[tip: core/rcu] rcu: React to callback overload by boosting RCU readers

[tip-bot2 for Paul E. McKenney]

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     8c14263d351b4766a040346ee917b8d81583a460
Gitweb:        https://git.kernel.org/tip/8c14263d351b4766a040346ee917b8d81583a460
Author:        Paul E. McKenney <paulmck@xxxxxxxxxx>
AuthorDate:    Thu, 07 Nov 2019 01:10:55 -08:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Thu, 20 Feb 2020 16:00:20 -08:00

rcu: React to callback overload by boosting RCU readers

RCU priority boosting currently is not applied until the grace period
is at least 250 milliseconds old (or the number of milliseconds specified
by the CONFIG_RCU_BOOST_DELAY Kconfig option).  Although this has worked
well, it can result in OOM under conditions of RCU callback flooding.
One can argue that the real-time systems using RCU priority boosting
should carefully avoid RCU callback flooding, but one can just as well
argue that an OOM is a rather obnoxious error message.

This commit therefore disables the RCU priority boosting delay when
there are excessive numbers of callbacks queued.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 kernel/rcu/tree_plugin.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 0be8fad..4d4637c 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1079,7 +1079,7 @@ static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
 	    (rnp->gp_tasks != NULL &&
 	     rnp->boost_tasks == NULL &&
 	     rnp->qsmask == 0 &&
-	     ULONG_CMP_GE(jiffies, rnp->boost_time))) {
+	     (ULONG_CMP_GE(jiffies, rnp->boost_time) || rcu_state.cbovld))) {
 		if (rnp->exp_tasks == NULL)
 			rnp->boost_tasks = rnp->gp_tasks;
 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);