[rfc] mm, thp: allow khugepaged to periodically compact memory synchronously

[David Rientjes <rientjes@google.com>]

We have seen a large benefit in the amount of hugepages that can be
allocated at fault and by khugepaged when memory is periodically
compacted in the background.

We trigger synchronous memory compaction over all memory every 15 minutes
to keep fragmentation low and to offset the lightweight compaction that
is done at page fault to keep latency low.

compact_sleep_millisecs controls how often khugepaged will compact all
memory.  Each scan_sleep_millisecs wakeup after this value has expired, a
node is synchronously compacted until all memory has been scanned.  Then,
khugepaged will restart the process compact_sleep_millisecs later.

This defaults to 0, which means no memory compaction is done.

Signed-off-by: David Rientjes <rientjes@google.com>
---
 RFC: this is for initial comment on whether it's appropriate to do this
 in khugepaged.  We already do to the background compaction for the
 benefit of thp, but others may feel like this belongs in a new per-node
 kcompactd thread as proposed by Vlastimil.

 Regardless, it appears there is a substantial need for periodic memory
 compaction in the background to reduce the latency of thp page faults
 and still have a reasonable chance of having the allocation succeed.

 We could also speed up this process in the case of alloc_sleep_millisecs
 timeout since allocation recently failed for khugepaged.

 Documentation/vm/transhuge.txt | 10 +++++++
 mm/huge_memory.c               | 65 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 75 insertions(+)

diff --git a/Documentation/vm/transhuge.txt b/Documentation/vm/transhuge.txt
--- a/Documentation/vm/transhuge.txt
+++ b/Documentation/vm/transhuge.txt
@@ -170,6 +170,16 @@ A lower value leads to gain less thp performance. Value of
 max_ptes_none can waste cpu time very little, you can
 ignore it.
 
+/sys/kernel/mm/transparent_hugepage/khugepaged/compact_sleep_millisecs
+
+controls how often khugepaged will utilize memory compaction to defragment
+memory.  This makes it easier to allocate hugepages both at page fault and
+by khugepaged since this compaction can be synchronous.
+
+This only occurs if scan_sleep_millisecs is configured.  One node per
+scan_sleep_millisecs wakeup is compacted when compact_sleep_millisecs
+expires until all memory has been compacted.
+
 == Boot parameter ==
 
 You can change the sysfs boot time defaults of Transparent Hugepage
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -23,6 +23,7 @@
 #include <linux/pagemap.h>
 #include <linux/migrate.h>
 #include <linux/hashtable.h>
+#include <linux/compaction.h>
 
 #include <asm/tlb.h>
 #include <asm/pgalloc.h>
@@ -65,6 +66,16 @@ static DECLARE_WAIT_QUEUE_HEAD(khugepaged_wait);
  */
 static unsigned int khugepaged_max_ptes_none __read_mostly = HPAGE_PMD_NR-1;
 
+/*
+ * Khugepaged may memory compaction over all memory at regular intervals.
+ * It round-robins through all nodes, compacting one at a time each
+ * scan_sleep_millisecs wakeup when triggered.
+ * May be set with compact_sleep_millisecs, which is disabled by default.
+ */
+static unsigned long khugepaged_compact_sleep_millisecs __read_mostly;
+static unsigned long khugepaged_compact_jiffies;
+static int next_compact_node = MAX_NUMNODES;
+
 static int khugepaged(void *none);
 static int khugepaged_slab_init(void);
 static void khugepaged_slab_exit(void);
@@ -463,6 +474,34 @@ static struct kobj_attribute alloc_sleep_millisecs_attr =
 	__ATTR(alloc_sleep_millisecs, 0644, alloc_sleep_millisecs_show,
 	       alloc_sleep_millisecs_store);
 
+static ssize_t compact_sleep_millisecs_show(struct kobject *kobj,
+					    struct kobj_attribute *attr,
+					    char *buf)
+{
+	return sprintf(buf, "%lu\n", khugepaged_compact_sleep_millisecs);
+}
+
+static ssize_t compact_sleep_millisecs_store(struct kobject *kobj,
+					     struct kobj_attribute *attr,
+					     const char *buf, size_t count)
+{
+	unsigned long msecs;
+	int err;
+
+	err = kstrtoul(buf, 10, &msecs);
+	if (err || msecs > ULONG_MAX)
+		return -EINVAL;
+
+	khugepaged_compact_sleep_millisecs = msecs;
+	khugepaged_compact_jiffies = jiffies + msecs_to_jiffies(msecs);
+	wake_up_interruptible(&khugepaged_wait);
+
+	return count;
+}
+static struct kobj_attribute compact_sleep_millisecs_attr =
+	__ATTR(compact_sleep_millisecs, 0644, compact_sleep_millisecs_show,
+	       compact_sleep_millisecs_store);
+
 static ssize_t pages_to_scan_show(struct kobject *kobj,
 				  struct kobj_attribute *attr,
 				  char *buf)
@@ -564,6 +603,7 @@ static struct attribute *khugepaged_attr[] = {
 	&full_scans_attr.attr,
 	&scan_sleep_millisecs_attr.attr,
 	&alloc_sleep_millisecs_attr.attr,
+	&compact_sleep_millisecs_attr.attr,
 	NULL,
 };
 
@@ -652,6 +692,10 @@ static int __init hugepage_init(void)
 		return 0;
 	}
 
+	if (khugepaged_compact_sleep_millisecs)
+		khugepaged_compact_jiffies = jiffies +
+			msecs_to_jiffies(khugepaged_compact_sleep_millisecs);
+
 	err = start_stop_khugepaged();
 	if (err)
 		goto err_khugepaged;
@@ -2834,6 +2878,26 @@ static void khugepaged_wait_work(void)
 		wait_event_freezable(khugepaged_wait, khugepaged_wait_event());
 }
 
+static void khugepaged_compact_memory(void)
+{
+	if (!khugepaged_compact_jiffies ||
+	    time_before(jiffies, khugepaged_compact_jiffies))
+		return;
+
+	get_online_mems();
+	if (next_compact_node == MAX_NUMNODES)
+		next_compact_node = first_node(node_states[N_MEMORY]);
+
+	compact_pgdat(NODE_DATA(next_compact_node), -1);
+
+	next_compact_node = next_node(next_compact_node, node_states[N_MEMORY]);
+	put_online_mems();
+
+	if (next_compact_node == MAX_NUMNODES)
+		khugepaged_compact_jiffies = jiffies +
+			msecs_to_jiffies(khugepaged_compact_sleep_millisecs);
+}
+
 static int khugepaged(void *none)
 {
 	struct mm_slot *mm_slot;
@@ -2842,6 +2906,7 @@ static int khugepaged(void *none)
 	set_user_nice(current, MAX_NICE);
 
 	while (!kthread_should_stop()) {
+		khugepaged_compact_memory();
 		khugepaged_do_scan();
 		khugepaged_wait_work();
 	}