mm: Proactive compaction

For some applications, we need to allocate almost all memory as hugepages. However, on a running system, higher-order allocations can fail if the memory is fragmented. Linux kernel currently does on-demand compaction as we request more hugepages, but this style of compaction incurs very high latency. Experiments with one-time full memory compaction (followed by hugepage allocations) show that kernel is able to restore a highly fragmented memory state to a fairly compacted memory state within <1 sec for a 32G system. Such data suggests that a more proactive compaction can help us allocate a large fraction of memory as hugepages keeping allocation latencies low.

For a more proactive compaction, the approach taken here is to define a new tunable called 'proactiveness' which dictates bounds for external fragmentation wrt HUGETLB_PAGE_ORDER order which kcompactd tries to maintain.

The tunable is exposed through sysctl:
/proc/sys/vm/compaction_proactiveness

It takes value in range [0, 100], with a default of 20.

For some applications, we need to allocate almost all memory as hugepages. However, on a running system, higher-order allocations can fail if the memory is fragmented. Linux kernel currently does on-demand compaction as we request more hugepages, but this style of compaction incurs very high latency. Experiments with one-time full memory compaction (followed by hugepage allocations) show that kernel is able to restore a highly fragmented memory state to a fairly compacted memory state within <1 sec for a 32G system. Such data suggests that a more proactive compaction can help us allocate a large fraction of memory as hugepages keeping allocation latencies low.

For a more proactive compaction, the approach taken here is to define a new tunable called 'proactiveness' which dictates bounds for external fragmentation wrt HUGETLB_PAGE_ORDER order which kcompactd tries to

maintain.

The tunable is exposed through sysctl:
/proc/sys/vm/compaction_proactiveness

It takes value in range [0, 100], with a default of 20.

Note that a previous version of this patch [1] was found to introduce too many tunables (per-order extfrag{low, high}), but this one reduces them to just one (proactiveness). Also, the new tunable is an opaque value instead of asking for specific bounds of "external fragmentation", which would have been difficult to estimate. The internal interpretation of this opaque value allows for future fine-tuning.

Currently, we use a simple translation from this tunable to [low, high] "fragmentation score" thresholds (low=100-proactiveness, high=low+10%). The score for a node is defined as weighted mean of per-zone external fragmentation wrt HUGETLB_PAGE_ORDER order. A zone's present_pages

determines its weight.

To periodically check per-node score, we reuse per-node kcompactd threads, which are woken up every 500 milliseconds to check the same. If a node's score exceeds its high threshold (as derived from user-provided proactiveness value), proactive compaction is started until its score reaches its low threshold value. By default, proactiveness is set to 20, which implies threshold values of low=80 and high=90.

This patch is largely based on ideas from Michal Hocko posted here:
https://lore.kernel.org/linux-mm/[email protected]/

Performance data
================

System: x64_64, 1T RAM, 80 CPU threads.
Kernel: 5.6.0-rc3 + this patch

echo madvise | sudo tee /sys/kernel/mm/transparent_hugepage/enabled echo madvise | sudo tee /sys/kernel/mm/transparent_hugepage/defrag

Before starting the driver, the system was fragmented from a userspace program that allocates all memory and then for each 2M aligned section, frees 3/4 of base pages using munmap. The workload is mainly anonymous userspace pages, which are easy to move around. I intentionally avoided unmovable pages in this test to see how much latency we incur when hugepage allocations hit direct compaction.

1. Kernel hugepage allocation latencies

With the system in such a fragmented state, a kernel driver then allocates as many hugepages as possible and measures allocation latency:

(all latency values are in microseconds)

- With vanilla 5.6.0-rc3

echo 0 | sudo tee /sys/kernel/mm/compaction/node-*/proactiveness

percentile latency
–––––––––– –––––––

	   5    7894
	  10    9496
	  25   12561
	  30   15295
	  40   18244
	  50   21229
	  60   27556
	  75   30147
	  80   31047
	  90   32859
	  95   33799

Total 2M hugepages allocated = 383859 (749G worth of hugepages out of 762G total free => 98% of free memory could be allocated as hugepages)

- With 5.6.0-rc3 + this patch, with proactiveness=20

echo 20 | sudo tee /sys/kernel/mm/compaction/node-*/proactiveness

percentile latency
–––––––––– –––––––

	   5       2
	  10       2
	  25       3
	  30       3
	  40       3
	  50       4
	  60       4
	  75       4
	  80       4
	  90       5
	  95     429

Total 2M hugepages allocated = 384105 (750G worth of hugepages out of 762G total free => 98% of free memory could be allocated as hugepages)

2. JAVA heap allocation

In this test, we first fragment memory using the same method as for (1).

Then, we start a Java process with a heap size set to 700G and request the heap to be allocated with THP hugepages. We also set THP to madvise to allow hugepage backing of this heap.

/usr/bin/time
java -Xms700G -Xmx700G -XX:+UseTransparentHugePages -XX:+AlwaysPreTouch

The above command allocates 700G of Java heap using hugepages.

- With vanilla 5.6.0-rc3

17.39user 1666.48system 27:37.89elapsed

- With 5.6.0-rc3 + this patch, with proactiveness=20

8.35user 194.58system 3:19.62elapsed

Elapsed time remains around 3:15, as proactiveness is further increased.

Note that proactive compaction happens throughout the runtime of these workloads. The situation of one-time compaction, sufficient to supply hugepages for following allocation stream, can probably happen for more extreme proactiveness values, like 80 or 90.

In the above Java workload, proactiveness is set to 20. The test starts with a node's score of 80 or higher, depending on the delay between the fragmentation step and starting the benchmark, which gives more-or-less time for the initial round of compaction. As the benchmark consumes hugepages, node's score quickly rises above the high threshold (90) and proactive compaction starts again, which brings down the score to the low threshold level (80). Repeat.

bpftrace also confirms proactive compaction running 20+ times during the runtime of this Java benchmark. kcompactd threads consume 100% of one of the CPUs while it tries to bring a node's score within thresholds.

Backoff behavior
================

Above workloads produce a memory state which is easy to compact. However, if memory is filled with unmovable pages, proactive compaction should essentially back off. To test this aspect:

- Created a kernel driver that allocates almost all memory as hugepages followed by freeing first 3/4 of each hugepage.
- Set proactiveness=40
- Note that proactive_compact_node() is deferred maximum number of times with HPAGE_FRAG_CHECK_INTERVAL_MSEC of wait between each check (=> ~30 seconds between retries).

