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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2010-02-22 17:04:59 -0800
committerIngo Molnar <mingo@elte.hu>2010-02-25 10:34:55 +0100
commit8bd93a2c5d4cab2ae17d06350daa7dbf546a4634 (patch)
tree3facbdbfbcc1b169fad20f456b0a2521adadfb25 /kernel/rcutree_plugin.h
parent998f2ac3fea93bfa8b55c279fff68f7c5b9ab93d (diff)
downloadlwn-8bd93a2c5d4cab2ae17d06350daa7dbf546a4634.tar.gz
lwn-8bd93a2c5d4cab2ae17d06350daa7dbf546a4634.zip
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>
Diffstat (limited to 'kernel/rcutree_plugin.h')
-rw-r--r--kernel/rcutree_plugin.h69
1 files changed, 69 insertions, 0 deletions
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index e77cdf321e13..a82566696b0b 100644
--- 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) */