mm: multigenerational lru: documentation

Add Documentation/vm/multigen_lru.rst.

Signed-off-by: Yu Zhao <yuzhao@google.com>
Tested-by: Konstantin Kharlamov <Hi-Angel@yandex.ru>

Documentation/vm/index.rst

@@ -17,6 +17,7 @@ various features of the Linux memory management
 
    swap_numa
    zswap
+   multigen_lru
 
 Kernel developers MM documentation
 ==================================

Documentation/vm/multigen_lru.rst

@@ -0,0 +1,143 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=====================
+Multigenerational LRU
+=====================
+
+Quick Start
+===========
+Build Options
+-------------
+:Required: Set ``CONFIG_LRU_GEN=y``.
+
+:Optional: Set ``CONFIG_LRU_GEN_ENABLED=y`` to turn the feature on by
+ default.
+
+:Optional: Change ``CONFIG_NR_LRU_GENS`` to a number ``X`` to support
+ a maximum of ``X`` generations.
+
+:Optional: Change ``CONFIG_TIERS_PER_GEN`` to a number ``Y`` to
+ support a maximum of ``Y`` tiers per generation.
+
+Runtime Options
+---------------
+:Required: Write ``1`` to ``/sys/kernel/mm/lru_gen/enable`` if the
+ feature was not turned on by default.
+
+:Optional: Change ``/sys/kernel/mm/lru_gen/spread`` to a number ``N``
+ to spread pages out across ``N+1`` generations. ``N`` should be less
+ than ``X``. Larger values make the background aging more aggressive.
+
+:Optional: Read ``/sys/kernel/debug/lru_gen`` to verify the feature.
+ This file has the following output:
+
+::
+
+  memcg  memcg_id  memcg_path
+    node  node_id
+      min_gen  birth_time  anon_size  file_size
+      ...
+      max_gen  birth_time  anon_size  file_size
+
+Given a memcg and a node, ``min_gen`` is the oldest generation
+(number) and ``max_gen`` is the youngest. Birth time is in
+milliseconds. The sizes of anon and file types are in pages.
+
+Recipes
+-------
+:Android on ARMv8.1+: ``X=4``, ``Y=3`` and ``N=0``.
+
+:Android on pre-ARMv8.1 CPUs: Not recommended due to the lack of
+ ``ARM64_HW_AFDBM``.
+
+:Laptops and workstations running Chrome on x86_64: Use the default
+ values.
+
+:Working set estimation: Write ``+ memcg_id node_id gen [swappiness]``
+ to ``/sys/kernel/debug/lru_gen`` to account referenced pages to
+ generation ``max_gen`` and create the next generation ``max_gen+1``.
+ ``gen`` should be equal to ``max_gen``. A swap file and a non-zero
+ ``swappiness`` are required to scan anon type. If swapping is not
+ desired, set ``vm.swappiness`` to ``0``.
+
+:Proactive reclaim: Write ``- memcg_id node_id gen [swappiness]
+ [nr_to_reclaim]`` to ``/sys/kernel/debug/lru_gen`` to evict
+ generations less than or equal to ``gen``. ``gen`` should be less
+ than ``max_gen-1`` as ``max_gen`` and ``max_gen-1`` are active
+ generations and therefore protected from the eviction. Use
+ ``nr_to_reclaim`` to limit the number of pages to evict. Multiple
+ command lines are supported, so does concatenation with delimiters
+ ``,`` and ``;``.
+
+Framework
+=========
+For each ``lruvec``, evictable pages are divided into multiple
+generations. The youngest generation number is stored in ``max_seq``
+for both anon and file types as they are aged on an equal footing. The
+oldest generation numbers are stored in ``min_seq[2]`` separately for
+anon and file types as clean file pages can be evicted regardless of
+swap and write-back constraints. These three variables are
+monotonically increasing. Generation numbers are truncated into
+``order_base_2(CONFIG_NR_LRU_GENS+1)`` bits in order to fit into
+``page->flags``. The sliding window technique is used to prevent
+truncated generation numbers from overlapping. Each truncated
+generation number is an index to an array of per-type and per-zone
+lists. Evictable pages are added to the per-zone lists indexed by
+``max_seq`` or ``min_seq[2]`` (modulo ``CONFIG_NR_LRU_GENS``),
+depending on their types.
+
+Each generation is then divided into multiple tiers. Tiers represent
+levels of usage from file descriptors only. Pages accessed N times via
+file descriptors belong to tier order_base_2(N). Each generation
+contains at most CONFIG_TIERS_PER_GEN tiers, and they require
+additional CONFIG_TIERS_PER_GEN-2 bits in page->flags. In contrast to
+moving across generations which requires the lru lock for the list
+operations, moving across tiers only involves an atomic operation on
+``page->flags`` and therefore has a negligible cost. A feedback loop
+modeled after the PID controller monitors the refault rates across all
+tiers and decides when to activate pages from which tiers in the
+reclaim path.
+
+The framework comprises two conceptually independent components: the
+aging and the eviction, which can be invoked separately from user
+space for the purpose of working set estimation and proactive reclaim.
+
+Aging
+-----
+The aging produces young generations. Given an ``lruvec``, the aging
+scans page tables for referenced pages of this ``lruvec``. Upon
+finding one, the aging updates its generation number to ``max_seq``.
+After each round of scan, the aging increments ``max_seq``.
+
+The aging maintains either a system-wide ``mm_struct`` list or
+per-memcg ``mm_struct`` lists, and it only scans page tables of
+processes that have been scheduled since the last scan.
+
+The aging is due when both of ``min_seq[2]`` reaches ``max_seq-1``,
+assuming both anon and file types are reclaimable.
+
+Eviction
+--------
+The eviction consumes old generations. Given an ``lruvec``, the
+eviction scans the pages on the per-zone lists indexed by either of
+``min_seq[2]``. It first tries to select a type based on the values of
+``min_seq[2]``. When anon and file types are both available from the
+same generation, it selects the one that has a lower refault rate.
+
+During a scan, the eviction sorts pages according to their new
+generation numbers, if the aging has found them referenced. It also
+moves pages from the tiers that have higher refault rates than tier 0
+to the next generation.
+
+When it finds all the per-zone lists of a selected type are empty, the
+eviction increments ``min_seq[2]`` indexed by this selected type.
+
+To-do List
+==========
+KVM Optimization
+----------------
+Support shadow page table scanning.
+
+NUMA Optimization
+-----------------
+Optimize page table scan for NUMA.