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>
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 4df5034..4a39d36 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -343,12 +343,17 @@
#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 { \