[1] https://patchwork.kernel.org/patch/11098289/

Signed-off-by Nitin Gupta <[email protected]>

To: Mel Gorman <[email protected]>
To: Michal Hocko <[email protected]>
To: Vlastimil Babka <[email protected]>
CC: Matthew Wilcox <[email protected]>
CC: Andrew Morton <[email protected]>
CC: Mike Kravetz <[email protected]>
CC: Joonsoo Kim <[email protected]>
CC: David Rientjes <[email protected]>
CC: Nitin Gupta <[email protected]>
CC: linux-kernel <[email protected]>
CC: linux-mm <[email protected]>
CC: Linux API <[email protected]>

---
Changelog v5 vs v4:
- Change tunable from sysfs to sysctl (Vlastimil)
- HUGETLB_PAGE_ORDER -> HPAGE_PMD_ORDER (Vlastimil)
- Minor cleanups (remove redundant initializations, ...)

Changelog v4 vs v3:
- Document various functions.
- Added admin-guide for the new tunable proactiveness.
- Rename proactive_compaction_score to fragmentation_score for clarity.

Changelog v3 vs v2:
- Make proactiveness a global tunable and not per-node. Also upadated the patch description to reflect the same (Vlastimil Babka).
- Don't start proactive compaction if kswapd is running (Vlastimil Babka).
- Clarified in the description that compaction runs in parallel with the workload, instead of a one-time compaction followed by a stream of hugepage allocations.

Changelog v2 vs v1:
- Introduce per-node and per-zone "proactive compaction score". This score is compared against watermarks which are set according to user provided proactiveness value.
- Separate code-paths for proactive compaction from targeted compaction i.e. where pgdat->kcompactd_max_order is non-zero.
- Renamed hpage_compaction_effort -> proactiveness. In future we may use more than extfrag wrt hugepage size to determine proactive compaction score.

---
 Documentation/admin-guide/sysctl/vm.rst |  13 ++
 include/linux/compaction.h              |   2 +
 kernel/sysctl.c                         |   9 ++
 mm/compaction.c                         | 165 +++++++++++++++++++++++-
 mm/internal.h                           |   1 +
 mm/vmstat.c                             |  17 +++
 6 files changed, 202 insertions(+), 5 deletions(-)

Documentation/admin-guide/sysctl/vm.rst +13 -0

index 0329a4d3fa9e..e5d88cabe980 100644
@@ -119,6 +119,19 @@ all zones are compacted such that free memory is available in contiguous
 blocks where possible. This can be important for example in the allocation of
 huge pages although processes will also directly compact memory as required.
 
+compaction_proactiveness
+========================
+
+This tunable takes a value in the range [0, 100] with a default value of
+20. This tunable determines how aggressively compaction is done in the
+background. Setting it to 0 disables proactive compaction.
+
+Note that compaction has a non-trivial system-wide impact as pages
+belonging to different processes are moved around, which could also lead
+to latency spikes in unsuspecting applications. The kernel employs
+various heuristics to avoid wasting CPU cycles if it detects that
+proactive compaction is not being effective.
+
 
 compact_unevictable_allowed
 ===========================

include/linux/compaction.h +2 -0

index 4b898cdbdf05..ccd28978b296 100644
@@ -85,11 +85,13 @@ static inline unsigned long compact_gap(unsigned int order)
 
 #ifdef CONFIG_COMPACTION
 extern int sysctl_compact_memory;
+extern int sysctl_compaction_proactiveness;
 extern int sysctl_compaction_handler(struct ctl_table *table, int write,
 			void __user *buffer, size_t *length, loff_t *ppos);
 extern int sysctl_extfrag_threshold;
 extern int sysctl_compact_unevictable_allowed;
 
+extern int extfrag_for_order(struct zone *zone, unsigned int order);
 extern int fragmentation_index(struct zone *zone, unsigned int order);
 extern enum compact_result try_to_compact_pages(gfp_t gfp_mask,
 		unsigned int order, unsigned int alloc_flags,

kernel/sysctl.c +9 -0

index 8a176d8727a3..51c90906efbc 100644
@@ -1458,6 +1458,15 @@ static struct ctl_table vm_table[] = {
 		.mode		= 0200,
 		.proc_handler	= sysctl_compaction_handler,
 	},
+	{
+		.procname	= "compaction_proactiveness",
+		.data		= &sysctl_compaction_proactiveness,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= &one_hundred,
+	},
 	{
 		.procname	= "extfrag_threshold",
 		.data		= &sysctl_extfrag_threshold,

mm/compaction.c +5 -0

index 46f0fcc93081..bf7f57a475ce 100644
@@ -50,6 +50,11 @@ static inline void count_compact_events(enum vm_event_item item, long delta)
 #define pageblock_start_pfn(pfn)	block_start_pfn(pfn, pageblock_order)
 #define pageblock_end_pfn(pfn)		block_end_pfn(pfn, pageblock_order)
 
+/*
+ * Fragmentation score check interval for proactive compaction purposes.
+ */
+static const int HPAGE_FRAG_CHECK_INTERVAL_MSEC = 500;
+
 static unsigned long release_freepages(struct list_head *freelist)
 {
 	struct page *page, *next;

