[PATCH 2/2] rcu/kvfree: Introduce KFREE_DRAIN_JIFFIES_[MAX/MIN] interval

["Uladzislau Rezki (Sony)" <urezki@gmail.com>]

Currently the monitor work is scheduled with a fixed interval that
is HZ/20 or each 50 milliseconds. The drawback of such approach is
a low utilization of page slot in some scenarios. The page can store
up to 512 records. For example on Android system it can look like:

<snip>
  kworker/3:0-13872   [003] .... 11286.007048: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000026522604 nr_records=1
  kworker/3:0-13872   [003] .... 11286.015638: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000095ed6fca nr_records=2
  kworker/1:2-20434   [001] .... 11286.051230: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000044872ffd nr_records=1
  kworker/1:2-20434   [001] .... 11286.059322: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000026522604 nr_records=2
  kworker/0:1-20052   [000] .... 11286.095295: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000044872ffd nr_records=2
  kworker/0:1-20052   [000] .... 11286.103418: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x00000000cbcf05db nr_records=1
  kworker/2:3-14372   [002] .... 11286.135155: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000095ed6fca nr_records=2
  kworker/2:3-14372   [002] .... 11286.135198: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000044872ffd nr_records=1
  kworker/1:2-20434   [001] .... 11286.155377: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x00000000cbcf05db nr_records=5
  kworker/2:3-14372   [002] .... 11286.167181: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000026522604 nr_records=5
  kworker/1:2-20434   [001] .... 11286.179202: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x000000008ef95e14 nr_records=1
  kworker/2:3-14372   [002] .... 11286.187398: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x00000000c597d297 nr_records=6
  kworker/3:0-13872   [003] .... 11286.187445: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000050bf92e2 nr_records=3
  kworker/1:2-20434   [001] .... 11286.198975: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x00000000cbcf05db nr_records=4
  kworker/1:2-20434   [001] .... 11286.207203: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000095ed6fca nr_records=4
<snip>

where a page only carries few records to reclaim a memory. In order to
improve batching and make utilization more efficient the patch introduces
a drain interval that can be set either to KFREE_DRAIN_JIFFIES_MAX or
KFREE_DRAIN_JIFFIES_MIN. It is adjusted if a flood is detected, in this
case a memory reclaim occurs more often whereas in mostly idle cases the
interval is set to its maximum timeout that improves the utilization of
page slots.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
 kernel/rcu/tree.c | 29 +++++++++++++++++++++++++----
 1 file changed, 25 insertions(+), 4 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index fd16c0b46d9e..c02a64995b85 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3249,7 +3249,8 @@ EXPORT_SYMBOL_GPL(call_rcu);
 
 
 /* Maximum number of jiffies to wait before draining a batch. */
-#define KFREE_DRAIN_JIFFIES (HZ / 50)
+#define KFREE_DRAIN_JIFFIES_MAX (HZ)
+#define KFREE_DRAIN_JIFFIES_MIN (HZ / 50)
 #define KFREE_N_BATCHES 2
 #define FREE_N_CHANNELS 2
 
@@ -3510,6 +3511,26 @@ need_offload_krc(struct kfree_rcu_cpu *krcp)
 	return !!krcp->head;
 }
 
+static void
+schedule_delayed_monitor_work(struct kfree_rcu_cpu *krcp)
+{
+	long delay, delay_left;
+
+	delay = READ_ONCE(krcp->count) >= KVFREE_BULK_MAX_ENTR ?
+		KFREE_DRAIN_JIFFIES_MIN:KFREE_DRAIN_JIFFIES_MAX;
+
+	if (delayed_work_pending(&krcp->monitor_work)) {
+		delay_left = krcp->monitor_work.timer.expires - jiffies;
+
+		if (delay < delay_left)
+			mod_delayed_work(system_wq, &krcp->monitor_work, delay);
+
+		return;
+	}
+
+	queue_delayed_work(system_wq, &krcp->monitor_work, delay);
+}
+
 /*
  * This function is invoked after the KFREE_DRAIN_JIFFIES timeout.
  */
@@ -3567,7 +3588,7 @@ static void kfree_rcu_monitor(struct work_struct *work)
 	// work to repeat an attempt. Because previous batches are
 	// still in progress.
 	if (need_offload_krc(krcp))
-		schedule_delayed_work(&krcp->monitor_work, KFREE_DRAIN_JIFFIES);
+		schedule_delayed_monitor_work(krcp);
 
 	raw_spin_unlock_irqrestore(&krcp->lock, flags);
 }
@@ -3755,7 +3776,7 @@ void kvfree_call_rcu(struct rcu_head *head, rcu_callback_t func)
 
 	// Set timer to drain after KFREE_DRAIN_JIFFIES.
 	if (rcu_scheduler_active == RCU_SCHEDULER_RUNNING)
-		schedule_delayed_work(&krcp->monitor_work, KFREE_DRAIN_JIFFIES);
+		schedule_delayed_monitor_work(krcp);
 
 unlock_return:
 	krc_this_cpu_unlock(krcp, flags);
@@ -3831,7 +3852,7 @@ void __init kfree_rcu_scheduler_running(void)
 
 		raw_spin_lock_irqsave(&krcp->lock, flags);
 		if (need_offload_krc(krcp))
-			schedule_delayed_work_on(cpu, &krcp->monitor_work, KFREE_DRAIN_JIFFIES);
+			schedule_delayed_monitor_work(krcp);
 		raw_spin_unlock_irqrestore(&krcp->lock, flags);
 	}
 }
-- 
2.30.2