Re: [PATCH 07/34] mm, vmscan: make kswapd reclaim in terms of nodes

From: Srikar Dronamraju
Date: Mon Aug 29 2016 - 05:40:23 EST

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> Patch "mm: vmscan: Begin reclaiming pages on a per-node basis" started
> thinking of reclaim in terms of nodes but kswapd is still zone-centric. This
> patch gets rid of many of the node-based versus zone-based decisions.
>
> o A node is considered balanced when any eligible lower zone is balanced.
> This eliminates one class of age-inversion problem because we avoid
> reclaiming a newer page just because it's in the wrong zone
> o pgdat_balanced disappears because we now only care about one zone being
> balanced.
> o Some anomalies related to writeback and congestion tracking being based on
> zones disappear.
> o kswapd no longer has to take care to reclaim zones in the reverse order
> that the page allocator uses.
> o Most importantly of all, reclaim from node 0 with multiple zones will
> have similar aging and reclaiming characteristics as every
> other node.
>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

This patch seems to hurt FA_DUMP functionality. This behaviour is not
seen on v4.7 but only after this patch.

So when a kernel on a multinode machine with memblock_reserve() such
that most of the nodes have zero available memory, kswapd seems to be
consuming 100% of the time.

This is independent of CONFIG_DEFERRED_STRUCT_PAGE, i.e this problem is
seen even with parallel page struct initialization disabled.

top - 13:48:52 up 1:07, 3 users, load average: 15.25, 15.32, 21.18
Tasks: 11080 total, 16 running, 11064 sleeping, 0 stopped, 0 zombie
%Cpu(s): 0.0 us, 2.7 sy, 0.0 ni, 97.3 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
KiB Mem: 15929941+total, 8637824 used, 15843563+free, 2304 buffers
KiB Swap: 91898816 total, 0 used, 91898816 free. 1381312 cached Mem

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
10824 root 20 0 0 0 0 R 100.000 0.000 65:30.76 kswapd2
10837 root 20 0 0 0 0 R 100.000 0.000 65:31.17 kswapd15
10823 root 20 0 0 0 0 R 97.059 0.000 65:30.85 kswapd1
10825 root 20 0 0 0 0 R 97.059 0.000 65:31.10 kswapd3
10826 root 20 0 0 0 0 R 97.059 0.000 65:31.18 kswapd4
10827 root 20 0 0 0 0 R 97.059 0.000 65:31.08 kswapd5
10828 root 20 0 0 0 0 R 97.059 0.000 65:30.91 kswapd6
10829 root 20 0 0 0 0 R 97.059 0.000 65:31.17 kswapd7
10830 root 20 0 0 0 0 R 97.059 0.000 65:31.17 kswapd8
10831 root 20 0 0 0 0 R 97.059 0.000 65:31.18 kswapd9
10832 root 20 0 0 0 0 R 97.059 0.000 65:31.12 kswapd10
10833 root 20 0 0 0 0 R 97.059 0.000 65:31.19 kswapd11
10834 root 20 0 0 0 0 R 97.059 0.000 65:31.13 kswapd12
10835 root 20 0 0 0 0 R 97.059 0.000 65:31.09 kswapd13
10836 root 20 0 0 0 0 R 97.059 0.000 65:31.18 kswapd14
277155 srikar 20 0 16960 13760 3264 R 52.941 0.001 0:00.37 top

top - 13:48:55 up 1:07, 3 users, load average: 15.23, 15.32, 21.15
Tasks: 11080 total, 16 running, 11064 sleeping, 0 stopped, 0 zombie
%Cpu(s): 0.0 us, 1.0 sy, 0.0 ni, 99.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
KiB Mem: 15929941+total, 8637824 used, 15843563+free, 2304 buffers
KiB Swap: 91898816 total, 0 used, 91898816 free. 1381312 cached Mem

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
10836 root 20 0 0 0 0 R 100.000 0.000 65:33.39 kswapd14
10823 root 20 0 0 0 0 R 100.000 0.000 65:33.05 kswapd1
10824 root 20 0 0 0 0 R 100.000 0.000 65:32.96 kswapd2
10825 root 20 0 0 0 0 R 100.000 0.000 65:33.30 kswapd3
10826 root 20 0 0 0 0 R 100.000 0.000 65:33.38 kswapd4
10827 root 20 0 0 0 0 R 100.000 0.000 65:33.28 kswapd5
10828 root 20 0 0 0 0 R 100.000 0.000 65:33.11 kswapd6
10829 root 20 0 0 0 0 R 100.000 0.000 65:33.37 kswapd7
10830 root 20 0 0 0 0 R 100.000 0.000 65:33.37 kswapd8
10831 root 20 0 0 0 0 R 100.000 0.000 65:33.38 kswapd9
10832 root 20 0 0 0 0 R 100.000 0.000 65:33.32 kswapd10
10833 root 20 0 0 0 0 R 100.000 0.000 65:33.39 kswapd11
10834 root 20 0 0 0 0 R 100.000 0.000 65:33.33 kswapd12
10835 root 20 0 0 0 0 R 100.000 0.000 65:33.29 kswapd13
10837 root 20 0 0 0 0 R 100.000 0.000 65:33.37 kswapd15
277155 srikar 20 0 17536 14912 3264 R 9.091 0.001 0:00.57 top
1092 root rt 0 0 0 0 S 0.455 0.000 0:00.08 watchdog/178

Please see that there is no used swap space. However 15 kswapd threads
corresponding to 15 out of the 16 nodes are running full throttle. The
only node 0 has memory, other nodes memory is fully reserved.

git bisect output
I tried git bisect between v4.7 and v4.8-rc3 filtered to mm/vmscan.c

