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rcu/nohz: Turn off tick for offloaded CPUs

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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>
This commit is contained in:
Paul E. McKenney
2019-08-12 10:28:08 -07:00
parent 969974e5c5
commit 0bd55c6936
1 changed files with 9 additions and 7 deletions
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16
kernel/rcu/tree_plugin.h
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@@ -1226,10 +1226,10 @@ static void rcu_prepare_kthreads(int cpu)
#if !defined(CONFIG_RCU_FAST_NO_HZ) #if !defined(CONFIG_RCU_FAST_NO_HZ)
/* /*
* Check to see if any future RCU-related work will need to be done * Check to see if any future non-offloaded RCU-related work will need
* by the current CPU, even if none need be done immediately, returning * to be done by the current CPU, even if none need be done immediately,
* 1 if so. This function is part of the RCU implementation; it is -not- * returning 1 if so. This function is part of the RCU implementation;
* an exported member of the RCU API. * it is -not- an exported member of the RCU API.
* *
* Because we not have RCU_FAST_NO_HZ, just check whether or not this * Because we not have RCU_FAST_NO_HZ, just check whether or not this
* CPU has RCU callbacks queued. * CPU has RCU callbacks queued.
@@ -1237,7 +1237,8 @@ static void rcu_prepare_kthreads(int cpu)
int rcu_needs_cpu(u64 basemono, u64 *nextevt) int rcu_needs_cpu(u64 basemono, u64 *nextevt)
{ {
*nextevt = KTIME_MAX; *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(); lockdep_assert_irqs_disabled();
/* If no callbacks, RCU doesn't need the CPU. */ /* If no non-offloaded callbacks, RCU doesn't need the CPU. */
if (rcu_segcblist_empty(&rdp->cblist)) { if (rcu_segcblist_empty(&rdp->cblist) ||
rcu_segcblist_is_offloaded(&this_cpu_ptr(&rcu_data)->cblist)) {
*nextevt = KTIME_MAX; *nextevt = KTIME_MAX;
return 0; return 0;
} }