@@ -1855,6 +1860,71 @@ static inline bool is_via_compact_memory(int order)
 	return order == -1;

mm/compaction.c +112 -0

+ * range [0, 100].
+
+ * The scaling factor ensures that proactive compaction focuses on larger
+ * zones like ZONE_NORMAL, rather than smaller, specialized zones like
+ * ZONE_DMA32. For smaller zones, the score value remains close to zero,
+ * and thus never exceeds the high threshold for proactive compaction.
+ */
+static int fragmentation_score_zone(struct zone *zone)
+{
+	unsigned long score;
+
+	score = zone->present_pages *
+			extfrag_for_order(zone, HPAGE_PMD_ORDER);
+	return div64_ul(score, zone->zone_pgdat->node_present_pages + 1);
+}
+
+/*
+ * The per-node proactive (background) compaction process is started by its
+ * corresponding kcompactd thread when the node's fragmentation score
+ * exceeds the high threshold. The compaction process remains active till
+ * the node's score falls below the low threshold, or one of the back-off
+ * conditions is met.
+ */
+static int fragmentation_score_node(pg_data_t *pgdat)
+{
+	unsigned long score = 0;
+	int zoneid;
+
+	for (zoneid = 0; zoneid < MAX_NR_ZONES; zoneid++) {
+		struct zone *zone;
+
+		zone = &pgdat->node_zones[zoneid];
+		score += fragmentation_score_zone(zone);
+	}
+
+	return score;
+}
+
+static int fragmentation_score_wmark(pg_data_t *pgdat, bool low)
+{
+	int wmark_low;
+
+	wmark_low = 100 - sysctl_compaction_proactiveness;
+	return low ? wmark_low : min(wmark_low + 10, 100);
+}
+
+static bool should_proactive_compact_node(pg_data_t *pgdat)
+{
+	int wmark_high;
+
+	if (!sysctl_compaction_proactiveness || kswapd_is_running(pgdat))
+		return false;
+
+	wmark_high = fragmentation_score_wmark(pgdat, false);
+	return fragmentation_score_node(pgdat) > wmark_high;
+}
+
 static enum compact_result __compact_finished(struct compact_control *cc)
 {
 	unsigned int order;

@@ -1881,6 +1951,25 @@ static enum compact_result __compact_finished(struct compact_control *cc)
 			return COMPACT_PARTIAL_SKIPPED;
 	}
 
+	if (cc->proactive_compaction) {
+		int score, wmark_low;
+		pg_data_t *pgdat;
+
+		pgdat = cc->zone->zone_pgdat;
+		if (kswapd_is_running(pgdat))
+			return COMPACT_PARTIAL_SKIPPED;
+
+		score = fragmentation_score_zone(cc->zone);
+		wmark_low = fragmentation_score_wmark(pgdat, true);
+
+		if (score > wmark_low)
+			ret = COMPACT_CONTINUE;
+		else
+			ret = COMPACT_SUCCESS;
+
+		goto out;
+	}
+
 	if (is_via_compact_memory(cc->order))
 		return COMPACT_CONTINUE;
 

@@ -1939,6 +2028,7 @@ static enum compact_result __compact_finished(struct compact_control *cc)
 		}
 	}
 
+out:
 	if (cc->contended || fatal_signal_pending(current))
 		ret = COMPACT_CONTENDED;
 

@@ -2412,6 +2502,41 @@ enum compact_result try_to_compact_pages(gfp_t gfp_mask, unsigned int order,
 	return rc;
 }
 
+/*
+ * Compact all zones within a node till each zone's fragmentation score
+ * reaches within proactive compaction thresholds (as determined by the
+ * proactiveness tunable).
+ *
+ * It is possible that the function returns before reaching score targets
+ * due to various back-off conditions, such as, contention on per-node or
+ * per-zone locks.
+ */
+static void proactive_compact_node(pg_data_t *pgdat)
+{
+	int zoneid;
+	struct zone *zone;
+	struct compact_control cc = {
+		.order = -1,
+		.mode = MIGRATE_SYNC_LIGHT,
+		.ignore_skip_hint = true,
+		.whole_zone = true,
+		.gfp_mask = GFP_KERNEL,
+		.proactive_compaction = true,
+	};
+
+	for (zoneid = 0; zoneid < MAX_NR_ZONES; zoneid++) {
+		zone = &pgdat->node_zones[zoneid];
+		if (!populated_zone(zone))
+			continue;
+
+		cc.zone = zone;
+
+		compact_zone(&cc, NULL);
+
+		VM_BUG_ON(!list_empty(&cc.freepages));
+		VM_BUG_ON(!list_empty(&cc.migratepages));
+	}
+}
 
 /* Compact all zones within a node */
 static void compact_node(int nid)

mm/compaction.c +2 -0

+
 /*
  * This is the entry point for compacting all nodes via
  * /proc/sys/vm/compact_memory

@@ -2637,6 +2769,7 @@ static int kcompactd(void *p)
 {
 	pg_data_t *pgdat = (pg_data_t*)p;
 	struct task_struct *tsk = current;
+	unsigned int proactive_defer = 0;
 
 	const struct cpumask *cpumask = cpumask_of_node(pgdat->node_id);
 

mm/compaction.c +24 -3

+
+			psi_memstall_enter(&pflags);
+			kcompactd_do_work(pgdat);
+			psi_memstall_leave(&pflags);
+			continue;
+		}
 
-		psi_memstall_enter(&pflags);
-		kcompactd_do_work(pgdat);
-		psi_memstall_leave(&pflags);
+		/* kcompactd wait timeout */
+		if (should_proactive_compact_node(pgdat)) {
+			unsigned int prev_score, score;
+
+			if (proactive_defer) {
+				proactive_defer--;
+				continue;
+			}
+			prev_score = fragmentation_score_node(pgdat);
+			proactive_compact_node(pgdat);
+			score = fragmentation_score_node(pgdat);
+			/*
+			 * Defer proactive compaction if the fragmentation
+			 * score did not go down i.e. no progress made.
+			 */
+			proactive_defer = score < prev_score ?
+					0 : 1 << COMPACT_MAX_DEFER_SHIFT;
+		}
 	}
 
 	return 0;

mm/internal.h +1 -0

index b5634e78f01d..9671bccd97d5 100644
@@ -228,6 +228,7 @@ struct compact_control {
 	bool no_set_skip_hint;		/* Don't mark blocks for skipping */
 	bool ignore_block_suitable;	/* Scan blocks considered unsuitable */
 	bool direct_compaction;		/* False from kcompactd or /proc/... */
+	bool proactive_compaction;	/* kcompactd proactive compaction */
 	bool whole_zone;		/* Whole zone should/has been scanned */
 	bool contended;			/* Signal lock or sched contention */
 	bool rescan;			/* Rescanning the same pageblock */

mm/vmstat.c +5 -0

index 96d21a792b57..d7ab7dbdc3a5 100644
@@ -1074,6 +1074,23 @@ static int __fragmentation_index(unsigned int order, struct contig_page_info *in
 	return 1000 - div_u64( (1000+(div_u64(info->free_pages * 1000ULL, requested))), info->free_blocks_total);
 }
 
