|author||Paul E. McKenney <firstname.lastname@example.org>||2010-02-22 17:04:59 -0800|
|committer||Ingo Molnar <email@example.com>||2010-02-25 10:34:55 +0100|
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 <firstname.lastname@example.org> Cc: email@example.com Cc: firstname.lastname@example.org Cc: email@example.com Cc: firstname.lastname@example.org Cc: email@example.com Cc: firstname.lastname@example.org Cc: email@example.com Cc: firstname.lastname@example.org Cc: Valdis.Kletnieks@vt.edu Cc: email@example.com LKML-Reference: <firstname.lastname@example.org> Signed-off-by: Ingo Molnar <email@example.com>
Diffstat (limited to 'init/Kconfig')
1 files changed, 16 insertions, 0 deletions
diff --git a/init/Kconfig b/init/Kconfig
index d95ca7cd5d4..42bf914b325 100644
@@ -396,6 +396,22 @@ config RCU_FANOUT_EXACT
Say N if unsure.
+ bool "Accelerate last non-dyntick-idle CPU's grace periods"
+ depends on TREE_RCU && NO_HZ && SMP
+ default n
+ 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.
def_bool RCU_TRACE && ( TREE_RCU || TREE_PREEMPT_RCU )