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Subject[PATCH 4.19 104/110] rcu: Make need_resched() respond to urgent RCU-QS needs
4.19-stable review patch.  If anyone has any objections, please let me know.


From: Paul E. McKenney <>

commit 92aa39e9dc77481b90cbef25e547d66cab901496 upstream.

The per-CPU rcu_dynticks.rcu_urgent_qs variable communicates an urgent
need for an RCU quiescent state from the force-quiescent-state processing
within the grace-period kthread to context switches and to cond_resched().
Unfortunately, such urgent needs are not communicated to need_resched(),
which is sometimes used to decide when to invoke cond_resched(), for
but one example, within the KVM vcpu_run() function. As of v4.15, this
can result in synchronize_sched() being delayed by up to ten seconds,
which can be problematic, to say nothing of annoying.

This commit therefore checks rcu_dynticks.rcu_urgent_qs from within
rcu_check_callbacks(), which is invoked from the scheduling-clock
interrupt handler. If the current task is not an idle task and is
not executing in usermode, a context switch is forced, and either way,
the rcu_dynticks.rcu_urgent_qs variable is set to false. If the current
task is an idle task, then RCU's dyntick-idle code will detect the
quiescent state, so no further action is required. Similarly, if the
task is executing in usermode, other code in rcu_check_callbacks() and
its called functions will report the corresponding quiescent state.

Reported-by: Marius Hillenbrand <>
Reported-by: David Woodhouse <>
Suggested-by: Peter Zijlstra <>
Signed-off-by: Paul E. McKenney <>
[ paulmck: Backported to make patch apply cleanly on older versions. ]
Tested-by: Marius Hillenbrand <>
Cc: <> # 4.12.x - 4.19.x
Signed-off-by: Greg Kroah-Hartman <>

kernel/rcu/tree.c | 9 +++++++++
1 file changed, 9 insertions(+)

--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2662,6 +2662,15 @@ void rcu_check_callbacks(int user)
+ /* The load-acquire pairs with the store-release setting to true. */
+ if (smp_load_acquire(this_cpu_ptr(&rcu_dynticks.rcu_urgent_qs))) {
+ /* Idle and userspace execution already are quiescent states. */
+ if (!rcu_is_cpu_rrupt_from_idle() && !user) {
+ set_tsk_need_resched(current);
+ set_preempt_need_resched();
+ }
+ __this_cpu_write(rcu_dynticks.rcu_urgent_qs, false);
+ }
if (rcu_pending())

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