+/*
+ * Calculates external fragmentation within a zone wrt the given order.
+ * It is defined as the percentage of pages found in blocks of size
+ * less than 1 << order. It returns values in range [0, 100].
+ */

mm/vmstat.c +2 -0

+}
+
 /* Same as __fragmentation index but allocs contig_page_info on stack */
 int fragmentation_index(struct zone *zone, unsigned int order)
 {

Quoted 11 lines · Nitin Gupta

For some applications, we need to allocate almost all memory as hugepages. However, on a running system, higher-order allocations can fail if the memory is fragmented. Linux kernel currently does on-demand compaction as we request more hugepages, but this style of compaction incurs very high latency. Experiments with one-time full memory compaction (followed by hugepage allocations) show that kernel is able to restore a highly fragmented memory state to a fairly compacted memory state within <1 sec for a 32G system. Such data suggests that a more proactive compaction can help us allocate a large fraction of memory as hugepages keeping allocation latencies low.

Quoted 3 lines

For a more proactive compaction, the approach taken here is to define a new tunable called 'proactiveness' which dictates bounds for external fragmentation wrt HUGETLB_PAGE_ORDER order which kcompactd tries to

HPAGE_PMD_ORDER

Quoted 9 lines

The tunable is exposed through sysctl:
/proc/sys/vm/compaction_proactiveness

It takes value in range [0, 100], with a default of 20.

Quoted 2 lines

This patch is largely based on ideas from Michal Hocko posted here:
https://lore.kernel.org/linux-mm/[email protected]/

Make this link a [2] reference? I would also add: "See also the LWN article [3]." where [3] is https://lwn.net/Articles/817905/

Reviewed-by Vlastimil Babka <[email protected]>

With some smaller nitpicks below.

But as we are adding a new API, I would really appreciate others comment about the approach at least.

Quoted 3 lines

+/*
+ * A zone's fragmentation score is the external fragmentation wrt to the
+ * HUGETLB_PAGE_ORDER scaled by the zone's size. It returns a value in the

HPAGE_PMD_ORDER

mm/compaction.c static void compact_nodes(void) · line 2591

Quoted 6 lines

mm/compaction.c +6 -0

+/*
+ * Tunable for proactive compaction. It determines how
+ * aggressively the kernel should compact memory in the
+ * background. It takes values in the range [0, 100].
+ */
+int sysctl_compaction_proactiveness = 20;

These are usually __read_mostly

Quoted 15 lines

mm/compaction.c +2 -0

+
 /*
  * This is the entry point for compacting all nodes via
  * /proc/sys/vm/compact_memory

@@ -2637,6 +2769,7 @@ static int kcompactd(void *p)
 {
      pg_data_t *pgdat = (pg_data_t*)p;
      struct task_struct *tsk = current;
+     unsigned int proactive_defer = 0;
 
      const struct cpumask *cpumask = cpumask_of_node(pgdat->node_id);
 

@@ -2652,12 +2785,34 @@ static int kcompactd(void *p)
              unsigned long pflags;
 

mm/compaction.c static int kcompactd(void *p) · line 2790

Quoted 6 lines

mm/compaction.c +3 -2

              trace_mm_compaction_kcompactd_sleep(pgdat->node_id);
-             wait_event_freezable(pgdat->kcompactd_wait,
-                             kcompactd_work_requested(pgdat));
+             if (wait_event_freezable_timeout(pgdat->kcompactd_wait,
+                     kcompactd_work_requested(pgdat),
+                     msecs_to_jiffies(HPAGE_FRAG_CHECK_INTERVAL_MSEC))) {

Hmm perhaps the wakeups should also backoff if there's nothing to do?

Quoted 9 lines

mm/vmstat.c +9 -0

+/*
+ * Calculates external fragmentation within a zone wrt the given order.
+ * It is defined as the percentage of pages found in blocks of size
+ * less than 1 << order. It returns values in range [0, 100].
+ */
+int extfrag_for_order(struct zone *zone, unsigned int order)
+{
+     struct contig_page_info info;
+

mm/vmstat.c static int __fragmentation_index(unsigned int order, struct contig_page_info *in · line 1091

Quoted 6 lines

mm/vmstat.c +6 -0

+     fill_contig_page_info(zone, order, &info);
+     if (info.free_pages == 0)
+             return 0;
+
+     return (info.free_pages - (info.free_blocks_suitable << order)) * 100
+                                                     / info.free_pages;

I guess this should also use div_u64() like __fragmentation_index() does.

mm/vmstat.c static int __fragmentation_index(unsigned int order, struct contig_page_info *in

Quoted 6 lines

mm/vmstat.c +2 -0

+}
+
 /* Same as __fragmentation index but allocs contig_page_info on stack */
 int fragmentation_index(struct zone *zone, unsigned int order)
 {
 

On 5/18/20 8:14 PM, Nitin Gupta wrote:
[patch v5 :)]

I've been successfully using this in my tree and it works great, but a friend who also uses my tree just found a bug (actually an improvement ;) due to the change from HUGETLB_PAGE_ORDER to HPAGE_PMD_ORDER in v5.

When building with CONFIG_TRANSPARENT_HUGEPAGE=n (for some reason it was off) HPAGE_PMD_SHIFT expands to BUILD_BUG() and compilation fails like this:

...
./include/linux/huge_mm.h:284:28: note: in expansion of macro ‘BUILD_BUG’
284 | #define HPAGE_PMD_SHIFT ({ BUILD_BUG(); 0; })

       |                            ^~~~~~~~~

./include/linux/huge_mm.h:78:26: note: in expansion of macro ‘HPAGE_PMD_SHIFT’
78 | #define HPAGE_PMD_ORDER (HPAGE_PMD_SHIFT-PAGE_SHIFT)
| ^~~~~~~~~~~~~~~
mm/compaction.c:1874:28: note: in expansion of macro ‘HPAGE_PMD_ORDER’

  1874 |    extfrag_for_order(zone, HPAGE_PMD_ORDER);
       |                            ^~~~~~~~~~~~~~~

...

