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Subject[PATCH tip/core/rcu 07/23] rcu: Provide OOM handler to motivate lazy RCU callbacks
Date
From: "Paul E. McKenney" <paul.mckenney@linaro.org>

In kernels built with CONFIG_RCU_FAST_NO_HZ=y, CPUs can accumulate a
large number of lazy callbacks, which as the name implies will be slow
to be invoked. This can be a problem on small-memory systems, where the
default 6-second sleep for CPUs having only lazy RCU callbacks could well
be fatal. This commit therefore installs an OOM hander that ensures that
every CPU with non-lazy callbacks has at least one non-lazy callback,
in turn ensuring timely advancement for these callbacks.

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Sasha Levin <levinsasha928@gmail.com>
---
kernel/rcutree.h | 5 ++-
kernel/rcutree_plugin.h | 80 +++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 84 insertions(+), 1 deletions(-)
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 117a150..effb273 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -315,8 +315,11 @@ struct rcu_data {
unsigned long n_rp_need_fqs;
unsigned long n_rp_need_nothing;

- /* 6) _rcu_barrier() callback. */
+ /* 6) _rcu_barrier() and OOM callbacks. */
struct rcu_head barrier_head;
+#ifdef CONFIG_RCU_FAST_NO_HZ
+ struct rcu_head oom_head;
+#endif /* #ifdef CONFIG_RCU_FAST_NO_HZ */

int cpu;
struct rcu_state *rsp;
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 7f3244c..bac8cc1 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -25,6 +25,7 @@
*/

#include <linux/delay.h>
+#include <linux/oom.h>

#define RCU_KTHREAD_PRIO 1

@@ -2112,6 +2113,85 @@ static void rcu_idle_count_callbacks_posted(void)
__this_cpu_add(rcu_dynticks.nonlazy_posted, 1);
}

+/*
+ * Data for flushing lazy RCU callbacks at OOM time.
+ */
+static atomic_t oom_callback_count;
+static DECLARE_WAIT_QUEUE_HEAD(oom_callback_wq);
+
+/*
+ * RCU OOM callback -- decrement the outstanding count and deliver the
+ * wake-up if we are the last one.
+ */
+static void rcu_oom_callback(struct rcu_head *rhp)
+{
+ if (atomic_dec_and_test(&oom_callback_count))
+ wake_up(&oom_callback_wq);
+}
+
+/*
+ * Post an rcu_oom_notify callback on the current CPU if it has at
+ * least one lazy callback. This will unnecessarily post callbacks
+ * to CPUs that already have a non-lazy callback at the end of their
+ * callback list, but this is an infrequent operation, so accept some
+ * extra overhead to keep things simple.
+ */
+static void rcu_oom_notify_cpu(void *flavor)
+{
+ struct rcu_state *rsp = flavor;
+ struct rcu_data *rdp = __this_cpu_ptr(rsp->rda);
+
+ if (rdp->qlen_lazy != 0) {
+ atomic_inc(&oom_callback_count);
+ rsp->call(&rdp->oom_head, rcu_oom_callback);
+ }
+}
+
+/*
+ * If low on memory, ensure that each CPU has a non-lazy callback.
+ * This will wake up CPUs that have only lazy callbacks, in turn
+ * ensuring that they free up the corresponding memory in a timely manner.
+ */
+static int rcu_oom_notify(struct notifier_block *self,
+ unsigned long notused, void *nfreed)
+{
+ int cpu;
+ struct rcu_state *rsp;
+
+ /* Wait for callbacks from earlier instance to complete. */
+ wait_event(oom_callback_wq, atomic_read(&oom_callback_count) == 0);
+
+ /*
+ * Prevent premature wakeup: ensure that all increments happen
+ * before there is a chance of the counter reaching zero.
+ */
+ atomic_set(&oom_callback_count, 1);
+
+ get_online_cpus();
+ for_each_online_cpu(cpu)
+ for_each_rcu_flavor(rsp)
+ smp_call_function_single(cpu, rcu_oom_notify_cpu,
+ rsp, 1);
+ put_online_cpus();
+
+ /* Unconditionally decrement: no need to wake ourselves up. */
+ atomic_dec(&oom_callback_count);
+
+ *(unsigned long *)nfreed = 1;
+ return NOTIFY_OK;
+}
+
+static struct notifier_block rcu_oom_nb = {
+ .notifier_call = rcu_oom_notify
+};
+
+static int __init rcu_register_oom_notifier(void)
+{
+ register_oom_notifier(&rcu_oom_nb);
+ return 0;
+}
+early_initcall(rcu_register_oom_notifier);
+
#endif /* #else #if !defined(CONFIG_RCU_FAST_NO_HZ) */

#ifdef CONFIG_RCU_CPU_STALL_INFO
--
1.7.8


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