[PATCH v3 00/13] mm, swap: rework of swap allocator locks

Kairui Song
2024-12-30T17:46:35Z

From: Kairui Song <kasong@tencent.com>

This series greatly improved swap performance by reworking
the locking design and simplify a lot of code path. Test showed
a up to 400% vm-scalability improvement with pmem as SWAP, and
up to 37% reduce of kernel compile real time with ZRAM as SWAP
(up to 60% improvement in system time).

This is part of the new swap allocator discussed during
the "Swap Abstraction" discussion at LSF/MM 2024, and
"mTHP and swap allocator" discussion at LPC 2024.

This is a follow up of previous swap cluster allocator series:
https://lore.kernel.org/linux-mm/20240730-swap-allocator-v5-0-cb9c148b9297@kernel.org/
Also enables further optimizations which will come later.

Previous series introduced a fully cluster based allocator, this
series completely get rid of the old allocator and makes the new
allocator avoid touching the si->lock unless needed. This bring huge
performance gain and get rid of slot cache for freeing path.

Currently, swap locking is mainly composed of two locks, cluster lock
(ci->lock) and device lock (si->lock). The device lock is widely used
to protect many things, causing it to be the main bottleneck for SWAP.

Cluster lock is much more fine-grained, so it will be best to use
ci->lock instead of si->lock as much as possible.

`perf lock` indicates this issue clearly. Doing linux kernel build
using tmpfs and ZRAM with limited memory (make -j64 with 1G memcg and 4k
pages), result of "perf lock contention -ab sleep 3" shows:

contended total wait max wait avg wait type caller
34948 53.63 s 7.11 ms 1.53 ms spinlock free_swap_and_cache_nr+0x350
16569 40.05 s 6.45 ms 2.42 ms spinlock get_swap_pages+0x231
11191 28.41 s 7.03 ms 2.54 ms spinlock swapcache_free_entries+0x59
4147 22.78 s 122.66 ms 5.49 ms spinlock page_vma_mapped_walk+0x6f3
4595 7.17 s 6.79 ms 1.56 ms spinlock swapcache_free_entries+0x59
406027 2.74 s 2.59 ms 6.74 us spinlock list_lru_add+0x39
...snip...

The top 5 caller are all users of si->lock, total wait time sums to several
minutes in the 3 seconds time window.

Following the new allocator design, many operation doesn't need to touch
si->lock at all. We only need to take si->lock when doing operations
across multiple clusters (changing the cluster list). So ideally
allocator should always take ci->lock first, then take si->lock only if
needed. But due to historical reasons, ci->lock is used inside si->lock
critical section, causing lock inversion if we simply try to acquire
si->lock after acquiring ci->lock.

This series audited all si->lock usage, clean up legacy codes, eliminate
usage of si->lock as much as possible by introducing new designs based
on the new cluster allocator.

Old HDD allocation codes are removed, cluster allocator is adapted
with small changes for HDD usage, test is looking OK.

And this also removed slot cache for freeing path. The performance is
even better without it now, and this enables other clean up and
optimizations as discussed before:

https://lore.kernel.org/all/CAMgjq7ACohT_uerSz8E_994ZZCv709Zor+43hdmesW_59W1BWw@mail.gmail.com/

After this series, lock contention on si->lock is nearly unobservable
with `perf lock` with the same test above:

contended total wait max wait avg wait type caller
... snip ...
91 204.62 us 4.51 us 2.25 us spinlock cluster_move+0x2e
... snip ...
47 125.62 us 4.47 us 2.67 us spinlock cluster_move+0x2e
... snip ...
23 63.15 us 3.95 us 2.74 us spinlock cluster_move+0x2e
... snip ...
17 41.26 us 4.58 us 2.43 us spinlock cluster_isolate_lock+0x1d
... snip ...

`cluster_move` and `cluster_isolate_lock` (two new introduced helper)
are basically the only users of si->lock now, performance gain is huge,
and LOC is reduced.

Tests Results:

vm-scalability
==============
Running `usemem --init-time -O -y -x -R -31 1G` from vm-scalability
in a 12G memory cgroup using simulated pmem as SWAP backend (32G pmem,
32 CPUs).

Using 4K folio by default, 64k mTHP and sequential access (!-R) results
are also provided. 6 test runs for each case, Total Throughput:

Test Before (KB/s) (stdev) After (KB/s) (stdev) Delta
---------------------------------------------------------------------------
Random (4K): 69937.11 (16449.77) 369816.17 (24476.68) +428.78%
Random (64k): 123442.83 (13207.51) 216379.00 (25024.83) +75.28%
Sequential (4K): 6313909.83 (148856.12) 6419860.66 (183563.38) +1.7%

Sequential access will cause lower stress for the allocator so the gain is
limited, but with random access (which is much closer to real workloads)
the performance gain is huge.

Build kernel with defconfig on tmpfs with ZRAM
==============================================
Below results shows a test matrix using different memory cgroup limit
and job numbets, and scaled up progressive for a intuitive result.
Done on a 48c96t system.