It would be great if the whole thing would compile without THP; the only occurrence is in fragmentation_score_zone(). Unfortunately I'm not familiar enough with how to properly check for THP and properly calculate whatever you're doing there, otherwise I would ifdef this away myself. ;)

Thanks for an otherwise great patch!

cheers,
Holger

Quoted 8 lines · Holger Hoffstätte

On 5/18/20 8:14 PM, Nitin Gupta wrote:
[patch v5 :)]

I've been successfully using this in my tree and it works great, but a friend who also uses my tree just found a bug (actually an improvement ;) due to the change from HUGETLB_PAGE_ORDER to HPAGE_PMD_ORDER in v5.

Quoted 2 lines

When building with CONFIG_TRANSPARENT_HUGEPAGE=n (for some reason it was off) HPAGE_PMD_SHIFT expands to BUILD_BUG() and compilation fails like this:

Oops, I forgot about this. Still I believe HPAGE_PMD_ORDER is the best choice as long as THP's are enabled. I guess fallback to HUGETLB_PAGE_ORDER would be possible if THPS are not enabled, but AFAICS some architectures don't define that. Such architectures perhaps won't benefit from proactive compaction anyway?

For some applications, we need to allocate almost all memory as hugepages. However, on a running system, higher-order allocations can fail if the memory is fragmented. Linux kernel currently does on- demand
compaction as we request more hugepages, but this style of compaction incurs very high latency. Experiments with one-time full memory compaction (followed by hugepage allocations) show that kernel is able
to restore a highly fragmented memory state to a fairly compacted memory
state within <1 sec for a 32G system. Such data suggests that a more proactive compaction can help us allocate a large fraction of memory as
hugepages keeping allocation latencies low.

For a more proactive compaction, the approach taken here is to define a new tunable called 'proactiveness' which dictates bounds for external
fragmentation wrt HUGETLB_PAGE_ORDER order which kcompactd tries to maintain.

The tunable is exposed through sysctl:
/proc/sys/vm/compaction_proactiveness

It takes value in range [0, 100], with a default of 20.

Note that a previous version of this patch [1] was found to introduce too
many tunables (per-order extfrag{low, high}), but this one reduces them
to just one (proactiveness). Also, the new tunable is an opaque value instead of asking for specific bounds of "external fragmentation", which
would have been difficult to estimate. The internal interpretation of this opaque value allows for future fine-tuning.

Currently, we use a simple translation from this tunable to [low, high]
"fragmentation score" thresholds (low=100-proactiveness, high=low+10%).
The score for a node is defined as weighted mean of per-zone external fragmentation wrt HUGETLB_PAGE_ORDER order. A zone's present_pages determines its weight.

To periodically check per-node score, we reuse per-node kcompactd threads, which are woken up every 500 milliseconds to check the same. If
a node's score exceeds its high threshold (as derived from user- provided
proactiveness value), proactive compaction is started until its score reaches its low threshold value. By default, proactiveness is set to 20,
which implies threshold values of low=80 and high=90.

This patch is largely based on ideas from Michal Hocko posted here:
https://lore.kernel.org/linux-mm/[email protected]/

Performance data
================

System: x64_64, 1T RAM, 80 CPU threads.
Kernel: 5.6.0-rc3 + this patch

echo madvise | sudo tee /sys/kernel/mm/transparent_hugepage/enabled echo madvise | sudo tee /sys/kernel/mm/transparent_hugepage/defrag

Before starting the driver, the system was fragmented from a userspace
program that allocates all memory and then for each 2M aligned section,
frees 3/4 of base pages using munmap. The workload is mainly anonymous
userspace pages, which are easy to move around. I intentionally avoided
unmovable pages in this test to see how much latency we incur when hugepage allocations hit direct compaction.

1. Kernel hugepage allocation latencies

With the system in such a fragmented state, a kernel driver then allocates
as many hugepages as possible and measures allocation latency:

(all latency values are in microseconds)

- With vanilla 5.6.0-rc3

echo 0 | sudo tee /sys/kernel/mm/compaction/node-*/proactiveness

percentile latency
–––––––––– –––––––

	   5    7894
	  10    9496
	  25   12561
	  30   15295
	  40   18244
	  50   21229
	  60   27556
	  75   30147
	  80   31047
	  90   32859
	  95   33799

Total 2M hugepages allocated = 383859 (749G worth of hugepages out of 762G total free => 98% of free memory could be allocated as hugepages)

- With 5.6.0-rc3 + this patch, with proactiveness=20

echo 20 | sudo tee /sys/kernel/mm/compaction/node-*/proactiveness

percentile latency
–––––––––– –––––––

	   5       2
	  10       2
	  25       3
	  30       3
	  40       3
	  50       4
	  60       4
	  75       4
	  80       4
	  90       5
	  95     429

Total 2M hugepages allocated = 384105 (750G worth of hugepages out of 762G total free => 98% of free memory could be allocated as hugepages)

2. JAVA heap allocation

In this test, we first fragment memory using the same method as for (1).

Then, we start a Java process with a heap size set to 700G and request
the heap to be allocated with THP hugepages. We also set THP to madvise
to allow hugepage backing of this heap.

/usr/bin/time
java -Xms700G -Xmx700G -XX:+UseTransparentHugePages
-XX:+AlwaysPreTouch

The above command allocates 700G of Java heap using hugepages.

- With vanilla 5.6.0-rc3

17.39user 1666.48system 27:37.89elapsed

- With 5.6.0-rc3 + this patch, with proactiveness=20

8.35user 194.58system 3:19.62elapsed

Elapsed time remains around 3:15, as proactiveness is further increased.

Note that proactive compaction happens throughout the runtime of these
workloads. The situation of one-time compaction, sufficient to supply hugepages for following allocation stream, can probably happen for more
extreme proactiveness values, like 80 or 90.

In the above Java workload, proactiveness is set to 20. The test starts
with a node's score of 80 or higher, depending on the delay between the
fragmentation step and starting the benchmark, which gives more-or- less
time for the initial round of compaction. As the benchmark consumes hugepages, node's score quickly rises above the high threshold (90) and
proactive compaction starts again, which brings down the score to the low threshold level (80). Repeat.

bpftrace also confirms proactive compaction running 20+ times during the
runtime of this Java benchmark. kcompactd threads consume 100% of one of
the CPUs while it tries to bring a node's score within thresholds.

