mm: make alloc_demote_folio externally invokable for migration

Patch series "DAMON based tiered memory management for CXL memory", v6.

Introduction
============

With the advent of CXL/PCIe attached DRAM, which will be called simply as
CXL memory in this cover letter, some systems are becoming more
heterogeneous having memory systems with different latency and bandwidth
characteristics.  They are usually handled as different NUMA nodes in
separate memory tiers and CXL memory is used as slow tiers because of its
protocol overhead compared to local DRAM.

In this kind of systems, we need to be careful placing memory pages on
proper NUMA nodes based on the memory access frequency.  Otherwise, some
frequently accessed pages might reside on slow tiers and it makes
performance degradation unexpectedly.  Moreover, the memory access
patterns can be changed at runtime.

To handle this problem, we need a way to monitor the memory access
patterns and migrate pages based on their access temperature.  The
DAMON(Data Access MONitor) framework and its DAMOS(DAMON-based Operation
Schemes) can be useful features for monitoring and migrating pages.  DAMOS
provides multiple actions based on DAMON monitoring results and it can be
used for proactive reclaim, which means swapping cold pages out with
DAMOS_PAGEOUT action, but it doesn't support migration actions such as
demotion and promotion between tiered memory nodes.

This series supports two new DAMOS actions; DAMOS_MIGRATE_HOT for
promotion from slow tiers and DAMOS_MIGRATE_COLD for demotion from fast
tiers.  This prevents hot pages from being stuck on slow tiers, which
makes performance degradation and cold pages can be proactively demoted to
slow tiers so that the system can increase the chance to allocate more hot
pages to fast tiers.

The DAMON provides various tuning knobs but we found that the proactive
demotion for cold pages is especially useful when the system is running
out of memory on its fast tier nodes.

Our evaluation result shows that it reduces the performance slowdown
compared to the default memory policy from 11% to 3~5% when the system
runs under high memory pressure on its fast tier DRAM nodes.

DAMON configuration
===================

The specific DAMON configuration doesn't have to be in the scope of this
patch series, but some rough idea is better to be shared to explain the
evaluation result.

The DAMON provides many knobs for fine tuning but its configuration file
is generated by HMSDK[3].  It includes gen_config.py script that generates
a json file with the full config of DAMON knobs and it creates multiple
kdamonds for each NUMA node when the DAMON is enabled so that it can run
hot/cold based migration for tiered memory.

Evaluation Workload
===================

The performance evaluation is done with redis[4], which is a widely used
in-memory database and the memory access patterns are generated via
YCSB[5].  We have measured two different workloads with zipfian and latest
distributions but their configs are slightly modified to make memory usage
higher and execution time longer for better evaluation.

The idea of evaluation using these migrate_{hot,cold} actions covers
system-wide memory management rather than partitioning hot/cold pages of a
single workload.  The default memory allocation policy creates pages to
the fast tier DRAM node first, then allocates newly created pages to the
slow tier CXL node when the DRAM node has insufficient free space.  Once
the page allocation is done then those pages never move between NUMA
nodes.  It's not true when using numa balancing, but it is not the scope
of this DAMON based tiered memory management support.

If the working set of redis can be fit fully into the DRAM node, then the
redis will access the fast DRAM only.  Since the performance of DRAM only
is faster than partially accessing CXL memory in slow tiers, this
environment is not useful to evaluate this patch series.

To make pages of redis be distributed across fast DRAM node and slow CXL
node to evaluate our migrate_{hot,cold} actions, we pre-allocate some cold
memory externally using mmap and memset before launching redis-server.  We
assumed that there are enough amount of cold memory in datacenters as
TMO[6] and TPP[7] papers mentioned.

The evaluation sequence is as follows.

1. Turn on DAMON with DAMOS_MIGRATE_COLD action for DRAM node and
   DAMOS_MIGRATE_HOT action for CXL node.  It demotes cold pages on DRAM
   node and promotes hot pages on CXL node in a regular interval.
2. Allocate a huge block of cold memory by calling mmap and memset at
   the fast tier DRAM node, then make the process sleep to make the fast
   tier has insufficient space for redis-server.
3. Launch redis-server and load prebaked snapshot image, dump.rdb.  The
   redis-server consumes 52GB of anon pages and 33GB of file pages, but
   due to the cold memory allocated at 2, it fails allocating the entire
   memory of redis-server on the fast tier DRAM node so it partially
   allocates the remaining on the slow tier CXL node.  The ratio of
   DRAM:CXL depends on the size of the pre-allocated cold memory.
4. Run YCSB to make zipfian or latest distribution of memory accesses to
   redis-server, then measure its execution time when it's completed.
5. Repeat 4 over 50 times to measure the average execution time for each
   run.
6. Increase the cold memory size then repeat goes to 2.