# bad: [d7f05528eedb047efe2288cff777676b028747b6] mm, vmscan: account for skipped pages as a partial scan
# good: [b1123ea6d3b3da25af5c8a9d843bd07ab63213f4] mm: balloon: use general non-lru movable page feature
git bisect start 'HEAD' 'b1123ea6' '--' 'mm/vmscan.c'
# bad: [c4a25635b60d08853a3e4eaae3ab34419a36cfa2] mm: move vmscan writes and file write accounting to the node
git bisect bad c4a25635b60d08853a3e4eaae3ab34419a36cfa2
# bad: [38087d9b0360987a6db46c2c2c4ece37cd048abe] mm, vmscan: simplify the logic deciding whether kswapd sleeps
git bisect bad 38087d9b0360987a6db46c2c2c4ece37cd048abe
# good: [b2e18757f2c9d1cdd746a882e9878852fdec9501] mm, vmscan: begin reclaiming pages on a per-node basis
git bisect good b2e18757f2c9d1cdd746a882e9878852fdec9501
# bad: [1d82de618ddde0f1164e640f79af152f01994c18] mm, vmscan: make kswapd reclaim in terms of nodes
git bisect bad 1d82de618ddde0f1164e640f79af152f01994c18
# good: [f7b60926ebc05944f73d93ffaf6690503b796a88] mm, vmscan: have kswapd only scan based on the highest requested zone
git bisect good f7b60926ebc05944f73d93ffaf6690503b796a88
# first bad commit: [1d82de618ddde0f1164e640f79af152f01994c18] mm, vmscan: make kswapd reclaim in terms of nodes

Here is perf top output on the kernel where kswapd is hogging cpu.

- 93.50% 0.01% [kernel] [k] kswapd
- kswapd
- 114.31% shrink_node
- 111.51% shrink_node_memcg
- pgdat_reclaimable
- 95.51% pgdat_reclaimable_pages
- 86.34% pgdat_reclaimable_pages
- 6.69% _find_next_bit.part.0
- 2.47% find_next_bit
- 14.46% pgdat_reclaimable
1.13% _find_next_bit.part.0
+ 0.30% find_next_bit
- 2.38% shrink_slab
- super_cache_count
- 0
- __list_lru_count_one.isra.1
_raw_spin_lock
- 28.04% pgdat_reclaimable
- 23.97% pgdat_reclaimable_pages
- 21.66% pgdat_reclaimable_pages
- 1.69% _find_next_bit.part.0
0.63% find_next_bit
- 3.70% pgdat_reclaimable
0.29% _find_next_bit.part.0
- 16.33% zone_balanced
- zone_watermark_ok_safe
- 14.86% zone_watermark_ok_safe
1.15% _find_next_bit.part.0
0.31% find_next_bit
- 2.72% prepare_kswapd_sleep
- zone_balanced
- zone_watermark_ok_safe
zone_watermark_ok_safe
- 80.72% 10.51% [kernel] [k] pgdat_reclaimable
- 140.49% pgdat_reclaimable
- 138.40% pgdat_reclaimable_pages
- 125.10% pgdat_reclaimable_pages
- 9.71% _find_next_bit.part.0
- 3.59% find_next_bit
1.64% _find_next_bit.part.0
+ 0.44% find_next_bit
- 21.03% ret_from_kernel_thread
kthread
- kswapd
- 16.75% shrink_node
shrink_node_memcg
- 4.28% pgdat_reclaimable
pgdat_reclaimable
- 69.17% 62.48% [kernel] [k] pgdat_reclaimable_pages
- 145.91% ret_from_kernel_thread
kthread
- 15.61% pgdat_reclaimable_pages
- 11.33% _find_next_bit.part.0
- 4.19% find_next_bit
- 66.18% 0.01% [kernel] [k] shrink_node
- shrink_node
- 157.54% shrink_node_memcg
- pgdat_reclaimable
- 134.94% pgdat_reclaimable_pages
- 121.99% pgdat_reclaimable_pages
- 9.46% _find_next_bit.part.0
- 3.49% find_next_bit
- 20.44% pgdat_reclaimable
1.59% _find_next_bit.part.0
+ 0.42% find_next_bit
- 3.37% shrink_slab
- super_cache_count
- 0
- __list_lru_count_one.isra.1
_raw_spin_lock
- 64.56% 0.03% [kernel] [k] shrink_node_memcg
- shrink_node_memcg
- pgdat_reclaimable
- 138.31% pgdat_reclaimable_pages
- 125.04% pgdat_reclaimable_pages
- 9.69% _find_next_bit.part.0
- 3.58% find_next_bit
- 20.95% pgdat_reclaimable
1.63% _find_next_bit.part.0
+ 0.43% find_next_bit
53.73% 0.00% [kernel] [k] kthread
53.73% 0.00% [kernel] [k] ret_from_kernel_thread
- 11.04% 10.04% [kernel] [k] zone_watermark_ok_safe
- 146.80% ret_from_kernel_thread
kthread
- kswapd
- 125.81% zone_balanced
zone_watermark_ok_safe
- 20.97% prepare_kswapd_sleep
zone_balanced
zone_watermark_ok_safe
- 14.55% zone_watermark_ok_safe
11.38% _find_next_bit.part.0
3.06% find_next_bit
- 11.03% 0.00% [kernel] [k] zone_balanced
- zone_balanced
- zone_watermark_ok_safe
145.84% zone_watermark_ok_safe
11.31% _find_next_bit.part.0
3.04% find_next_bit

--
Thanks and Regards
Srikar Dronamraju

Next message: Daniel Wagner: "Re: [PATCH v3 2/3] tracing: Add trace_irqsoff tracepoints"
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Next in thread: Mel Gorman: "Re: [PATCH 07/34] mm, vmscan: make kswapd reclaim in terms of nodes"
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