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mm: multi-gen LRU: improve design doc

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This patch improves the design doc. Specifically,
  1. add a section for the per-memcg mm_struct list, and
  2. add a section for the PID controller.

Link: https://lkml.kernel.org/r/20230214035445.1250139-2-talumbau@google.com
Signed-off-by: T.J. Alumbaugh <talumbau@google.com>
Cc: Yu Zhao <yuzhao@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
T.J. Alumbaugh 2023-02-14 03:54:45 +00:00 committed by Andrew Morton
parent 9a52b2f32a
commit 32d32ef140
2 changed files with 40 additions and 6 deletions
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44
Documentation/mm/multigen_lru.rst
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@ -103,7 +103,8 @@ moving across tiers only involves atomic operations on
``folio->flags`` and therefore has a negligible cost. A feedback loop
modeled after the PID controller monitors refaults over all the tiers
from anon and file types and decides which tiers from which types to
evict or protect.
evict or protect. The desired effect is to balance refault percentages
between anon and file types proportional to the swappiness level.
There are two conceptually independent procedures: the aging and the
eviction. They form a closed-loop system, i.e., the page reclaim.
@ -156,6 +157,27 @@ This time-based approach has the following advantages:
and memory sizes.
2. It is more reliable because it is directly wired to the OOM killer.
``mm_struct`` list
------------------
An ``mm_struct`` list is maintained for each memcg, and an
``mm_struct`` follows its owner task to the new memcg when this task
is migrated.
A page table walker iterates ``lruvec_memcg()->mm_list`` and calls
``walk_page_range()`` with each ``mm_struct`` on this list to scan
PTEs. When multiple page table walkers iterate the same list, each of
them gets a unique ``mm_struct``, and therefore they can run in
parallel.
Page table walkers ignore any misplaced pages, e.g., if an
``mm_struct`` was migrated, pages left in the previous memcg will be
ignored when the current memcg is under reclaim. Similarly, page table
walkers will ignore pages from nodes other than the one under reclaim.
This infrastructure also tracks the usage of ``mm_struct`` between
context switches so that page table walkers can skip processes that
have been sleeping since the last iteration.
Rmap/PT walk feedback
---------------------
Searching the rmap for PTEs mapping each page on an LRU list (to test
@ -170,7 +192,7 @@ promotes hot pages. If the scan was done cacheline efficiently, it
adds the PMD entry pointing to the PTE table to the Bloom filter. This
forms a feedback loop between the eviction and the aging.
Bloom Filters
Bloom filters
-------------
Bloom filters are a space and memory efficient data structure for set
membership test, i.e., test if an element is not in the set or may be
@ -186,6 +208,18 @@ is false positive, the cost is an additional scan of a range of PTEs,
which may yield hot pages anyway. Parameters of the filter itself can
control the false positive rate in the limit.
PID controller
--------------
A feedback loop modeled after the Proportional-Integral-Derivative
(PID) controller monitors refaults over anon and file types and
decides which type to evict when both types are available from the
same generation.
The PID controller uses generations rather than the wall clock as the
time domain because a CPU can scan pages at different rates under
varying memory pressure. It calculates a moving average for each new
generation to avoid being permanently locked in a suboptimal state.
Memcg LRU
---------
An memcg LRU is a per-node LRU of memcgs. It is also an LRU of LRUs,
@ -223,9 +257,9 @@ parts:
* Generations
* Rmap walks
* Page table walks
* Bloom filters
* PID controller
* Page table walks via ``mm_struct`` list
* Bloom filters for rmap/PT walk feedback
* PID controller for refault feedback
The aging and the eviction form a producer-consumer model;
specifically, the latter drives the former by the sliding window over

2
mm/vmscan.c
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@ -3604,7 +3604,7 @@ static bool iterate_mm_list_nowalk(struct lruvec *lruvec, unsigned long max_seq)
}
/******************************************************************************
* refault feedback loop
* PID controller
******************************************************************************/
/*