[PATCH 1/2] mm/migrate: optimize hotplug-time demotion order updates

[Dave Hansen]

From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
== tl;dr ==

Automatic demotion opted for a simple, lazy approach to handling
hotplug events.  This noticeably slows down memory hotplug[1].
Optimize away updates to the demotion order when CPU and memory
hotplug events should have no effect.

== Background ==

Automatic demotion is a memory migration strategy to ensure that
new allocations have room in faster memory tiers on tiered memory
systems.  The kernel maintains an array (node_demotion[]) to
drive these migrations.

The node_demotion[] path is calculated by starting at nodes with
CPUs and then "walking" to nodes with memory.  Only hotplug
events which online or offline a node with memory (N_ONLINE) or
CPUs (N_CPU) will actually affect the migration order.

== Problem ==

However, the current code is lazy.  It completely regenerates the
migration order on *any* CPU or memory hotplug event.  The logic
was that these events are extremely rare and that the overhead
from indiscriminate order regeneration is minimal.

Part of the update logic involves a synchronize_rcu(), which is a
pretty big hammer.  Its overhead was large enough to be detected
by some 0day tests that watch memory hotplug performance[1].

== Solution ==

Add a new helper (node_demotion_topo_changed()) which can
differentiate between superfluous and impactful hotplug events.
Skip the expensive update operation for superfluous events.

== Aside: Locking ==

It took me a few moments to declare the locking to be safe enough
for node_demotion_topo_changed() to work.  It all hinges on the
memory hotplug lock:

During memory hotplug events, 'mem_hotplug_lock' is held for
write.  This ensures that two memory hotplug events can not be
called simultaneously.

CPU hotplug has a similar lock (cpuhp_state_mutex) which also
provides mutual exclusion between CPU hotplug events.  In
addition, the demotion code acquire and hold the mem_hotplug_lock
for read during its CPU hotplug handlers.  This provides mutual
exclusion between the demotion memory hotplug callbacks and the
CPU hotplug callbacks.

This effectively allows treating the migration target generation
code to act as if it is single-threaded.

1. https://lore.kernel.org/all/20210905135932.GE15026@xsang-OptiPlex-9020/

Fixes: 884a6e5d1f93 ("mm/migrate: update node demotion order on hotplug events")
Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: "Huang, Ying" <ying.huang@xxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Wei Xu <weixugc@xxxxxxxxxx>
Cc: Oscar Salvador <osalvador@xxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Greg Thelen <gthelen@xxxxxxxxxx>
Cc: Yang Shi <yang.shi@xxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

 b/mm/migrate.c |   40 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 40 insertions(+)

diff -puN mm/migrate.c~faster-node-order mm/migrate.c
--- a/mm/migrate.c~faster-node-order	2021-09-14 11:05:04.998951737 -0700
+++ b/mm/migrate.c	2021-09-14 11:05:05.002951737 -0700
@@ -3122,6 +3122,36 @@ static int establish_migrate_target(int
 }
 
 /*
+ * The node_demotion[] path is calculated by starting at
+ * nodes with CPUs and then "walking" to nodes with memory.
+ * Only hotplug events which online or offline a node with
+ * memory (N_ONLINE) or CPUs (N_CPU) will actually affect
+ * the migration order.
+ *
+ * Differentiate between hotplug events which are impactful
+ * or superfluous to node_demotion[].
+ *
+ * Must only be called once per hotplug event.  Callers
+ * must not make concurrent calls.
+ */
+static bool node_demotion_topo_changed(void)
+{
+	static int prev_topo_cpus = -1;
+	static int prev_topo_mems = -1;
+	int now_topo_cpus = num_node_state(N_CPU);
+	int now_topo_mems = num_node_state(N_ONLINE);
+
+	if ((now_topo_cpus == prev_topo_cpus) &&
+	    (now_topo_mems == prev_topo_mems))
+	   return false;
+
+	prev_topo_cpus = now_topo_cpus;
+	prev_topo_mems = now_topo_mems;
+
+	return true;
+}
+
+/*
  * When memory fills up on a node, memory contents can be
  * automatically migrated to another node instead of
  * discarded at reclaim.
@@ -3147,6 +3177,16 @@ static void __set_migration_target_nodes
 	int node;
 
 	/*
+	 * The "migration path" array is heavily optimized
+	 * for reads.  This is the write side which incurs a
+	 * very heavy synchronize_rcu().  Avoid this overhead
+	 * when nothing of consequence has changed since the
+	 * last write.
+	 */
+	if (!node_demotion_topo_changed())
+		return;
+
+	/*
 	 * Avoid any oddities like cycles that could occur
 	 * from changes in the topology.  This will leave
 	 * a momentary gap when migration is disabled.
_