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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2018-07-09 22:47:30 +0200
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2018-08-31 01:03:39 +0200
commit92aa39e9dc77481b90cbef25e547d66cab901496 (patch)
tree71b231d7fcddf06c0d6f8a4b6e8ab414ac0f0756 /kernel
parentrcu: Inline _rcu_barrier() into its sole remaining caller (diff)
downloadlinux-92aa39e9dc77481b90cbef25e547d66cab901496.tar.xz
linux-92aa39e9dc77481b90cbef25e547d66cab901496.zip
rcu: Make need_resched() respond to urgent RCU-QS needs
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>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/rcu/tree.c9
1 files changed, 9 insertions, 0 deletions
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index ce16b8da2c6f..f47ac7a4719f 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2471,6 +2471,15 @@ void rcu_check_callbacks(int user)
{
trace_rcu_utilization(TPS("Start scheduler-tick"));
raw_cpu_inc(rcu_data.ticks_this_gp);
+ /* 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);
+ }
rcu_flavor_check_callbacks(user);
if (rcu_pending())
invoke_rcu_core();