Regression in mobility grouping?

From: Johannes Weiner
Date: Tue Sep 27 2016 - 21:42:10 EST

Hi guys,

we noticed what looks like a regression in page mobility grouping
during an upgrade from 3.10 to 4.0. Identical machines, workloads, and
uptime, but /proc/pagetypeinfo on 3.10 looks like this:

Number of blocks type Unmovable Reclaimable Movable Reserve Isolate
Node 1, zone Normal 815 433 31518 2 0

and on 4.0 like this:

Number of blocks type Unmovable Reclaimable Movable Reserve CMA Isolate
Node 1, zone Normal 3880 3530 25356 2 0 0

4.0 is either polluting pageblocks more aggressively at allocation, or
is not able to make pageblocks movable again when the reclaimable and
unmovable allocations are released. Invoking compaction manually
(/proc/sys/vm/compact_memory) is not bringing them back, either.

The problem we are debugging is that these machines have a very high
rate of order-3 allocations (fdtable during fork, network rx), and
after the upgrade allocstalls have increased dramatically. I'm not
entirely sure this is the same issue, since even order-0 allocations
are struggling, but the mobility grouping in itself looks problematic.

I'm still going through the changes relevant to mobility grouping in
that timeframe, but if this rings a bell for anyone, it would help. I
hate blaming random patches, but these caught my eye:

9c0415e mm: more aggressive page stealing for UNMOVABLE allocations
3a1086f mm: always steal split buddies in fallback allocations
99592d5 mm: when stealing freepages, also take pages created by splitting buddy page

The changelog states that by aggressively stealing split buddy pages
during a fallback allocation we avoid subsequent stealing. But since
there are generally more movable/reclaimable pages available, and so
less falling back and stealing freepages on behalf of movable, won't
this mean that we could expect exactly that result - growing numbers
of unmovable blocks, while rarely stealing them back in movable alloc
fallbacks? And the expansion of !MOVABLE blocks would over time make
compaction less and less effective too, seeing as it doesn't consider
anything !MOVABLE suitable migration targets?

Attached are the full /proc/pagetypeinfo and /proc/buddyinfo from both
kernels on machines with similar uptimes and directly after invoking
compaction. As you can see, the buddy lists are much more fragmented
on 4.0, with unmovable/reclaimable allocations polluting more blocks.

Any thoughts on this would be greatly appreciated. I can test patches.

Thanks!
Node 0, zone DMA 0 0 0 1 2 1 1 0 1 1 3
Node 0, zone DMA32 1062 1491 1641 1725 478 77 5 1 0 0 0
Node 0, zone Normal 10436 16239 5903 696 130 729 1298 550 109 0 0
Node 1, zone Normal 5956 15 5 28 11 8 2 0 0 0 0
Node 0, zone DMA 1 1 0 1 1 1 1 0 0 1 3
Node 0, zone DMA32 9462 6148 2297 27 0 0 0 0 0 0 0
Node 0, zone Normal 130376 36589 3777 94 1 0 0 0 0 0 0
Node 1, zone Normal 190988 77269 3896 332 6 0 0 0 0 0 0
Page block order: 9
Pages per block: 512

Free pages count per migrate type at order 0 1 2 3 4 5 6 7 8 9 10
Node 0, zone DMA, type Unmovable 0 0 0 1 2 1 1 0 1 0 0
Node 0, zone DMA, type Reclaimable 0 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA, type Movable 0 0 0 0 0 0 0 0 0 0 3
Node 0, zone DMA, type Reserve 0 0 0 0 0 0 0 0 0 1 0
Node 0, zone DMA, type Isolate 0 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32, type Unmovable 488 221 286 0 0 0 0 0 0 0 0
Node 0, zone DMA32, type Reclaimable 1 725 741 0 0 0 0 0 0 0 0
Node 0, zone DMA32, type Movable 431 1735 1073 105 0 0 0 0 0 0 0
Node 0, zone DMA32, type Reserve 0 0 0 17 1 0 0 0 0 0 0
Node 0, zone DMA32, type Isolate 0 0 0 0 0 0 0 0 0 0 0
Node 0, zone Normal, type Unmovable 1922 16 1 19 0 0 0 0 0 0 0
Node 0, zone Normal, type Reclaimable 4549 0 0 1 0 0 0 0 0 0 0
Node 0, zone Normal, type Movable 3 0 2 3 0 0 0 0 0 0 0
Node 0, zone Normal, type Reserve 0 0 1 22 1 2 1 0 0 0 0
Node 0, zone Normal, type Isolate 0 0 0 0 0 0 0 0 0 0 0

