From 3d6ea130af2832bea1d6dac9f9709f034c43a1c9 Mon Sep 17 00:00:00 2001
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Date: Mon, 9 Jul 2018 13:47:30 -0700
Subject: [PATCH] rcu: Make need_resched() respond to urgent RCU-QS needs
References: git fixes (kernel/rcu)
Patch-mainline: v4.20
Git-commit: 92aa39e9dc77481b90cbef25e547d66cab901496
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 <mhillenb@amazon.de>
Reported-by: David Woodhouse <dwmw2@infradead.org>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
kernel/rcu/tree.c | 11 +++++++++++
1 file changed, 11 insertions(+)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index a4db6d1574f3..930caa0cc3d0 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2856,6 +2856,17 @@ void rcu_check_callbacks(int user)
{
trace_rcu_utilization(TPS("Start scheduler-tick"));
increment_cpu_stall_ticks();
+
+ /* 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 (!is_idle_task(current) && !user) {
+ set_tsk_need_resched(current);
+ set_preempt_need_resched();
+ }
+ __this_cpu_write(rcu_dynticks.rcu_urgent_qs, false);
+ }
+
if (user || rcu_is_cpu_rrupt_from_idle()) {
/*