rcu/nohz: Turn off tick for offloaded CPUs
Historically, no-CBs CPUs allowed the scheduler-clock tick to be
unconditionally disabled on any transition to idle or nohz_full userspace
execution (see the rcu_needs_cpu() implementations). Unfortunately,
the checks used by rcu_needs_cpu() are defeated now that no-CBs CPUs
use ->cblist, which might make users of battery-powered devices rather
unhappy. This commit therefore adds explicit rcu_segcblist_is_offloaded()
checks to return to the historical energy-efficient semantics.
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 379cb7e..91cefa3 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1226,10 +1226,10 @@ static void rcu_prepare_kthreads(int cpu)
#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.
+ * Check to see if any future non-offloaded 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 not have RCU_FAST_NO_HZ, just check whether or not this
* CPU has RCU callbacks queued.
@@ -1237,7 +1237,8 @@ static void rcu_prepare_kthreads(int cpu)
int rcu_needs_cpu(u64 basemono, u64 *nextevt)
{
*nextevt = KTIME_MAX;
- return !rcu_segcblist_empty(&this_cpu_ptr(&rcu_data)->cblist);
+ return !rcu_segcblist_empty(&this_cpu_ptr(&rcu_data)->cblist) &&
+ !rcu_segcblist_is_offloaded(&this_cpu_ptr(&rcu_data)->cblist);
}
/*
@@ -1338,8 +1339,9 @@ int rcu_needs_cpu(u64 basemono, u64 *nextevt)
lockdep_assert_irqs_disabled();
- /* If no callbacks, RCU doesn't need the CPU. */
- if (rcu_segcblist_empty(&rdp->cblist)) {
+ /* If no non-offloaded callbacks, RCU doesn't need the CPU. */
+ if (rcu_segcblist_empty(&rdp->cblist) ||
+ rcu_segcblist_is_offloaded(&this_cpu_ptr(&rcu_data)->cblist)) {
*nextevt = KTIME_MAX;
return 0;
}
