kdamond_split_regions() bails out early when nr_regions is already
above max_nr_regions / 2. A large region that picks up new internal
variation after that point never gets split, so we lose visibility
into its hot/cold structure.
We hit this with damon-paddr on hugepage workloads and damon-vaddr
on processes that mmap a large anonymous range.
Example with max_nr_regions == 1500. A target ends up with 799
small hot/cold regions plus one big region (an earlier merge
collapsed a uniformly-accessed range into a single piece):
H:hot
C:cold
r1 r2 r3 r800
HHHHHH|CCCCCC|HHHHHH|...|HHHHHH..........................|
nr_regions = 800 > max_nr_regions / 2 = 750
Now a cold subarea shows up inside r800:
r1 r2 r3 r800
HHHHHH|CCCCCC|HHHHHH|...|HHHHHH........CCCCCC.............|
The small regions can't merge with each other (their access counts
differ), so budget never frees up. r800 can't be split because
nr_regions > max_nr_regions / 2 returns early. The cold subarea
stays invisible.
Patch 1 keeps refining on this path: when nr_regions is above
max_nr_regions / 2 but still under the maximum, it splits a fraction
of the regions instead of returning. The fraction shrinks as the
remaining budget shrinks, so the count approaches max_nr_regions
smoothly. A useless split is undone by the next merge cycle.
Patch 2 adds a KUnit test for the case where nr_regions is already
above max_nr_regions / 2.
Thanks to SJ for the suggestion to drive the split fraction
from the remaining budget rather than an age-based filter.
Changes from v2
- v2: https://lore.kernel.org/[email protected]
- Collect R-b: from SJ.
- Rebase to latest mm-new.
Changes from v1
- v1:
https://lore.kernel.org/damon/[email protected]/
- Some feedback from SJ.
Jiayuan Chen (2):
mm/damon/core: split a fraction of regions when nr_regions exceeds
max/2
mm/damon/tests/core-kunit: test split above max_nr_regions/2
mm/damon/core.c | 49 +++++++++++++++++++++++++---
mm/damon/tests/core-kunit.h | 64 +++++++++++++++++++++++++++++++++++++
2 files changed, 108 insertions(+), 5 deletions(-)
base-commit: 86100fb7e27ebb5da22fc8a2810eebcf8cc897e8
--
2.47.3
