cond_resched() is used to provide urgent quiescent states for
read-side critical sections on PREEMPT_RCU=n configurations.
This was necessary because lacking preempt_count, there was no
way for the tick handler to know if we were executing in RCU
read-side critical section or not.
An always-on CONFIG_PREEMPT_COUNT, however, allows the tick to
reliably report quiescent states.
Accordingly, evaluate preempt_count() based quiescence in
rcu_flavor_sched_clock_irq().
Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
---
kernel/rcu/tree_plugin.h | 3 ++-
kernel/sched/core.c | 15 +--------------
2 files changed, 3 insertions(+), 15 deletions(-)
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index f87191e008ff..618f055f8028 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -963,7 +963,8 @@ static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
*/
static void rcu_flavor_sched_clock_irq(int user)
{
- if (user || rcu_is_cpu_rrupt_from_idle()) {
+ if (user || rcu_is_cpu_rrupt_from_idle() ||
+ !(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK))) {
/*
* Get here if this CPU took its interrupt from user
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index bf5df2b866df..15db5fb7acc7 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -8588,20 +8588,7 @@ int __sched _cond_resched(void)
preempt_schedule_common();
return 1;
}
- /*
- * In preemptible kernels, ->rcu_read_lock_nesting tells the tick
- * whether the current CPU is in an RCU read-side critical section,
- * so the tick can report quiescent states even for CPUs looping
- * in kernel context. In contrast, in non-preemptible kernels,
- * RCU readers leave no in-memory hints, which means that CPU-bound
- * processes executing in kernel context might never report an
- * RCU quiescent state. Therefore, the following code causes
- * cond_resched() to report a quiescent state, but only when RCU
- * is in urgent need of one.
- */
-#ifndef CONFIG_PREEMPT_RCU
- rcu_all_qs();
-#endif
+
return 0;
}
EXPORT_SYMBOL(_cond_resched);
--
2.31.1
