rcu: Accelerate grace period if last non-dynticked CPU

Currently, rcu_needs_cpu() simply checks whether the current CPU
has an outstanding RCU callback, which means that the last CPU
to go into dyntick-idle mode might wait a few ticks for the
relevant grace periods to complete.  However, if all the other
CPUs are in dyntick-idle mode, and if this CPU is in a quiescent
state (which it is for RCU-bh and RCU-sched any time that we are
considering going into dyntick-idle mode), then the grace period
is instantly complete.

This patch therefore repeatedly invokes the RCU grace-period
machinery in order to force any needed grace periods to complete
quickly.  It does so a limited number of times in order to
prevent starvation by an RCU callback function that might pass
itself to call_rcu().

However, if any CPU other than the current one is not in
dyntick-idle mode, fall back to simply checking (with fix to bug
noted by Lai Jiangshan).  Also, take advantage of last
grace-period forcing, the opportunity to do so noted by Steve
Rostedt.  And apply simplified #ifdef condition suggested by
Frederic Weisbecker.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: Valdis.Kletnieks@vt.edu
Cc: dhowells@redhat.com
LKML-Reference: <1266887105-1528-15-git-send-email-paulmck@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 14 insertions, 0 deletions

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index d77b547..dbcee76 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -143,6 +143,8 @@ static inline unsigned int cpumask_any_but(const struct cpumask *mask,
 
 #define for_each_cpu(cpu, mask)			\
 	for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask)
+#define for_each_cpu_not(cpu, mask)		\
+	for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask)
 #define for_each_cpu_and(cpu, mask, and)	\
 	for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask, (void)and)
 #else
@@ -203,6 +205,18 @@ int cpumask_any_but(const struct cpumask *mask, unsigned int cpu);
 		(cpu) < nr_cpu_ids;)
 
 /**
+ * for_each_cpu_not - iterate over every cpu in a complemented mask
+ * @cpu: the (optionally unsigned) integer iterator
+ * @mask: the cpumask pointer
+ *
+ * After the loop, cpu is >= nr_cpu_ids.
+ */
+#define for_each_cpu_not(cpu, mask)				\
+	for ((cpu) = -1;					\
+		(cpu) = cpumask_next_zero((cpu), (mask)),	\
+		(cpu) < nr_cpu_ids;)
+
+/**
  * for_each_cpu_and - iterate over every cpu in both masks
  * @cpu: the (optionally unsigned) integer iterator
  * @mask: the first cpumask pointer