Backoff behavior
================

Above workloads produce a memory state which is easy to compact. However, if memory is filled with unmovable pages, proactive compaction
should essentially back off. To test this aspect:

- Created a kernel driver that allocates almost all memory as hugepages
followed by freeing first 3/4 of each hugepage.
- Set proactiveness=40
- Note that proactive_compact_node() is deferred maximum number of times
with HPAGE_FRAG_CHECK_INTERVAL_MSEC of wait between each check (=> ~30 seconds between retries).

[1] https://patchwork.kernel.org/patch/11098289/

Signed-off-by Nitin Gupta <[email protected]>

To: Mel Gorman <[email protected]>
To: Michal Hocko <[email protected]>
To: Vlastimil Babka <[email protected]>
CC: Matthew Wilcox <[email protected]>
CC: Andrew Morton <[email protected]>
CC: Mike Kravetz <[email protected]>
CC: Joonsoo Kim <[email protected]>
CC: David Rientjes <[email protected]>
CC: Nitin Gupta <[email protected]>
CC: linux-kernel <[email protected]>
CC: linux-mm <[email protected]>
CC: Linux API <[email protected]>

---
Changelog v5 vs v4:
- Change tunable from sysfs to sysctl (Vlastimil)
- HUGETLB_PAGE_ORDER -> HPAGE_PMD_ORDER (Vlastimil)
- Minor cleanups (remove redundant initializations, ...)

Changelog v4 vs v3:
- Document various functions.
- Added admin-guide for the new tunable proactiveness.
- Rename proactive_compaction_score to fragmentation_score for
clarity.

Changelog v3 vs v2:
- Make proactiveness a global tunable and not per-node. Also
upadated the
patch description to reflect the same (Vlastimil Babka).
- Don't start proactive compaction if kswapd is running (Vlastimil
Babka).
- Clarified in the description that compaction runs in parallel with the workload, instead of a one-time compaction followed by a
stream of
hugepage allocations.

Changelog v2 vs v1:
- Introduce per-node and per-zone "proactive compaction score". This score is compared against watermarks which are set according to user provided proactiveness value.
- Separate code-paths for proactive compaction from targeted
compaction
i.e. where pgdat->kcompactd_max_order is non-zero.
- Renamed hpage_compaction_effort -> proactiveness. In future we may use more than extfrag wrt hugepage size to determine proactive compaction score.

---
 Documentation/admin-guide/sysctl/vm.rst |  13 ++
 include/linux/compaction.h              |   2 +
 kernel/sysctl.c                         |   9 ++
 mm/compaction.c                         | 165
+++++++++++++++++++++++-
 mm/internal.h                           |   1 +
 mm/vmstat.c                             |  17 +++
 6 files changed, 202 insertions(+), 5 deletions(-)

Documentation/admin-guide/sysctl/vm.rst +0 -0

index 0329a4d3fa9e..e5d88cabe980 100644
@@ -119,6 +119,19 @@ all zones are compacted such that free memory is

available in contiguous
blocks where possible. This can be important for example in the
allocation of
huge pages although processes will also directly compact memory as
required.

Documentation/admin-guide/sysctl/vm.rst +4 -0

+compaction_proactiveness
+========================
+
+This tunable takes a value in the range [0, 100] with a default

value of
+20. This tunable determines how aggressively compaction is done in the

Documentation/admin-guide/sysctl/vm.rst +4 -0

+background. Setting it to 0 disables proactive compaction.
+
+Note that compaction has a non-trivial system-wide impact as pages
+belonging to different processes are moved around, which could also

lead

compact_unevictable_allowed
===========================

include/linux/compaction.h +2 -0

index 4b898cdbdf05..ccd28978b296 100644
@@ -85,11 +85,13 @@ static inline unsigned long compact_gap(unsigned
 int order)
 
 #ifdef CONFIG_COMPACTION
 extern int sysctl_compact_memory;
+extern int sysctl_compaction_proactiveness;
 extern int sysctl_compaction_handler(struct ctl_table *table, int
 write,
 			void __user *buffer, size_t *length, loff_t
 *ppos);
 extern int sysctl_extfrag_threshold;
 extern int sysctl_compact_unevictable_allowed;
 
+extern int extfrag_for_order(struct zone *zone, unsigned int order);

extern int fragmentation_index(struct zone *zone, unsigned int
order);
extern enum compact_result try_to_compact_pages(gfp_t gfp_mask,

include/linux/compaction.h +0 -0

 		unsigned int order, unsigned int alloc_flags,

kernel/sysctl.c +9 -0

index 8a176d8727a3..51c90906efbc 100644
@@ -1458,6 +1458,15 @@ static struct ctl_table vm_table[] = {
 		.mode		= 0200,
 		.proc_handler	= sysctl_compaction_handler,
 	},
+	{
+		.procname	= "compaction_proactiveness",
+		.data		= &sysctl_compaction_proactiveness,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= &one_hundred,
+	},
 	{
 		.procname	= "extfrag_threshold",
 		.data		= &sysctl_extfrag_threshold,

mm/compaction.c +2 -0

index 46f0fcc93081..bf7f57a475ce 100644
@@ -50,6 +50,11 @@ static inline void count_compact_events(enum
 vm_event_item item, long delta)
 #define pageblock_start_pfn(pfn)	block_start_pfn(pfn,
 pageblock_order)
 #define pageblock_end_pfn(pfn)		block_end_pfn(pfn,
 pageblock_order)
 
+/*
+ * Fragmentation score check interval for proactive compaction

purposes.
+ */

mm/compaction.c +9 -0

+static const int HPAGE_FRAG_CHECK_INTERVAL_MSEC = 500;
+
 static unsigned long release_freepages(struct list_head *freelist)
 {
 	struct page *page, *next;

@@ -1855,6 +1860,71 @@ static inline bool is_via_compact_memory(int
 order)
 	return order == -1;
 }
 
