[PATCH V3 01/10] mm: page_alloc: freelist migratetype hygiene

From: Johannes Weiner
Date: Tue Mar 05 2024 - 23:16:35 EST

Hello! This is version 3 of the freelist hygiene patches. Sorry it's
been a while. The short of it is I fixed the bugs uncovered last time
by Mike and Vlastimil, incorporated Vlastimils suggested optimizations
and reworked a good chunk of the page isolation code with Zi.

I also iterated on the benchmarks somewhat and grabbed the extfrag
stats Mel suggested, as well as stats on mismatches between requested
and received migratetypes from the allocator. I think the results of
this are pretty good.

Rationale, test results, and detailed changelog follow.

---

This is a breakout series from the huge page allocator patches[1].

While testing and benchmarking the series more incrementally, as per
reviewer request, it became apparent that there are several sources of
freelist migratetype violations that later patches in the series hid.

Those violations occur when pages of one migratetype end up on the
freelists of another type. This encourages incompatible page mixing
down the line, where allocation requests ask for one migrate type, but
receives pages of another. This defeats the mobility grouping.

The series addresses those causes. The last patch adds type checks on
all freelist movements to rule out any violations. I used these checks
to identify the violations fixed up in the preceding patches.

The series is a breakout, but has merit on its own: Less type mixing
means improved grouping, means less work for compaction, means higher
THP success rate and lower allocation latencies.

The results can be seen in a mixed workload that stresses the machine
with a memcache-type workload and a kernel build job while
periodically attempting to allocate batches of THP. The following data
is aggregated over 50 consecutive defconfig builds:

VANILLA PATCHED
Hugealloc Time mean 165843.93 ( +0.00%) 113025.88 ( -31.85%)
Hugealloc Time stddev 158957.35 ( +0.00%) 114716.07 ( -27.83%)
Kbuild Real time 310.24 ( +0.00%) 300.73 ( -3.06%)
Kbuild User time 1271.13 ( +0.00%) 1259.42 ( -0.92%)
Kbuild System time 582.02 ( +0.00%) 559.79 ( -3.81%)
THP fault alloc 30585.14 ( +0.00%) 40853.62 ( +33.57%)
THP fault fallback 36626.46 ( +0.00%) 26357.62 ( -28.04%)
THP fault fail rate % 54.49 ( +0.00%) 39.22 ( -27.53%)
Pagealloc fallback 1328.00 ( +0.00%) 1.00 ( -99.85%)
Pagealloc type mismatch 181009.50 ( +0.00%) 0.00 ( -100.00%)
Direct compact stall 434.56 ( +0.00%) 257.66 ( -40.61%)
Direct compact fail 421.70 ( +0.00%) 249.94 ( -40.63%)
Direct compact success 12.86 ( +0.00%) 7.72 ( -37.09%)
Direct compact success rate % 2.86 ( +0.00%) 2.82 ( -0.96%)
Compact daemon scanned migrate 3370059.62 ( +0.00%) 3612054.76 ( +7.18%)
Compact daemon scanned free 7718439.20 ( +0.00%) 5386385.02 ( -30.21%)
Compact direct scanned migrate 309248.62 ( +0.00%) 176721.04 ( -42.85%)
Compact direct scanned free 433582.84 ( +0.00%) 315727.66 ( -27.18%)
Compact migrate scanned daemon % 91.20 ( +0.00%) 94.48 ( +3.56%)
Compact free scanned daemon % 94.58 ( +0.00%) 94.42 ( -0.16%)
Compact total migrate scanned 3679308.24 ( +0.00%) 3788775.80 ( +2.98%)
Compact total free scanned 8152022.04 ( +0.00%) 5702112.68 ( -30.05%)
Alloc stall 872.04 ( +0.00%) 5156.12 ( +490.71%)
Pages kswapd scanned 510645.86 ( +0.00%) 3394.94 ( -99.33%)
Pages kswapd reclaimed 134811.62 ( +0.00%) 2701.26 ( -98.00%)
Pages direct scanned 99546.06 ( +0.00%) 376407.52 ( +278.12%)
Pages direct reclaimed 62123.40 ( +0.00%) 289535.70 ( +366.06%)
Pages total scanned 610191.92 ( +0.00%) 379802.46 ( -37.76%)
Pages scanned kswapd % 76.36 ( +0.00%) 0.10 ( -98.58%)
Swap out 12057.54 ( +0.00%) 15022.98 ( +24.59%)
Swap in 209.16 ( +0.00%) 256.48 ( +22.52%)
File refaults 17701.64 ( +0.00%) 11765.40 ( -33.53%)

Huge page success rate is higher, allocation latencies are shorter and
more predictable.

