mm/damon: introduce DAMOS_QUOTA_HUGEPAGE auto tuning

Patch series "mm/damon: Introduce a huge page collapsing mechanism using
auto tuning", v4.

Overview
========
This patchset introduces a new autotuning which allows to collapse hot
regions into hugepages.

Motivation
==========
Since TLB is a bottleneck for many systems[1], a way to optimize TLB
misses (or hits) is to use huge pages.  Unfortunately, using "always" in
THP leads to memory fragmentation and memory waste.  For this reason, most
application guides and system administrators suggest to disable THP.

Selective huge page collapse per process is possible using prctl and a
launcher.  However, this does not solve the issue with hot region
detection.  Additionally, it the sysadmin should create a launcher that
uses PRCTL to enable THP for a particular process.

We can use the DAMON support for DAMOS_HUGEPAGE and DAMOS_COLLAPSE, to
target a certain process.  DAMOS_COLLAPSE can also target the hot regions
in that process.

Still, there is an issue with the amount of huge page consumption.  Since
huge pages can lead to memory fragmentation and waste, there should be a
way to limit the amount of huge page consumption.  There is hugetlbfs, but
it requires changes to the application code or the use of libhugetlbfs.

DAMON has now a way to autotune some of the variables and adjust quotas
automatically, so that DAMON is fired only under the right circumstances. 
It would be nice to have something similar, but for huge pages.

Solution
========
A new autotuning quota goal[2], damos_hugepage_mem_bp, is introduced,
which checks the huge page consumption to total memory consumption.  This
new quota mechanism reuses current autotuning architecture.

In order to test this new mechanism, a sample module[3] was created, but
not included in this patch series.  To demonstrate the tool, damo user
space tool was modified[4], which sets up huge pages collapse autotuning.

Benchmarks
==========
Setup: physical server with arm64 processor with 4 NUMA nodes, 1 TB RAM
and running mariaDB 10.5.29.  Sysbench was used for the benchmark, with 20
tables and 3 million rows per table.  The database was pinned to one of
the nodes, and the benchmark framework to a different node.  No network
traffic involved in the benchmark.

Damo user space tool was forked and hugepage_mem_bp support added[4].

DAMON was lauched using this command line:

sudo ./damo start $(pidof mariadbd) \
--monitoring_nr_regions_range 10 1000 \
--monitoring_intervals 5000 100000 60000000 \
--damos_quota_time 0 --damos_quota_space 128000000 \
--damos_quota_interval 1000 \
--damos_quota_weights 0 1 1 \
--damos_quota_goal hugepage_mem_bp <target> \
--damos_quota_goal_tuner temporal \
--damos_apply_interval 50000 \
--damos_access_rate 0 max --damos_age 50 max \
--damos_action collapse --debug_damon

<target> was 1000 to taget 10% hugepage to total memory ratio, or 2500 to
target 25%.  Tuner was also tested with consistent and temporal.

Results
=======
After the last timestamp, there was no change in huge page use, and the
total huge page to memory consumption ratio barely moved.

hugepage_mem_bp: 1000
goal tuner: temporal

+-----------+----------------+----------------+----------------------+
| timestamp | total mem used | huge page used | percentage hugepage  |
+-----------+----------------+----------------+----------------------+
| 0         | 16945.04297    | 0              | 0                    |
| 7         | 17008.69531    | 74             | 0.435071583          |
| 8         | 17036.40234    | 194            | 1.138738074          |
| 9         | 17017.01563    | 314            | 1.845211916          |
| 10        | 17029.67969    | 434            | 2.548491856          |
| 61        | 17111.30859    | 584            | 3.412947623          |
| 120       | 17071.05859    | 694            | 4.065360072          |
| 180       | 17133.88281    | 804            | 4.692456513          |
| 203       | 17088.16406    | 916            | 5.360435426          |
| 204       | 17126.34766    | 1046           | 6.107548562          |
| 205       | 17093.84375    | 1176           | 6.879669764          |
| 206       | 17142.77734    | 1298           | 7.571701913          |
| 209       | 17149.17969    | 1686           | 9.831374041          |
| 210       | 17097.30859    | 1754           | 10.25892462          |
+-----------+----------------+----------------+----------------------+

hugepage_mem_bp: 1000
goal tuner: consistent

+-----------+----------------+----------------+----------------------+
| timestamp | total mem used | huge page used | percentage hugepage  |
+-----------+----------------+----------------+----------------------+
| 0         | 16955.24609    | 0              | 0                    |
| 34        | 17039.71875    | 106            | 0.622075995          |
| 78        | 17009.47656    | 554            | 3.257007927          |
| 90        | 17048.92188    | 596            | 3.495822225          |
| 150       | 17092.90625    | 706            | 4.130368409          |
| 180       | 17053.08984    | 764            | 4.480126517          |
| 233       | 17100.50391    | 1496           | 8.748280216          |
| 239       | 17098.89063    | 2216           | 12.95990511          |
| 240       | 17135.44531    | 2334           | 13.62088908          |
| 245       | 17132.55078    | 2932           | 17.11362212          |
| 246       | 17117.95313    | 3052           | 17.82923448          |
| 250       | 17163.12109    | 3532           | 20.57900763          |
+-----------+----------------+----------------+----------------------+