+static bool kswapd_is_running(pg_data_t *pgdat)
+{
+	return pgdat->kswapd && (pgdat->kswapd->state == TASK_RUNNING);
+}
+
+/*
+ * A zone's fragmentation score is the external fragmentation wrt to

the
+ * HUGETLB_PAGE_ORDER scaled by the zone's size. It returns a value in the
+ * range [0, 100].
+
+ * The scaling factor ensures that proactive compaction focuses on larger
+ * zones like ZONE_NORMAL, rather than smaller, specialized zones like
+ * ZONE_DMA32. For smaller zones, the score value remains close to zero,
+ * and thus never exceeds the high threshold for proactive compaction.
+ */

mm/compaction.c +7 -0

+static int fragmentation_score_zone(struct zone *zone)
+{
+	unsigned long score;
+
+	score = zone->present_pages *
+			extfrag_for_order(zone, HPAGE_PMD_ORDER);
+	return div64_ul(score, zone->zone_pgdat->node_present_pages +

1);

mm/compaction.c +4 -0

+}
+
+/*
+ * The per-node proactive (background) compaction process is started

by its
+ * corresponding kcompactd thread when the node's fragmentation score
+ * exceeds the high threshold. The compaction process remains active till
+ * the node's score falls below the low threshold, or one of the back-off
+ * conditions is met.
+ */

mm/compaction.c +28 -0

+static int fragmentation_score_node(pg_data_t *pgdat)
+{
+	unsigned long score = 0;
+	int zoneid;
+
+	for (zoneid = 0; zoneid < MAX_NR_ZONES; zoneid++) {
+		struct zone *zone;
+
+		zone = &pgdat->node_zones[zoneid];
+		score += fragmentation_score_zone(zone);
+	}
+
+	return score;
+}
+
+static int fragmentation_score_wmark(pg_data_t *pgdat, bool low)
+{
+	int wmark_low;
+
+	wmark_low = 100 - sysctl_compaction_proactiveness;
+	return low ? wmark_low : min(wmark_low + 10, 100);
+}
+
+static bool should_proactive_compact_node(pg_data_t *pgdat)
+{
+	int wmark_high;
+
+	if (!sysctl_compaction_proactiveness ||

kswapd_is_running(pgdat))

mm/compaction.c +6 -0

+		return false;
+
+	wmark_high = fragmentation_score_wmark(pgdat, false);
+	return fragmentation_score_node(pgdat) > wmark_high;
+}
+
 static enum compact_result __compact_finished(struct compact_control

*cc)
{

mm/compaction.c +22 -0

 	unsigned int order;

@@ -1881,6 +1951,25 @@ static enum compact_result
 __compact_finished(struct compact_control *cc)
 			return COMPACT_PARTIAL_SKIPPED;
 	}
 
+	if (cc->proactive_compaction) {
+		int score, wmark_low;
+		pg_data_t *pgdat;
+
+		pgdat = cc->zone->zone_pgdat;
+		if (kswapd_is_running(pgdat))
+			return COMPACT_PARTIAL_SKIPPED;
+
+		score = fragmentation_score_zone(cc->zone);
+		wmark_low = fragmentation_score_wmark(pgdat, true);
+
+		if (score > wmark_low)
+			ret = COMPACT_CONTINUE;
+		else
+			ret = COMPACT_SUCCESS;
+
+		goto out;
+	}
+
 	if (is_via_compact_memory(cc->order))
 		return COMPACT_CONTINUE;

@@ -1939,6 +2028,7 @@ static enum compact_result
 __compact_finished(struct compact_control *cc)
 		}
 	}
 
+out:
 	if (cc->contended || fatal_signal_pending(current))
 		ret = COMPACT_CONTENDED;

@@ -2412,6 +2502,41 @@ enum compact_result try_to_compact_pages(gfp_t
 gfp_mask, unsigned int order,
 	return rc;
 }
 
+/*
+ * Compact all zones within a node till each zone's fragmentation

score
+ * reaches within proactive compaction thresholds (as determined by the
+ * proactiveness tunable).
+ *
+ * It is possible that the function returns before reaching score targets
+ * due to various back-off conditions, such as, contention on per- node or
+ * per-zone locks.
+ */

mm/compaction.c +56 -5

+static void proactive_compact_node(pg_data_t *pgdat)
+{
+	int zoneid;
+	struct zone *zone;
+	struct compact_control cc = {
+		.order = -1,
+		.mode = MIGRATE_SYNC_LIGHT,
+		.ignore_skip_hint = true,
+		.whole_zone = true,
+		.gfp_mask = GFP_KERNEL,
+		.proactive_compaction = true,
+	};
+
+	for (zoneid = 0; zoneid < MAX_NR_ZONES; zoneid++) {
+		zone = &pgdat->node_zones[zoneid];
+		if (!populated_zone(zone))
+			continue;
+
+		cc.zone = zone;
+
+		compact_zone(&cc, NULL);
+
+		VM_BUG_ON(!list_empty(&cc.freepages));
+		VM_BUG_ON(!list_empty(&cc.migratepages));
+	}
+}
 
 /* Compact all zones within a node */
 static void compact_node(int nid)

@@ -2458,6 +2583,13 @@ static void compact_nodes(void)
 /* The written value is actually unused, all memory is compacted */
 int sysctl_compact_memory;
 
+/*
+ * Tunable for proactive compaction. It determines how
+ * aggressively the kernel should compact memory in the
+ * background. It takes values in the range [0, 100].
+ */
+int sysctl_compaction_proactiveness = 20;
+
 /*
  * This is the entry point for compacting all nodes via
  * /proc/sys/vm/compact_memory

@@ -2637,6 +2769,7 @@ static int kcompactd(void *p)
 {
 	pg_data_t *pgdat = (pg_data_t*)p;
 	struct task_struct *tsk = current;
+	unsigned int proactive_defer = 0;
 
 	const struct cpumask *cpumask = cpumask_of_node(pgdat-
 >node_id);

@@ -2652,12 +2785,34 @@ static int kcompactd(void *p)
 		unsigned long pflags;
 
 		trace_mm_compaction_kcompactd_sleep(pgdat->node_id);
-		wait_event_freezable(pgdat->kcompactd_wait,
-				kcompactd_work_requested(pgdat));
+		if (wait_event_freezable_timeout(pgdat->kcompactd_wait,
+			kcompactd_work_requested(pgdat),
+			msecs_to_jiffies(HPAGE_FRAG_CHECK_INTERVAL_MSEC
 ))) {
+
+			psi_memstall_enter(&pflags);
+			kcompactd_do_work(pgdat);
+			psi_memstall_leave(&pflags);
+			continue;
+		}
 
