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Subject[PATCH 00/12] introduce percpu block scan_hint
Hi everyone,

It was reported a while [1] that an increase in allocation alignment
requirement [2] caused the percpu memory allocator to do significantly
more work.

After spending quite a bit of time diving into it, it seems the crux was
the following:
1) chunk management by free_bytes caused allocations to scan over
chunks that could not fit due to fragmentation
2) per block fragmentation required scanning from an early first_free
bit causing allocations to repeat work

This series introduces a scan_hint for pcpu_block_md and merges the
paths used to manage the hints. The scan_hint represents the largest
known free area prior to the contig_hint. There are some caveats to
this. First, it may not necessarily be the largest area as we do partial
updates based on freeing of regions and failed scanning in
pcpu_alloc_area(). Second, if contig_hint == scan_hint, then
scan_hint_start > contig_hint_start is possible. This is necessary
for scan_hint discovery when refreshing the hint of a block.

A necessary change is to enforce a block to be the size of a page. This
let's the management of nr_empty_pop_pages to be done by breaking and
making full contig_hints in the hint update paths. Prior, this was done
by piggy backing off of refreshing the chunk contig_hint as it performed
a full scan and counting empty full pages.

The following are the results found using the workload provided in [3].

branch | time
5.0-rc7 | 69s
[2] reverted | 44s
scan_hint | 39s

The times above represent the approximate average across multiple runs.
I tested based on a basic 1M 16-byte allocation pattern with no
alignment requirement and times did not differ between 5.0-rc7 and


This patchset contains the following 12 patches:

0001 fixes an issue where the chunk contig_hint was being updated
improperly with the new region's starting offset and possibly differing
contig_hint. 0002 fixes possibly scanning pass the end of the bitmap.
0003 introduces a helper to do region overlap comparison. 0004 switches
to chunk management by contig_hint rather than free_bytes. 0005 moves
chunks that fail to allocate to the empty block list to prevent excess
scanning with of chunks with small contig_hints and poor alignment.
0006 introduces the constraint PCPU_BITMAP_BLOCK_SIZE == PAGE_SIZE and
modifies nr_empty_pop_pages management to be a part of the hint updates.
0007-0009 introduces percpu block scan_hint. 0010 makes pcpu_block_md
generic so chunk hints can be managed as a pcpu_block_md responsible
for more bits. 0011-0012 add chunk scan_hints.

This patchset is on top of percpu#master a3b22b9f11d9.

diffstats below:

Dennis Zhou (12):
percpu: update free path with correct new free region
percpu: do not search past bitmap when allocating an area
percpu: introduce helper to determine if two regions overlap
percpu: manage chunks based on contig_bits instead of free_bytes
percpu: relegate chunks unusable when failing small allocations
percpu: add block level scan_hint
percpu: remember largest area skipped during allocation
percpu: use block scan_hint to only scan forward
percpu: make pcpu_block_md generic
percpu: convert chunk hints to be based on pcpu_block_md
percpu: use chunk scan_hint to skip some scanning

include/linux/percpu.h | 12 +-
mm/percpu-internal.h | 15 +-
mm/percpu-km.c | 2 +-
mm/percpu-stats.c | 5 +-
mm/percpu.c | 547 +++++++++++++++++++++++++++++------------
5 files changed, 404 insertions(+), 177 deletions(-)


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