Re: [PATCH v2 1/1] rcu/kvfree: Update KFREE_DRAIN_JIFFIES interval

[Uladzislau Rezki <urezki@gmail.com>]

On Tue, Jun 28, 2022 at 01:45:31PM +0200, Uladzislau Rezki wrote:
> > On Mon, Jun 27, 2022 at 09:53:53PM +0200, Uladzislau Rezki (Sony) wrote:
> > > 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:
> > 
> > I was looking at queuing this one, but we need a bit more data.  In
> > the meantime, here is my wordsmithing of the above paragraph:
> > 
> > Currently the monitor work is scheduled with a fixed interval of HZ/20,
> > which is roughly 50 milliseconds. The drawback of this approach is
> > low utilization of the 512 page slots in scenarios with infrequence
> > kvfree_rcu() calls.  For example on an Android system:
> > 
> Good i will update with your changes :)
> 
> > > <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.
> > 
> > And of the above paragraph:
> > 
> > Out of 512 slots, in all cases, fewer than 10 were actually used.
> > In order to improve batching and make utilization more efficient this
> > commit sets a drain interval to a fixed 1-second interval. Floods are
> > detected when a page fills quickly, and in that case, the reclaim work
> > is re-scheduled for the next scheduling-clock tick (jiffy).
> > 
> Same here, will apply this.
> 
> > ---
> > 
> > But what we need now is a trace like the one above showing higher utilization
> > of the pages.  Could you please supply this?
> > 
> Yep, i will add traces to show that utilization becomes better what is
> actually expectable.
> 
> --
> Uladzislau Rezki
Paul, see below the v3:

From ff2a402a59442ba575f13f660ee1b140d44483b4 Mon Sep 17 00:00:00 2001
From: "Uladzislau Rezki (Sony)" <urezki@gmail.com>
Date: Wed, 1 Jun 2022 11:28:53 +0200
Subject: [PATCH v3 1/1] rcu/kvfree: Update KFREE_DRAIN_JIFFIES interval

Currently the monitor work is scheduled with a fixed interval of HZ/20,
which is roughly 50 milliseconds. The drawback of this approach is
low utilization of the 512 page slots in scenarios with infrequence
kvfree_rcu() calls.  For example on an Android system:

<snip>
  kworker/3:3-507     [003] ....   470.286305: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x00000000d0f0dde5 nr_records=6
  kworker/6:1-76      [006] ....   470.416613: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x00000000ea0d6556 nr_records=1
  kworker/6:1-76      [006] ....   470.416625: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x000000003e025849 nr_records=9
  kworker/1:1-73      [001] ....   470.438895: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000091a38dc1 nr_records=1
  kworker/3:3-507     [003] ....   471.390000: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x00000000815a8713 nr_records=48
  kworker/1:1-73      [001] ....   471.725785: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x00000000fda9bf20 nr_records=3
  kworker/1:1-73      [001] ....   471.725833: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x00000000a425b67b nr_records=76
  kworker/3:3-507     [003] ....   471.958960: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x00000000dce5bdbd nr_records=78
  kworker/0:4-1411    [000] ....   472.085673: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x000000007996be9d nr_records=1
  kworker/0:4-1411    [000] ....   472.085728: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x00000000d0f0dde5 nr_records=5
  kworker/6:1-76      [006] ....   472.260340: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000065630ee4 nr_records=102
  kworker/2:1-78      [002] ....   472.527362: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x00000000815a8713 nr_records=1
<snip>

Out of 512 slots, in all cases, fewer than 10 were actually used.
In order to improve batching and make utilization more efficient this
commit sets a drain interval to a fixed 1-second interval. Floods are
detected when a page fills quickly, and in that case, the reclaim work
is re-scheduled for the next scheduling-clock tick (jiffy).

After this change:

<snip>
  kworker/0:5-1415    [000] ....   239.852727: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x000000005d2561ea nr_records=178
  kworker/3:0-38      [003] ....   239.855113: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000039c44d27 nr_records=93
  kworker/1:4-1747    [001] ....   239.872647: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x000000004abb2f7e nr_records=109
  kworker/4:59-288    [004] ....   239.884829: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000049e7a308 nr_records=182
  kworker/0:5-1415    [000] ....   240.028762: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000037eae6d6 nr_records=86
  kworker/3:0-38      [003] ....   240.040769: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x000000003be0ae55 nr_records=244
  kworker/3:0-38      [003] ....   240.068639: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000021905efe nr_records=45
        <...>-1415    [000] ....   240.868830: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x000000007c97e065 nr_records=461
        <...>-1415    [000] ....   241.068646: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x000000000725886e nr_records=25
  kworker/0:5-1415    [000] ....   241.100931: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x0000000021905efe nr_records=300
  worker/4:59-2880    [004] ....   241.801181: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x000000004abb2f7e nr_records=180
  worker/6:60-2554    [006] ....   242.000565: rcu_invoke_kfree_bulk_callback: rcu_preempt bulk=0x000000006d3084af nr_records=40
<snip>

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
 kernel/rcu/tree.c | 27 +++++++++++++++++++++++----
 1 file changed, 23 insertions(+), 4 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index fd16c0b46d9e..37f89bedf45e 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 +3510,25 @@ 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 +3586,7 @@ 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 +3774,7 @@ 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 +3850,7 @@ 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