6 test run for each case, it can be seen clearly that as concurrent job
number goes higher the performance gain is higher, but even -j6 is
showing slight improvement.

make -j<NR> | System Time (seconds) | Total Time (seconds)
(NR / Mem / ZRAM) | (Before / After / Delta) | (Before / After / Delta)
With 4k pages only:
6 / 192M / 3G | 1533 / 1522 / -0.7% | 1420 / 1414 / -0.3%
12 / 256M / 4G | 2275 / 2226 / -2.2% | 758 / 742 / -2.1%
24 / 384M / 5G | 3596 / 3154 / -12.3% | 476 / 422 / -11.3%
48 / 768M / 7G | 8159 / 3605 / -55.8% | 330 / 221 / -33.0%
96 / 1.5G / 10G | 18541 / 6462 / -65.1% | 283 / 180 / -36.4%
With 64k mTHP:
24 / 512M / 5G | 3585 / 3469 / -3.2% | 293 / 290 / -0.1%
48 / 1G / 7G | 8173 / 3607 / -55.9% | 251 / 158 / -37.0%
96 / 2G / 10G | 16305 / 7791 / -52.2% | 226 / 144 / -36.3%

The fragmentation are reduced too:
With: make -j96 / 1152M memcg, 64K mTHP:
(avg of 4 test run)
Before:
hugepages-64kB/stats/swpout: 1696184
hugepages-64kB/stats/swpout_fallback: 414318
After: (-63.2% mTHP swapout failure)
hugepages-64kB/stats/swpout: 1866267
hugepages-64kB/stats/swpout_fallback: 158330

There is a up to 65.1% improvement in sys time for build kernel test,
and lower fragmentation rate.

Build kernel with tinyconfig on tmpfs with HDD as swap:
=======================================================

This test is similar to above, but HDD test is very noisy and slow, the
deviation is huge, so just use tinyconfig instead and take the median test
result of 3 test run, which looks OK:

Before this series:
114.44user 29.11system 39:42.90elapsed 6%CPU
2901232inputs+0outputs (238877major+4227640minor)pagefaults

After this commit:
113.90user 23.81system 38:11.77elapsed 6%CPU
2548728inputs+0outputs (235471major+4238110minor)pagefaults

Single thread SWAP:
===================

Sequential SWAP should also be slightly faster as we removed a lot of
unnecessary parts. Test using micro benchmark for swapout/in 4G
zero memory using ZRAM, 10 test runs:

Swapout Before (avg. 3359304):
3353796 3358551 3371305 3356043 3367524 3355303 3355924 3354513 3360776

Swapin Before (avg. 1928698):
1920283 1927183 1934105 1921373 1926562 1938261 1927726 1928636 1934155

Swapout After (avg. 3347511, -0.4%):
3337863 3347948 3355235 3339081 3333134 3353006 3354917 3346055 3360359

Swapin After (avg. 1922290, -0.3%):
1919101 1925743 1916810 1917007 1923930 1935152 1917403 1923549 1921913

The gain is limited at noise level but seems slightly better.

V2: https://lore.kernel.org/linux-mm/202412250259.S5ew5ZrN-lkp@intel.com/T/
Updates since V2:
- Use atomic_long_try_cmpxchg instead of atomic_long_cmpxchg
[Uros Bizjak]
- Fix bot build error after previous rebase.

V1: https://lore.kernel.org/linux-mm/20241022192451.38138-1-ryncsn@gmail.com/
Updates since V1:
- Retest some tests after rebase on top of latest mm-unstable, the new
Cgroup lock removal increased the performance gain of this series too,
some results are basically same as before so unchanged:
https://lore.kernel.org/linux-mm/20241218114633.85196-1-ryncsn@gmail.com/
- Rework the off-list bit handling, make it easier to review and more
robust, also reduce LOC [Chris Li].
- Code style improvements and minor code optimizations. [Chris Li].
- Fixing a potential swapoff race issue due to missing SWP_WRITEOK check
[Huang Ying].
- Added vm-scalability test with pmem [Huang Ying].

Suggested-by: Chris Li <chrisl@kernel.org>
Signed-off-by: Kairui Song <kasong@tencent.com>

Kairui Song (13):
mm, swap: minor clean up for swap entry allocation
mm, swap: fold swap_info_get_cont in the only caller
mm, swap: remove old allocation path for HDD
mm, swap: use cluster lock for HDD
mm, swap: clean up device availability check
mm, swap: clean up plist removal and adding
mm, swap: hold a reference during scan and cleanup flag usage
mm, swap: use an enum to define all cluster flags and wrap flags
changes
mm, swap: reduce contention on device lock
mm, swap: simplify percpu cluster updating
mm, swap: introduce a helper for retrieving cluster from offset
mm, swap: use a global swap cluster for non-rotation devices
mm, swap_slots: remove slot cache for freeing path

fs/btrfs/inode.c | 1 -
fs/f2fs/data.c | 1 -
fs/iomap/swapfile.c | 1 -
include/linux/swap.h | 34 +-
include/linux/swap_slots.h | 3 -
mm/page_io.c | 1 -
mm/swap_slots.c | 78 +--
mm/swapfile.c | 1250 ++++++++++++++++--------------------
8 files changed, 595 insertions(+), 774 deletions(-)

--
2.47.1