-		psi_memstall_enter(&pflags);
-		kcompactd_do_work(pgdat);
-		psi_memstall_leave(&pflags);
+		/* kcompactd wait timeout */
+		if (should_proactive_compact_node(pgdat)) {
+			unsigned int prev_score, score;
+
+			if (proactive_defer) {
+				proactive_defer--;
+				continue;
+			}
+			prev_score = fragmentation_score_node(pgdat);
+			proactive_compact_node(pgdat);
+			score = fragmentation_score_node(pgdat);
+			/*
+			 * Defer proactive compaction if the

fragmentation

mm/compaction.c +4 -0

+			 * score did not go down i.e. no progress made.
+			 */
+			proactive_defer = score < prev_score ?
+					0 : 1 <<

COMPACT_MAX_DEFER_SHIFT;

mm/compaction.c +1 -0

+		}
 	}
 
 	return 0;

mm/internal.h +0 -0

index b5634e78f01d..9671bccd97d5 100644
@@ -228,6 +228,7 @@ struct compact_control {
 	bool no_set_skip_hint;		/* Don't mark blocks for

skipping */
bool ignore_block_suitable; /* Scan blocks considered
unsuitable */
bool direct_compaction; /* False from kcompactd or
/proc/... */

mm/internal.h +1 -0

+	bool proactive_compaction;	/* kcompactd proactive

compaction */
bool whole_zone; /* Whole zone should/has been scanned
*/
bool contended; /* Signal lock or sched
contention */
bool rescan; /* Rescanning the same
pageblock */

mm/vmstat.c +0 -0

index 96d21a792b57..d7ab7dbdc3a5 100644
@@ -1074,6 +1074,23 @@ static int __fragmentation_index(unsigned int

order, struct contig_page_info *in
return 1000 - div_u64( (1000+(div_u64(info->free_pages *
1000ULL, requested))), info->free_blocks_total);
}

+/*
+ * Calculates external fragmentation within a zone wrt the given order.
+ * It is defined as the percentage of pages found in blocks of size
+ * less than 1 << order. It returns values in range [0, 100].
+ */

mm/vmstat.c +9 -0

+int extfrag_for_order(struct zone *zone, unsigned int order)
+{
+	struct contig_page_info info;
+
+	fill_contig_page_info(zone, order, &info);
+	if (info.free_pages == 0)
+		return 0;
+
+	return (info.free_pages - (info.free_blocks_suitable << order))

* 100

mm/vmstat.c +1 -0

+							/

info.free_pages;

mm/vmstat.c +2 -0

+}
+
 /* Same as __fragmentation index but allocs contig_page_info on

stack */
int fragmentation_index(struct zone *zone, unsigned int order) {

This looks good to me. I like the idea overall of controlling aggressiveness of compaction with a single tunable for the whole system. I wonder how an end user could arrive at what a reasonable value would be for this based upon their workload. More comments below.

Quoted 24 lines

For some applications, we need to allocate almost all memory as hugepages. However, on a running system, higher-order allocations can fail if the memory is fragmented. Linux kernel currently does on- demand
compaction as we request more hugepages, but this style of compaction incurs very high latency. Experiments with one-time full memory compaction (followed by hugepage allocations) show that kernel is able
to restore a highly fragmented memory state to a fairly compacted memory
state within <1 sec for a 32G system. Such data suggests that a more proactive compaction can help us allocate a large fraction of memory as
hugepages keeping allocation latencies low.

For a more proactive compaction, the approach taken here is to define a new tunable called 'proactiveness' which dictates bounds for external
fragmentation wrt HUGETLB_PAGE_ORDER order which kcompactd tries to maintain.

The tunable is exposed through sysctl:
/proc/sys/vm/compaction_proactiveness

Quoted 1 line

It takes value in range [0, 100], with a default of 20.

Looking at the code, setting this to 100 would mean system would continuously strive to drive level of fragmentation down to 0 which can not be reasonable and would bog the system down. A cap lower than 100 might be a good idea to keep kcompactd from dragging system down.

Quoted 6 lines

Total 2M hugepages allocated = 383859 (749G worth of hugepages out of 762G total free => 98% of free memory could be allocated as hugepages)

Quoted 3 lines

- With 5.6.0-rc3 + this patch, with proactiveness=20

echo 20 | sudo tee /sys/kernel/mm/compaction/node-*/proactiveness

Should be "echo 20 | sudo tee /proc/sys/vm/compaction_proactiveness"

Quoted 26 lines

Documentation/admin-guide/sysctl/vm.rst +0 -0

index 0329a4d3fa9e..e5d88cabe980 100644
@@ -119,6 +119,19 @@ all zones are compacted such that free memory is

available in contiguous
blocks where possible. This can be important for example in the
allocation of
huge pages although processes will also directly compact memory as
required.

Documentation/admin-guide/sysctl/vm.rst +4 -0

+compaction_proactiveness
+========================
+
+This tunable takes a value in the range [0, 100] with a default

value of
+20. This tunable determines how aggressively compaction is done in the

Documentation/admin-guide/sysctl/vm.rst +4 -0

+background. Setting it to 0 disables proactive compaction.
+
+Note that compaction has a non-trivial system-wide impact as pages
+belonging to different processes are moved around, which could also

lead

Documentation/admin-guide/sysctl/vm.rst all zones are compacted such that free memory is available in contiguous · line 134

Quoted 4 lines

Documentation/admin-guide/sysctl/vm.rst +4 -0

+to latency spikes in unsuspecting applications. The kernel employs
+various heuristics to avoid wasting CPU cycles if it detects that
+proactive compaction is not being effective.
+

Value of 100 would cause kcompactd to try to bring fragmentation down to 0. If hugepages are being consumed and released continuously by the workload, it is possible that kcompactd keeps making progress (and hence passes the test "proactive_defer = score < prev_score ?") continuously but can not reach a fragmentation score of 0 and hence gets stuck in compact_zone() for a long time. Page migration for compaction is not inexpensive. Maybe either cap the value to something less than 100 or set a floor for wmark_low above 0.

Some more guidance regarding the value for this tunable might be helpful here, something along the lines of what does a value of 100 mean in terms of how kcompactd will behave. It can then give end user a better idea of what they are getting at what cost. You touch upon the cost above. Just add some more details so an end user can get a better idea of size of the cost for higher values of this tunable.