path: root/init
diff options
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 /init
parent998f2ac3fea93bfa8b55c279fff68f7c5b9ab93d (diff)
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 'init')
1 files changed, 16 insertions, 0 deletions
diff --git a/init/Kconfig b/init/Kconfig
index d95ca7cd5d4..42bf914b325 100644
--- 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.
select DEBUG_FS