Number of blocks type Unmovable Reclaimable Movable Reserve Isolate
Node 0, zone DMA 1 0 6 1 0
Node 0, zone DMA32 96 21 898 1 0
Node 0, zone Normal 1105 497 30140 2 0
Page block order: 9
Pages per block: 512

Free pages count per migrate type at order 0 1 2 3 4 5 6 7 8 9 10
Node 1, zone Normal, type Unmovable 5746 3 0 0 0 0 0 0 0 0 0
Node 1, zone Normal, type Reclaimable 53 10 0 0 0 0 0 0 0 0 0
Node 1, zone Normal, type Movable 1 2919 1131 0 0 0 0 0 0 0 0
Node 1, zone Normal, type Reserve 0 0 0 0 0 1 2 0 0 0 0
Node 1, zone Normal, type Isolate 0 0 0 0 0 0 0 0 0 0 0

Number of blocks type Unmovable Reclaimable Movable Reserve Isolate
Node 1, zone Normal 868 433 31465 2 0
Page block order: 9
Pages per block: 512

Free pages count per migrate type at order 0 1 2 3 4 5 6 7 8 9 10
Node 0, zone DMA, type Unmovable 1 1 0 1 1 1 1 0 0 0 0
Node 0, zone DMA, type Reclaimable 0 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA, type Movable 0 0 0 0 0 0 0 0 0 0 3
Node 0, zone DMA, type Reserve 0 0 0 0 0 0 0 0 0 1 0
Node 0, zone DMA, type CMA 0 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA, type Isolate 0 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32, type Unmovable 2717 4401 895 1 0 0 0 0 0 0 0
Node 0, zone DMA32, type Reclaimable 6004 1784 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32, type Movable 1 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32, type Reserve 0 0 3 0 0 0 0 0 0 0 0
Node 0, zone DMA32, type CMA 0 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32, type Isolate 0 0 0 0 0 0 0 0 0 0 0
Node 0, zone Normal, type Unmovable 115050 40237 3785 0 0 0 0 0 0 0 0
Node 0, zone Normal, type Reclaimable 51921 14109 659 0 0 0 0 0 0 0 0
Node 0, zone Normal, type Movable 1 41954 984 0 0 0 0 0 0 0 0
Node 0, zone Normal, type Reserve 0 0 0 0 0 0 0 0 0 0 0
Node 0, zone Normal, type CMA 0 0 0 0 0 0 0 0 0 0 0
Node 0, zone Normal, type Isolate 0 0 0 0 0 0 0 0 0 0 0

Number of blocks type Unmovable Reclaimable Movable Reserve CMA Isolate
Node 0, zone DMA 1 0 6 1 0 0
Node 0, zone DMA32 620 184 211 1 0 0
Node 0, zone Normal 6634 3757 21351 2 0 0
Page block order: 9
Pages per block: 512

Free pages count per migrate type at order 0 1 2 3 4 5 6 7 8 9 10
Node 1, zone Normal, type Unmovable 58723 366 15 6 0 0 0 0 0 0 0
Node 1, zone Normal, type Reclaimable 163 74 5 1 0 0 0 0 0 0 0
Node 1, zone Normal, type Movable 1217 283 10 0 0 0 0 0 0 0 0
Node 1, zone Normal, type Reserve 0 0 0 3 0 0 0 0 0 0 0
Node 1, zone Normal, type CMA 0 0 0 0 0 0 0 0 0 0 0
Node 1, zone Normal, type Isolate 0 0 0 0 0 0 0 0 0 0 0

Number of blocks type Unmovable Reclaimable Movable Reserve CMA Isolate
Node 1, zone Normal 3903 3518 25345 2 0 0
Node 0, zone DMA -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 -1.000
Node 0, zone DMA32 -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 0.981 0.991 0.996 0.998
Node 0, zone Normal -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 0.977 0.989 0.995 0.998
Node 1, zone Normal -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 0.982 0.991 0.996 0.998
Node 0, zone DMA -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 -1.000
Node 0, zone DMA32 -1.000 -1.000 -1.000 -1.000 0.868 0.934 0.967 0.984 0.992 0.996 0.998
Node 0, zone Normal -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 0.972 0.986 0.993 0.997 0.999
Node 1, zone Normal -1.000 -1.000 -1.000 -1.000 -1.000 -1.000 0.972 0.986 0.993 0.997 0.999