Commit-ID: c3ce910b1456a45fa88959af3735bd6b285e54af
Gitweb: http://git.kernel.org/tip/c3ce910b1456a45fa88959af3735bd6b285e54af
Author: Paul E. McKenney <paul.mckenney@xxxxxxxxxx>
AuthorDate: Tue, 14 Feb 2012 10:12:54 -0800
Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
CommitDate: Tue, 21 Feb 2012 09:42:11 -0800
rcu: Eliminate softirq-mediated RCU_FAST_NO_HZ idle-entry loop
If a softirq is pending, the current CPU has RCU callbacks pending,
and RCU does not immediately need anything from this CPU, then the
current code resets the RCU_FAST_NO_HZ state machine. This means that
upon exit from the subsequent softirq handler, RCU_FAST_NO_HZ will
try really hard to force RCU into dyntick-idle mode. And if the same
conditions hold after a few tries (determined by RCU_IDLE_OPT_FLUSHES),
the same situation can repeat, possibly endlessly. This scenario is
not particularly good for battery lifetime.
This commit therefore suppresses the early exit from the RCU_FAST_NO_HZ
state machine in the case where there is a softirq pending. This change
forces the state machine to retain its memory, and to enter holdoff if
this condition persists.
Reported-by: "Abou Gazala, Neven M" <neven.m.abou.gazala@xxxxxxxxx>
Signed-off-by: Paul E. McKenney <paul.mckenney@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
kernel/rcutree_plugin.h | 3 ++-
1 files changed, 2 insertions(+), 1 deletions(-)
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index f7ceadf..392a651 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -2133,7 +2133,8 @@ static void rcu_prepare_for_idle(int cpu)
/* First time through, initialize the counter. */
per_cpu(rcu_dyntick_drain, cpu) = RCU_IDLE_FLUSHES;
} else if (per_cpu(rcu_dyntick_drain, cpu) <= RCU_IDLE_OPT_FLUSHES &&
- !rcu_pending(cpu)) {
+ !rcu_pending(cpu) &&
+ !local_softirq_pending()) {
/* Can we go dyntick-idle despite still having callbacks? */
trace_rcu_prep_idle("Dyntick with callbacks");
per_cpu(rcu_dyntick_drain, cpu) = 0;
--
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