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>

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index d77b547..dbcee76 100644 (file)
--- 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

diff --git a/init/Kconfig b/init/Kconfig
index d95ca7c..42bf914 100644 (file)
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -396,6 +396,22 @@ config RCU_FANOUT_EXACT
 
          Say N if unsure.
 
+config RCU_FAST_NO_HZ
+       bool "Accelerate last non-dyntick-idle CPU's grace periods"
+       depends on TREE_RCU && NO_HZ && SMP
+       default n
+       help
+         This option causes RCU to attempt to accelerate grace periods
+         in order to allow the final CPU to enter dynticks-idle state
+         more quickly.  On the other hand, this option increases the
+         overhead of the dynticks-idle checking, particularly on systems
+         with large numbers of CPUs.
+
+         Say Y if energy efficiency is critically important, particularly
+               if you have relatively few CPUs.
+
+         Say N if you are unsure.
+
 config TREE_RCU_TRACE
        def_bool RCU_TRACE && ( TREE_RCU || TREE_PREEMPT_RCU )
        select DEBUG_FS

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 099a255..29d88c0 100644 (file)
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1550,10 +1550,9 @@ static int rcu_pending(int cpu)
 /*
  * Check to see if any future RCU-related work will need to be done
  * by the current CPU, even if none need be done immediately, returning
- * 1 if so.  This function is part of the RCU implementation; it is -not-
- * an exported member of the RCU API.
+ * 1 if so.
  */
-int rcu_needs_cpu(int cpu)
+static int rcu_needs_cpu_quick_check(int cpu)
 {
        /* RCU callbacks either ready or pending? */
        return per_cpu(rcu_sched_data, cpu).nxtlist ||

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index e77cdf3..a825666 100644 (file)
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -906,3 +906,72 @@ static void __init __rcu_init_preempt(void)
 }
 
 #endif /* #else #ifdef CONFIG_TREE_PREEMPT_RCU */
+
+#if !defined(CONFIG_RCU_FAST_NO_HZ)
+
+/*
+ * Check to see if any future RCU-related work will need to be done
+ * by the current CPU, even if none need be done immediately, returning
+ * 1 if so.  This function is part of the RCU implementation; it is -not-
+ * an exported member of the RCU API.
+ *
+ * Because we have preemptible RCU, just check whether this CPU needs
+ * any flavor of RCU.  Do not chew up lots of CPU cycles with preemption
+ * disabled in a most-likely vain attempt to cause RCU not to need this CPU.
+ */
+int rcu_needs_cpu(int cpu)
+{
+       return rcu_needs_cpu_quick_check(cpu);
+}
+
+#else /* #if !defined(CONFIG_RCU_FAST_NO_HZ) */
+
+#define RCU_NEEDS_CPU_FLUSHES 5
+
+/*
+ * Check to see if any future RCU-related work will need to be done
+ * by the current CPU, even if none need be done immediately, returning
+ * 1 if so.  This function is part of the RCU implementation; it is -not-
+ * an exported member of the RCU API.
+ *
+ * Because we are not supporting preemptible RCU, attempt to accelerate
+ * any current grace periods so that RCU no longer needs this CPU, but
+ * only if all other CPUs are already in dynticks-idle mode.  This will
+ * allow the CPU cores to be powered down immediately, as opposed to after
+ * waiting many milliseconds for grace periods to elapse.
+ */
+int rcu_needs_cpu(int cpu)
+{
+       int c = 1;
+       int i;
+       int thatcpu;
+
+       /* Don't bother unless we are the last non-dyntick-idle CPU. */
+       for_each_cpu_not(thatcpu, nohz_cpu_mask)
+               if (thatcpu != cpu)
+                       return rcu_needs_cpu_quick_check(cpu);
+
+       /* Try to push remaining RCU-sched and RCU-bh callbacks through. */
+       for (i = 0; i < RCU_NEEDS_CPU_FLUSHES && c; i++) {
+               c = 0;
+               if (per_cpu(rcu_sched_data, cpu).nxtlist) {
+                       rcu_sched_qs(cpu);
+                       force_quiescent_state(&rcu_sched_state, 0);
+                       __rcu_process_callbacks(&rcu_sched_state,
+                                               &per_cpu(rcu_sched_data, cpu));
+                       c = !!per_cpu(rcu_sched_data, cpu).nxtlist;
+               }
+               if (per_cpu(rcu_bh_data, cpu).nxtlist) {
+                       rcu_bh_qs(cpu);
+                       force_quiescent_state(&rcu_bh_state, 0);
+                       __rcu_process_callbacks(&rcu_bh_state,
+                                               &per_cpu(rcu_bh_data, cpu));
+                       c = !!per_cpu(rcu_bh_data, cpu).nxtlist;
+               }
+       }
+
+       /* If RCU callbacks are still pending, RCU still needs this CPU. */
+       return c;
+}
+
+#endif /* #else #if !defined(CONFIG_RCU_FAST_NO_HZ) */