mm: page_alloc: defrag_mode

The page allocator groups requests by migratetype to stave off
fragmentation. However, in practice this is routinely defeated by the
fact that it gives up *before* invoking reclaim and compaction - which may
well produce suitable pages. As a result, fragmentation of physical
memory is a common ongoing process in many load scenarios.

Fragmentation deteriorates compaction's ability to produce huge pages.
Depending on the lifetime of the fragmenting allocations, those effects
can be long-lasting or even permanent, requiring drastic measures like
forcible idle states or even reboots as the only reliable ways to recover
the address space for THP production.

In a kernel build test with supplemental THP pressure, the THP allocation
rate steadily declines over 15 runs:

thp_fault_alloc
61988
56474
57258
50187
52388
55409
52925
47648
43669
40621
36077
41721
36685
34641
33215

This is a hurdle in adopting THP in any environment where hosts are shared
between multiple overlapping workloads (cloud environments), and rarely
experience true idle periods. To make THP a reliable and predictable
optimization, there needs to be a stronger guarantee to avoid such
fragmentation.

Introduce defrag_mode. When enabled, reclaim/compaction is invoked to its
full extent *before* falling back. Specifically, ALLOC_NOFRAGMENT is
enforced on the allocator fastpath and the reclaiming slowpath.

For now, fallbacks are permitted to avert OOMs. There is a plan to add
defrag_mode=2 to prefer OOMs over fragmentation, but this requires
additional prep work in compaction and the reserve management to make it
ready for all possible allocation contexts.

The following test results are from a kernel build with periodic bursts of
THP allocations, over 15 runs:

vanilla defrag_mode=1
@claimer[unmovable]: 189 103
@claimer[movable]: 92 103
@claimer[reclaimable]: 207 61
@pollute[unmovable from movable]: 25 0
@pollute[unmovable from reclaimable]: 28 0
@pollute[movable from unmovable]: 38835 0
@pollute[movable from reclaimable]: 147136 0
@pollute[reclaimable from unmovable]: 178 0
@pollute[reclaimable from movable]: 33 0
@steal[unmovable from movable]: 11 0
@steal[unmovable from reclaimable]: 5 0
@steal[reclaimable from unmovable]: 107 0
@steal[reclaimable from movable]: 90 0
@steal[movable from reclaimable]: 354 0
@steal[movable from unmovable]: 130 0

Both types of polluting fallbacks are eliminated in this workload.

Interestingly, whole block conversions are reduced as well. This is
because once a block is claimed for a type, its empty space remains
available for future allocations, instead of being padded with fallbacks;
this allows the native type to group up instead of spreading out to new
blocks. The assumption in the allocator has been that pollution from
movable allocations is less harmful than from other types, since they can
be reclaimed or migrated out should the space be needed. However, since
fallbacks occur *before* reclaim/compaction is invoked, movable pollution
will still cause non-movable allocations to spread out and claim more
blocks.

Without fragmentation, THP rates hold steady with defrag_mode=1:

thp_fault_alloc
32478
20725
45045
32130
14018
21711
40791
29134
34458
45381
28305
17265
22584
28454
30850

While the downward trend is eliminated, the keen reader will of course
notice that the baseline rate is much smaller than the vanilla kernel's to
begin with. This is due to deficiencies in how reclaim and compaction are
currently driven: ALLOC_NOFRAGMENT increases the extent to which smaller
allocations are competing with THPs for pageblocks, while making no effort
themselves to reclaim or compact beyond their own request size. This
effect already exists with the current usage of ALLOC_NOFRAGMENT, but is
amplified by defrag_mode insisting on whole block stealing much more
strongly.

Subsequent patches will address defrag_mode reclaim strategy to raise the
THP success baseline above the vanilla kernel.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Johannes Weiner <[e-mail]>
Cc: Mel Gorman <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Cc: Zi Yan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

Documentation/admin-guide/sysctl/vm.rst

···
 - compact_memory
 - compaction_proactiveness
 - compact_unevictable_allowed
+- defrag_mode
 - dirty_background_bytes
 - dirty_background_ratio
 - dirty_bytes
···
 to compaction, which would block the task from becoming active until the fault
 is resolved.
 
+defrag_mode
+===========
+
+When set to 1, the page allocator tries harder to avoid fragmentation
+and maintain the ability to produce huge pages / higher-order pages.
+
+It is recommended to enable this right after boot, as fragmentation,
+once it occurred, can be long-lasting or even permanent.
 
 dirty_background_bytes
 ======================

mm/page_alloc.c

···
 int user_min_free_kbytes = -1;
 static int watermark_boost_factor __read_mostly = 15000;
 static int watermark_scale_factor = 10;
+static int defrag_mode;
 
 /* movable_zone is the "real" zone pages in ZONE_MOVABLE are taken from */
 int movable_zone;
···
 	 */
 	alloc_flags = (__force int) (gfp_mask & __GFP_KSWAPD_RECLAIM);
 
+	if (defrag_mode) {
+		alloc_flags |= ALLOC_NOFRAGMENT;
+		return alloc_flags;
+	}
+
 #ifdef CONFIG_ZONE_DMA32
 	if (!zone)
 		return alloc_flags;
···
 				continue;
 		}
 
-		if (no_fallback && nr_online_nodes > 1 &&
+		if (no_fallback && !defrag_mode && nr_online_nodes > 1 &&
 		    zone != zonelist_zone(ac->preferred_zoneref)) {
 			int local_nid;
 
···
 	 * It's possible on a UMA machine to get through all zones that are
 	 * fragmented. If avoiding fragmentation, reset and try again.
 	 */
-	if (no_fallback) {
+	if (no_fallback && !defrag_mode) {
 		alloc_flags &= ~ALLOC_NOFRAGMENT;
 		goto retry;
 	}
···
 
 	alloc_flags = gfp_to_alloc_flags_cma(gfp_mask, alloc_flags);
 
+	if (defrag_mode)
+		alloc_flags |= ALLOC_NOFRAGMENT;
+
 	return alloc_flags;
 }
 
···
 				&compaction_retries))
 		goto retry;
 
+	/* Reclaim/compaction failed to prevent the fallback */
+	if (defrag_mode) {
+		alloc_flags &= ALLOC_NOFRAGMENT;
+		goto retry;
+	}
 
 	/*
 	 * Deal with possible cpuset update races or zonelist updates to avoid
···
 		.proc_handler	= watermark_scale_factor_sysctl_handler,
 		.extra1		= SYSCTL_ONE,
 		.extra2		= SYSCTL_THREE_THOUSAND,
+	},
+	{
+		.procname	= "defrag_mode",
+		.data		= &defrag_mode,
+		.maxlen		= sizeof(defrag_mode),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= SYSCTL_ZERO,
+		.extra2		= SYSCTL_ONE,
 	},
 	{
 		.procname	= "percpu_pagelist_high_fraction",