[PATCH v2 1/1] rcu/kvfree: Update KFREE_DRAIN_JIFFIES 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 sets
a drain interval to 1 second as default one. When a flood is detected
an interval is adjusted in a way that a reclaim work is re-scheduled
on a next timer jiffy.

- default -
Total time taken by all kfree'ers: 11510245312 ns, loops: 10000, batches: 1553, memory footprint: 70MB
Total time taken by all kfree'ers: 9813329636  ns, loops: 10000, batches: 1544, memory footprint: 67MB
Total time taken by all kfree'ers: 10085206318 ns, loops: 10000, batches: 1499, memory footprint: 156MB
Total time taken by all kfree'ers: 9582207782  ns, loops: 10000, batches: 1456, memory footprint: 103MB
Total time taken by all kfree'ers: 9872195750  ns, loops: 10000, batches: 1519, memory footprint: 105MB
Total time taken by all kfree'ers: 9574438300  ns, loops: 10000, batches: 1480, memory footprint: 101MB
Total time taken by all kfree'ers: 9990015265  ns, loops: 10000, batches: 1489, memory footprint: 127MB
Total time taken by all kfree'ers: 9978971689  ns, loops: 10000, batches: 1455, memory footprint: 94MB
Total time taken by all kfree'ers: 10357628090 ns, loops: 10000, batches: 1456, memory footprint: 64MB
Total time taken by all kfree'ers: 9838469975  ns, loops: 10000, batches: 1448, memory footprint: 131MB

- patch -
Total time taken by all kfree'ers: 8488575321  ns, loops: 10000, batches: 1735, memory footprint: 82MB
Total time taken by all kfree'ers: 9256401034  ns, loops: 10000, batches: 1762, memory footprint: 76MB
Total time taken by all kfree'ers: 9198011994  ns, loops: 10000, batches: 1751, memory footprint: 82MB
Total time taken by all kfree'ers: 10590540622 ns, loops: 10000, batches: 1540, memory footprint: 94MB
Total time taken by all kfree'ers: 9953902918  ns, loops: 10000, batches: 1638, memory footprint: 89MB
Total time taken by all kfree'ers: 10176669464 ns, loops: 10000, batches: 1613, memory footprint: 84MB
Total time taken by all kfree'ers: 9387490978  ns, loops: 10000, batches: 1762, memory footprint: 85MB
Total time taken by all kfree'ers: 9530535341  ns, loops: 10000, batches: 1781, memory footprint: 66MB
Total time taken by all kfree'ers: 9945442340  ns, loops: 10000, batches: 1758, memory footprint: 68MB
Total time taken by all kfree'ers: 9188848287  ns, loops: 10000, batches: 1781, memory footprint: 67MB

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
 kernel/rcu/tree.c | 28 ++++++++++++++++++++++++----
 1 file changed, 24 insertions(+), 4 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index fd16c0b46d9e..c11670ba008e 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3249,7 +3249,7 @@ 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 (HZ)
 #define KFREE_N_BATCHES 2
 #define FREE_N_CHANNELS 2
 
@@ -3510,6 +3510,26 @@ 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 ?
+		1:KFREE_DRAIN_JIFFIES;
+
+	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 +3587,7 @@ 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 +3775,7 @@ 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 +3851,7 @@ 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