions, 5 deletions

diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 4df503470e42..4a39d364493c 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -343,12 +343,17 @@ struct rcu_data {
 #define RCU_FORCE_QS            3       /* Need to force quiescent state. */
 #define RCU_SIGNAL_INIT         RCU_SAVE_DYNTICK
 
-#define RCU_JIFFIES_TILL_FORCE_QS        3      /* for rsp->jiffies_force_qs */
+#define RCU_JIFFIES_TILL_FORCE_QS (1 + (HZ > 250) + (HZ > 500))
+                                        /* For jiffies_till_first_fqs and */
+                                        /*  and jiffies_till_next_fqs. */
 
-#define RCU_STALL_RAT_DELAY             2       /* Allow other CPUs time */
-                                                /*  to take at least one */
-                                                /*  scheduling clock irq */
-                                                /*  before ratting on them. */
+#define RCU_JIFFIES_FQS_DIV     256     /* Very large systems need more */
+                                        /*  delay between bouts of */
+                                        /*  quiescent-state forcing. */
+
+#define RCU_STALL_RAT_DELAY     2       /* Allow other CPUs time to take */
+                                        /*  at least one scheduling clock */
+                                        /*  irq before ratting on them. */
 
 #define rcu_wait(cond)                                                  \
 do {                                                                    \