For each test at 4 took about a minute so repeating it 50 times almost
took about 1 hour for each test with a specific cold memory from 440GB to
500GB in 10GB increments for each evaluation.  So it took about more than
10 hours for both zipfian and latest workloads to get the entire
evaluation results.  Repeating the same test set multiple times doesn't
show much difference so I think it might be enough to make the result
reliable.

Evaluation Results
==================

All the result values are normalized to DRAM-only execution time because
the workload cannot be faster than DRAM-only unless the workload hits the
peak bandwidth but our redis test doesn't go beyond the bandwidth limit.

So the DRAM-only execution time is the ideal result without affected by
the gap between DRAM and CXL performance difference.  The NUMA node
environment is as follows.

  node0 - local DRAM, 512GB with a CPU socket (fast tier)
  node1 - disabled
  node2 - CXL DRAM, 96GB, no CPU attached (slow tier)

The following is the result of generating zipfian distribution to
redis-server and the numbers are averaged by 50 times of execution.

  1. YCSB zipfian distribution read only workload
  memory pressure with cold memory on node0 with 512GB of local DRAM.
  ====================+================================================+=========
                      |       cold memory occupied by mmap and memset  |
                      |   0G  440G  450G  460G  470G  480G  490G  500G |
  ====================+================================================+=========
  Execution time normalized to DRAM-only values                        | GEOMEAN
  --------------------+------------------------------------------------+---------
  DRAM-only           | 1.00     -     -     -     -     -     -     - | 1.00
  CXL-only            | 1.19     -     -     -     -     -     -     - | 1.19
  default             |    -  1.00  1.05  1.08  1.12  1.14  1.18  1.18 | 1.11
  DAMON tiered        |    -  1.03  1.03  1.03  1.03  1.03  1.07 *1.05 | 1.04
  DAMON lazy          |    -  1.04  1.03  1.04  1.05  1.06  1.06 *1.06 | 1.05
  ====================+================================================+=========
  CXL usage of redis-server in GB                                      | AVERAGE
  --------------------+------------------------------------------------+---------
  DRAM-only           |  0.0     -     -     -     -     -     -     - |  0.0
  CXL-only            | 51.4     -     -     -     -     -     -     - | 51.4
  default             |    -   0.6  10.6  20.5  30.5  40.5  47.6  50.4 | 28.7
  DAMON tiered        |    -   0.6   0.5   0.4   0.7   0.8   7.1   5.6 |  2.2
  DAMON lazy          |    -   0.5   3.0   4.5   5.4   6.4   9.4   9.1 |  5.5
  ====================+================================================+=========

Each test result is based on the execution environment as follows.

  DRAM-only:           redis-server uses only local DRAM memory.
  CXL-only:            redis-server uses only CXL memory.
  default:             default memory policy(MPOL_DEFAULT).
                       numa balancing disabled.
  DAMON tiered:        DAMON enabled with DAMOS_MIGRATE_COLD for DRAM
                       nodes and DAMOS_MIGRATE_HOT for CXL nodes.
  DAMON lazy:          same as DAMON tiered, but turn on DAMON just
                       before making memory access request via YCSB.

The above result shows the "default" execution time goes up as the size of
cold memory is increased from 440G to 500G because the more cold memory
used, the more CXL memory is used for the target redis workload and this
makes the execution time increase.

However, "DAMON tiered" and other DAMON results show less slowdown because
the DAMOS_MIGRATE_COLD action at DRAM node proactively demotes
pre-allocated cold memory to CXL node and this free space at DRAM
increases more chance to allocate hot or warm pages of redis-server to
fast DRAM node.  Moreover, DAMOS_MIGRATE_HOT action at CXL node also
promotes hot pages of redis-server to DRAM node actively.

As a result, it makes more memory of redis-server stay in DRAM node
compared to "default" memory policy and this makes the performance
improvement.

Please note that the result numbers of "DAMON tiered" and "DAMON lazy" at
500G are marked with * stars, which means their test results are replaced
with reproduced tests that didn't have OOM issue.

