[PATCH 0/6] mm: split underutilized THPs

Usama Arif
2024-07-30T12:54:10Z

The current upstream default policy for THP is always. However, Meta
uses madvise in production as the current THP=always policy vastly
overprovisions THPs in sparsely accessed memory areas, resulting in
excessive memory pressure and premature OOM killing.
Using madvise + relying on khugepaged has certain drawbacks over
THP=always. Using madvise hints mean THPs aren't "transparent" and
require userspace changes. Waiting for khugepaged to scan memory and
collapse pages into THP can be slow and unpredictable in terms of performance
(i.e. you dont know when the collapse will happen), while production
environments require predictable performance. If there is enough memory
available, its better for both performance and predictability to have
a THP from fault time, i.e. THP=always rather than wait for khugepaged
to collapse it, and deal with sparsely populated THPs when the system is
running out of memory.

This patch-series is an attempt to mitigate the issue of running out of
memory when THP is always enabled. During runtime whenever a THP is being
faulted in or collapsed by khugepaged, the THP is added to a list.
Whenever memory reclaim happens, the kernel runs the deferred_split
shrinker which goes through the list and checks if the THP was underutilized,
i.e. how many of the base 4K pages of the entire THP were zero-filled.
If this number goes above a certain threshold, the shrinker will attempt
to split that THP. Then at remap time, the pages that were zero-filled are
not remapped, hence saving memory. This method avoids the downside of
wasting memory in areas where THP is sparsely filled when THP is always
enabled, while still providing the upside THPs like reduced TLB misses without
having to use madvise.

Meta production workloads that were CPU bound (>99% CPU utilzation) were
tested with THP shrinker. The results after 2 hours are as follows:

| THP=madvise | THP=always | THP=always
| | | + shrinker series
| | | + max_ptes_none=409
-----------------------------------------------------------------------------
Performance improvement | - | +1.8% | +1.7%
(over THP=madvise) | | |
-----------------------------------------------------------------------------
Memory usage | 54.6G | 58.8G (+7.7%) | 55.9G (+2.4%)
-----------------------------------------------------------------------------
max_ptes_none=409 means that any THP that has more than 409 out of 512
(80%) zero filled filled pages will be split.

To test out the patches, the below commands without the shrinker will
invoke OOM killer immediately and kill stress, but will not fail with
the shrinker:

echo 450 > /sys/kernel/mm/transparent_hugepage/khugepaged/max_ptes_none
mkdir /sys/fs/cgroup/test
echo $$ > /sys/fs/cgroup/test/cgroup.procs
echo 20M > /sys/fs/cgroup/test/memory.max
echo 0 > /sys/fs/cgroup/test/memory.swap.max
# allocate twice memory.max for each stress worker and touch 40/512 of
# each THP, i.e. vm-stride 50K.
# With the shrinker, max_ptes_none of 470 and below won't invoke OOM
# killer.
# Without the shrinker, OOM killer is invoked immediately irrespective
# of max_ptes_none value and kill stress.
stress --vm 1 --vm-bytes 40M --vm-stride 50K

Patches 1-2 add back helper functions that were previously removed
to operate on page lists (needed by patch 3).
Patch 3 is an optimization to free zapped tail pages rather than
waiting for page reclaim or migration.
Patch 4 is a prerequisite for THP shrinker to not remap zero-filled
subpages when splitting THP.
Patches 6 adds support for THP shrinker.

(This patch-series restarts the work on having a THP shrinker in kernel
originally done in
https://lore.kernel.org/all/cover.1667454613.git.alexlzhu@fb.com/.
The THP shrinker in this series is significantly different than the
original one, hence its labelled v1 (although the prerequisite to not
remap clean subpages is the same).)

Alexander Zhu (1):
mm: add selftests to split_huge_page() to verify unmap/zap of zero
pages

Usama Arif (3):
Revert "memcg: remove mem_cgroup_uncharge_list()"
Revert "mm: remove free_unref_page_list()"
mm: split underutilized THPs

Yu Zhao (2):
mm: free zapped tail pages when splitting isolated thp
mm: don't remap unused subpages when splitting isolated thp

Documentation/admin-guide/mm/transhuge.rst | 6 +
include/linux/huge_mm.h | 4 +-
include/linux/khugepaged.h | 1 +
include/linux/memcontrol.h | 12 ++
include/linux/mm_types.h | 2 +
include/linux/rmap.h | 2 +-
include/linux/vm_event_item.h | 1 +
mm/huge_memory.c | 152 +++++++++++++++---
mm/hugetlb.c | 1 +
mm/internal.h | 5 +-
mm/khugepaged.c | 3 +-
mm/memcontrol.c | 22 ++-
mm/migrate.c | 76 +++++++--
mm/migrate_device.c | 4 +-
mm/page_alloc.c | 18 +++
mm/rmap.c | 2 +-
mm/vmscan.c | 3 +-
mm/vmstat.c | 1 +
.../selftests/mm/split_huge_page_test.c | 113 +++++++++++++
tools/testing/selftests/mm/vm_util.c | 22 +++
tools/testing/selftests/mm/vm_util.h | 1 +
21 files changed, 414 insertions(+), 37 deletions(-)

--
2.43.0