Stealing (fallback) rate is drastically reduced. Notably, while the
vanilla kernel keeps doing fallbacks on an ongoing basis, the patched
kernel enters a steady state once the distribution of block types is
adequate for the workload. Steals over 50 runs:

VANILLA PATCHED
1504.0 227.0
1557.0 6.0
1391.0 13.0
1080.0 26.0
1057.0 40.0
1156.0 6.0
805.0 46.0
736.0 20.0
1747.0 2.0
1699.0 34.0
1269.0 13.0
1858.0 12.0
907.0 4.0
727.0 2.0
563.0 2.0
3094.0 2.0
10211.0 3.0
2621.0 1.0
5508.0 2.0
1060.0 2.0
538.0 3.0
5773.0 2.0
2199.0 0.0
3781.0 2.0
1387.0 1.0
4977.0 0.0
2865.0 1.0
1814.0 1.0
3739.0 1.0
6857.0 0.0
382.0 0.0
407.0 1.0
3784.0 0.0
297.0 0.0
298.0 0.0
6636.0 0.0
4188.0 0.0
242.0 0.0
9960.0 0.0
5816.0 0.0
354.0 0.0
287.0 0.0
261.0 0.0
140.0 1.0
2065.0 0.0
312.0 0.0
331.0 0.0
164.0 0.0
465.0 1.0
219.0 0.0

Mismatches are down too. Those count every time an allocation request
asks for one migratetype and gets another. This still happens in the
patched kernel due to non-stealing fallbacks, but it's quite rare and
follows the pattern of overall fallbacks - once the block type
distribution settles, mismatches cease as well:

VANILLA: PATCHED:
182602.0 268.0
135794.0 20.0
88619.0 19.0
95973.0 0.0
129590.0 0.0
129298.0 0.0
147134.0 0.0
230854.0 0.0
239709.0 0.0
137670.0 0.0
132430.0 0.0
65712.0 0.0
57901.0 0.0
67506.0 0.0
63565.0 4.0
34806.0 0.0
42962.0 0.0
32406.0 0.0
38668.0 0.0
61356.0 0.0
57800.0 0.0
41435.0 0.0
83456.0 0.0
65048.0 0.0
28955.0 0.0
47597.0 0.0
75117.0 0.0
55564.0 0.0
38280.0 0.0
52404.0 0.0
26264.0 0.0
37538.0 0.0
19671.0 0.0
30936.0 0.0
26933.0 0.0
16962.0 0.0
44554.0 0.0
46352.0 0.0
24995.0 0.0
35152.0 0.0
12823.0 0.0
21583.0 0.0
18129.0 0.0
31693.0 0.0
28745.0 0.0
33308.0 0.0
31114.0 0.0
35034.0 0.0
12111.0 0.0
24885.0 0.0

Compaction work is markedly reduced despite a better outcome.

In the vanilla kernel, reclaim seems to have been driven primarily by
watermark boosting that happens as a result of fallbacks. With those
all but eliminated, watermarks average lower and kswapd does less
work. The uptick in direct reclaim is because THP requests have to
fend for themselves more often - which is intended policy right
now. Aggregate reclaim activity is lowered significantly, though.

V3:
- fixed freelist type violations from non-atomic page isolation
updates (Zi Yan)
- fixed incorrect migratetype update ordering during merge (Vlastimil Babka)
- reject moving a zone-straddling block altogether (Vlastimil Babka)
- fixed freelist type violations from lockless migratetype lookups in
cornercase freeing paths (Vlastimil Babka)
- fixed erroneous WARN in the bulk freeing path that was intended to catch
mistakes in the now-removed pcpcache (Mike Kravetz)
- fixed typo in patch 1's changelog (Zi Yan)
- optimized migratetype lookup in free_unref_page_list() (Vlastimil Babka)
- batched vmstat updates in page merging hot path (Vlastimil Babka)
- rebased to mm-everything-2024-03-05-20-43 (v6.8-rc5+)

V2:
- dropped the get_pfnblock_migratetype() optimization
patchlet since somebody else beat me to it (thanks Zi)
- broke out pcp bypass fix since somebody else reported the bug:
https://lore.kernel.org/linux-mm/20230911181108.GA104295@xxxxxxxxxxx/
- fixed the CONFIG_UNACCEPTED_MEMORY build (lkp)
- rebased to v6.6-rc1

include/linux/mm.h | 18 +-
include/linux/page-isolation.h | 5 +-
include/linux/vmstat.h | 8 -
mm/debug_page_alloc.c | 12 +-
mm/internal.h | 9 -
mm/page_alloc.c | 650 +++++++++++++++++++++------------------
mm/page_isolation.c | 122 +++-----
7 files changed, 410 insertions(+), 414 deletions(-)

Based on mm-everything-2024-03-05-20-43.

[1] https://lore.kernel.org/lkml/20230418191313.268131-1-hannes@xxxxxxxxxxx/