That was needed because sometimes the test processes get OOM when DRAM has
insufficient space.  The DAMOS_MIGRATE_HOT doesn't kick reclaim but just
gives up migration when there is not enough space at DRAM side.  The
problem happens when there is competition between normal allocation and
migration and the migration is done before normal allocation, then the
completely unrelated normal allocation can trigger reclaim, which incurs
OOM.

Because of this issue, I have also tested more cases with
"demotion_enabled" flag enabled to make such reclaim doesn't trigger OOM,
but just demote reclaimed pages.  The following test results show more
tests with "kswapd" marked.

  2. YCSB zipfian distribution read only workload (with demotion_enabled true)
  memory pressure with cold memory on node0 with 512GB of local DRAM.
  ====================+================================================+=========
                      |       cold memory occupied by mmap and memset  |
                      |   0G  440G  450G  460G  470G  480G  490G  500G |
  ====================+================================================+=========
  Execution time normalized to DRAM-only values                        | GEOMEAN
  --------------------+------------------------------------------------+---------
  DAMON tiered        |    -  1.03  1.03  1.03  1.03  1.03  1.07  1.05 | 1.04
  DAMON lazy          |    -  1.04  1.03  1.04  1.05  1.06  1.06  1.06 | 1.05
  DAMON tiered kswapd |    -  1.03  1.03  1.03  1.03  1.02  1.02  1.03 | 1.03
  DAMON lazy kswapd   |    -  1.04  1.04  1.04  1.03  1.05  1.04  1.05 | 1.04
  ====================+================================================+=========
  CXL usage of redis-server in GB                                      | AVERAGE
  --------------------+------------------------------------------------+---------
  DAMON tiered        |    -   0.6   0.5   0.4   0.7   0.8   7.1   5.6 |  2.2
  DAMON lazy          |    -   0.5   3.0   4.5   5.4   6.4   9.4   9.1 |  5.5
  DAMON tiered kswapd |    -   0.0   0.0   0.4   0.5   0.1   0.8   1.0 |  0.4
  DAMON lazy kswapd   |    -   4.2   4.6   5.3   1.7   6.8   8.1   5.8 |  5.2
  ====================+================================================+=========

Each test result is based on the exeuction environment as follows.

  DAMON tiered:        same as before
  DAMON lazy:          same as before
  DAMON tiered kswapd: same as DAMON tiered, but turn on
                       /sys/kernel/mm/numa/demotion_enabled to make
                       kswapd or direct reclaim does demotion.
  DAMON lazy kswapd:   same as DAMON lazy, but turn on
                       /sys/kernel/mm/numa/demotion_enabled to make
                       kswapd or direct reclaim does demotion.

The "DAMON tiered kswapd" and "DAMON lazy kswapd" didn't trigger OOM at
all unlike other tests because kswapd and direct reclaim from DRAM node
can demote reclaimed pages to CXL node independently from DAMON actions
and their results are slightly better than without having
"demotion_enabled".

In summary, the evaluation results show that DAMON memory management with
DAMOS_MIGRATE_{HOT,COLD} actions reduces the performance slowdown compared
to the "default" memory policy from 11% to 3~5% when the system runs with
high memory pressure on its fast tier DRAM nodes.

Having these DAMOS_MIGRATE_HOT and DAMOS_MIGRATE_COLD actions can make
tiered memory systems run more efficiently under high memory pressures.


This patch (of 7):

The alloc_demote_folio can be used out of vmscan.c so it'd be better to
remove static keyword from it.

Link: https://lkml.kernel.org/r/20240614030010.751-1-honggyu.kim@sk.com
Link: https://lkml.kernel.org/r/20240614030010.751-2-honggyu.kim@sk.com
Signed-off-by: Honggyu Kim <honggyu.kim@sk.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: SeongJae Park <sj@kernel.org>
Cc: Gregory Price <gregory.price@memverge.com>
Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc: Hyeongtak Ji <hyeongtak.ji@sk.com>
Cc: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 1 insertions, 2 deletions

diff --git a/mm/vmscan.c b/mm/vmscan.c
index d1a87ceef0dc..5713c29e70c4 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -919,8 +919,7 @@ static void folio_check_dirty_writeback(struct folio *folio,
 		mapping->a_ops->is_dirty_writeback(folio, dirty, writeback);
 }
 
-static struct folio *alloc_demote_folio(struct folio *src,
-		unsigned long private)
+struct folio *alloc_demote_folio(struct folio *src, unsigned long private)
 {
 	struct folio *dst;
 	nodemask_t *allowed_mask;