From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date: Mon, 25 Jun 2012 15:41:11 +0000 (-0700)
Subject: rcu: Allow RCU quiescent-state forcing to be preempted
X-Git-Tag: firefly_0821_release~3680^2~1841^2~2^2~15
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=b4be093fee0200789df59b6c90e2d099a20f55b3;p=firefly-linux-kernel-4.4.55.git

rcu: Allow RCU quiescent-state forcing to be preempted

RCU quiescent-state forcing is currently carried out without preemption
points, which can result in excessive latency spikes on large systems
(many hundreds or thousands of CPUs).  This patch therefore inserts
a voluntary preemption point into force_qs_rnp(), which should greatly
reduce the magnitude of these spikes.

Reported-by: Mike Galbraith <mgalbraith@suse.de>
Reported-by: Dimitri Sivanich <sivanich@sgi.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
---

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 6182686de4a6..723e2e723074 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1767,6 +1767,7 @@ static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *))
 	struct rcu_node *rnp;
 
 	rcu_for_each_leaf_node(rsp, rnp) {
+		cond_resched();
 		mask = 0;
 		raw_spin_lock_irqsave(&rnp->lock, flags);
 		if (!rcu_gp_in_progress(rsp)) {