To reduce grace periods and improve kfree() performance, we have done
batching recently dramatically bringing down the number of grace periods
while giving us the ability to use kfree_bulk() for efficient kfree'ing.
However, this has increased the likelihood of OOM condition under heavy
kfree_rcu() flood on small memory systems. This patch introduces a
shrinker which starts grace periods right away if the system is under
memory pressure due to existence of objects that have still not started
a grace period.
With this patch, I do not observe an OOM anymore on a system with 512MB
RAM and 8 CPUs, with the following rcuperf options:
rcuperf.kfree_loops=20000 rcuperf.kfree_alloc_num=8000
rcuperf.kfree_rcu_test=1 rcuperf.kfree_mult=2
NOTE:
On systems with no memory pressure, the patch has no effect as intended.
Cc: urezki@xxxxxxxxx
Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
---
kernel/rcu/tree.c | 58 +++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 58 insertions(+)
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index d91c9156fab2e..28ec35e15529d 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2723,6 +2723,8 @@ struct kfree_rcu_cpu {
struct delayed_work monitor_work;
bool monitor_todo;
bool initialized;
+ // Number of objects for which GP not started
+ int count;
};
static DEFINE_PER_CPU(struct kfree_rcu_cpu, krc);
@@ -2791,6 +2793,7 @@ static inline bool queue_kfree_rcu_work(struct kfree_rcu_cpu *krcp)
krwp->head_free = krcp->head;
krcp->head = NULL;
+ krcp->count = 0;
INIT_RCU_WORK(&krwp->rcu_work, kfree_rcu_work);
queue_rcu_work(system_wq, &krwp->rcu_work);
return true;
@@ -2864,6 +2867,7 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
head->func = func;
head->next = krcp->head;
krcp->head = head;
+ krcp->count++;
// Set timer to drain after KFREE_DRAIN_JIFFIES.
if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING &&
@@ -2879,6 +2883,58 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
}
EXPORT_SYMBOL_GPL(kfree_call_rcu);
+static unsigned long
+kfree_rcu_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
+{
+ int cpu;
+ unsigned long flags, count = 0;
+
+ /* Snapshot count of all CPUs */
+ for_each_online_cpu(cpu) {
+ struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
+
+ spin_lock_irqsave(&krcp->lock, flags);
+ count += krcp->count;
+ spin_unlock_irqrestore(&krcp->lock, flags);
+ }
+
+ return count;
+}
+
+static unsigned long
+kfree_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
+{
+ int cpu, freed = 0;
+ unsigned long flags;
+
+ for_each_online_cpu(cpu) {
+ int count;
+ struct kfree_rcu_cpu *krcp = per_cpu_ptr(&krc, cpu);
+
+ count = krcp->count;
+ spin_lock_irqsave(&krcp->lock, flags);
+ if (krcp->monitor_todo)
+ kfree_rcu_drain_unlock(krcp, flags);
+ else
+ spin_unlock_irqrestore(&krcp->lock, flags);
+
+ sc->nr_to_scan -= count;
+ freed += count;
+
+ if (sc->nr_to_scan <= 0)
+ break;
+ }
+
+ return freed;
+}
+
+static struct shrinker kfree_rcu_shrinker = {
+ .count_objects = kfree_rcu_shrink_count,
+ .scan_objects = kfree_rcu_shrink_scan,
+ .batch = 0,
+ .seeks = DEFAULT_SEEKS,
+};
+
void __init kfree_rcu_scheduler_running(void)
{
int cpu;
@@ -3774,6 +3830,8 @@ static void __init kfree_rcu_batch_init(void)
INIT_DELAYED_WORK(&krcp->monitor_work, kfree_rcu_monitor);
krcp->initialized = true;
}
+ if (register_shrinker(&kfree_rcu_shrinker))
+ pr_err("Failed to register kfree_rcu() shrinker!\n");
}
void __init rcu_init(void)
--
2.25.0.265.gbab2e86ba0-goog