hugepage_mem_bp: 2500
goal tuner: temporal

+-----------+----------------+----------------+----------------------+
| timestamp | total mem used | huge page used | percentage hugepage  |
+-----------+----------------+----------------+----------------------+
| 0         | 17010.31641    | 0              | 0                    |
| 9         | 17063.6875     | 50             | 0.2930199            |
| 10        | 17051.75781    | 170            | 0.996964664          |
| 60        | 17133.85547    | 572            | 3.338419663          |
| 90        | 17192.07813    | 626            | 3.641211932          |
| 120       | 17221.44531    | 682            | 3.960178647          |
| 181       | 17199.76172    | 790            | 4.593086886          |
| 208       | 17222.77734    | 1206           | 7.002354939          |
| 214       | 17245.17969    | 1904           | 11.04076637          |
| 215       | 17240.45703    | 2024           | 11.73982799          |
| 220       | 17234.79688    | 2624           | 15.22501262          |
| 228       | 17222.83594    | 3584           | 20.80958103          |
| 231       | 17247.55469    | 3944           | 22.86700968          |
| 235       | 17229.37109    | 4424           | 25.67708349          |
+-----------+----------------+----------------+----------------------+

hugepage_mem_bp: 1000
goal tuner: consist

+-----------+----------------+----------------+----------------------+
| timestamp | total mem used | huge page used | percentage hugepage  |
+-----------+----------------+----------------+----------------------+
| 0         | 17125.85156    | 0              | 0                    |
| 38        | 17081.23438    | 76             | 0.444932716          |
| 39        | 17133.11719    | 196            | 1.143983304          |
| 40        | 17119.83984    | 316            | 1.84581166           |
| 60        | 17109.72656    | 554            | 3.237924335          |
| 90        | 17164.11328    | 628            | 3.65879664           |
| 180       | 17177.66016    | 792            | 4.610639591          |
| 220       | 17180.86719    | 1378           | 8.020549749          |
| 226       | 17187.82031    | 1980           | 11.51978531          |
| 233       | 17143.48438    | 2818           | 16.4377319           |
| 240       | 17137.38281    | 3656           | 21.33347921          |
| 250       | 17175.5        | 4856           | 28.27283049          |
| 260       | 17199.66406    | 6056           | 35.20999002          |
| 270       | 17203.98438    | 7254           | 42.16465118          |
| 275       | 17207.21875    | 7762           | 45.10897498          |
+-----------+----------------+----------------+----------------------+

More detailed tables are provided here[5]

From this, we can conclude that the huge page autotuner works fine,
achieving the target.  When using consistent autotuner, it actually
over-achieves the target, which is expected, since quota esz_bp is not set
to 0 to cap the DAMOS policy.

Patches Sequence
================
Patch 1 -> Introduce DAMOS_QUOTA_HUGEPAGE_MEM_BP and autotuning
Patch 2 -> sysfs support for the new quota goal
Patch 3 -> Document hugepage_mem_bp parameter


This patch (of 3):

Introduce DAMOS_QUOTA_HUGEPAGE_MEM_BP auto tuning.  Add a new DAMOS quota
goal metric to measure the amount of huge page consumption to total memory
consumption ratio.

Vmstat may lag, which in some cases may lead to NR_FREE_PAGES being
greater than or equal to the amount of RAM in the system.  A guard is
added to avoid the extremely unlikely case [6].  In the case, return 100%
(10000 bp).

Link: https://lore.kernel.org/20260831144732.80910-1-sj@kernel.org
Link: https://lore.kernel.org/20260831144732.80910-2-sj@kernel.org
Link: https://dl.acm.org/doi/pdf/10.1145/3307650.3322227 [1]
Link: https://lore.kernel.org/e67f05ad-dbb9-45e6-ba30-b167a99ac67d@huawei-partners.com [2]
Link: https://lore.kernel.org/20260616150316.580819-3-gutierrez.asier@huawei-partners.com [3]
Link: https://github.com/asierHuawei/damo/commit/79ae1a4ab1c012a7161db85a000d14f08fa36736 [4]
Link: https://lore.kernel.org/all/03f678dd-9ef3-4b97-b753-c2e4554c5159@huawei-partners.com/ [5]
Link: https://lore.kernel.org/all/20260715151615.99767-1-sj@kernel.org/ [6]
Signed-off-by: Asier Gutierrez <gutierrez.asier@huawei-partners.com>
Signed-off-by: SJ Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: SJ Park <sj@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Lorenzo Stoakes <ljs@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/linux/damon.h b/include/linux/damon.h
index 6993dca0f355..955b9f614e5b 100644
--- a/include/linux/damon.h
+++ b/include/linux/damon.h
@@ -153,6 +153,7 @@ enum damos_action {
  * @DAMOS_QUOTA_INACTIVE_MEM_BP:	Inactive to total LRU memory ratio.
  * @DAMOS_QUOTA_NODE_ELIGIBLE_MEM_BP:	Scheme-eligible memory ratio of a
  *					node in basis points (0-10000).
+ * @DAMOS_QUOTA_HUGEPAGE_MEM_BP:	Huge page to total used memory ratio.
  * @NR_DAMOS_QUOTA_GOAL_METRICS:	Number of DAMOS quota goal metrics.
  *
  * Metrics equal to larger than @NR_DAMOS_QUOTA_GOAL_METRICS are unsupported.
@@ -167,6 +168,7 @@ enum damos_quota_goal_metric {
 	DAMOS_QUOTA_ACTIVE_MEM_BP,
 	DAMOS_QUOTA_INACTIVE_MEM_BP,
 	DAMOS_QUOTA_NODE_ELIGIBLE_MEM_BP,
+	DAMOS_QUOTA_HUGEPAGE_MEM_BP,
 	NR_DAMOS_QUOTA_GOAL_METRICS,
 };