introduce percpu block scan_hint
Status from this message: Applied to percpu/for-5.2.
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8 of 12 patches have a Reviewed-by or Acked-by.
| Patch | Peng Fan |
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| 1/12 | R-b |
| 2/12 | R-b |
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| 4/12 | R-b |
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| 6/12 | R-b |
| 7/12 | R-b |
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| 10/12 | R-b |
| 11/12 | R-b |
| 12/12 | R-b |
Trailers given in replies are filled; those carried in the patches from earlier versions are outlined with dashes.
Cover letter [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 | 39sThe 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 scan_hint.
[1] https://lore.kernel.org/netdev/CANn89iKb_vW+LA-91RV=zuAqbNycPFUYW54w_S=KZ3HdcWPw6Q@mail.gmail.com/
[2] https://lore.kernel.org/netdev/[email protected]/
[3] https://lore.kernel.org/netdev/[email protected]/
This patchset contains the following 12 patches:
0001-percpu-update-free-path-with-correct-new-free-region.patch 0002-percpu-do-not-search-past-bitmap-when-allocating-an-.patch 0003-percpu-introduce-helper-to-determine-if-two-regions-.patch 0004-percpu-manage-chunks-based-on-contig_bits-instead-of.patch 0005-percpu-relegate-chunks-unusable-when-failing-small-a.patch 0006-percpu-set-PCPU_BITMAP_BLOCK_SIZE-to-PAGE_SIZE.patch 0007-percpu-add-block-level-scan_hint.patch 0008-percpu-remember-largest-area-skipped-during-allocati.patch 0009-percpu-use-block-scan_hint-to-only-scan-forward.patch 0010-percpu-make-pcpu_block_md-generic.patch 0011-percpu-convert-chunk-hints-to-be-based-on-pcpu_block.patch 0012-percpu-use-chunk-scan_hint-to-skip-some-scanning.patch
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: set PCPU_BITMAP_BLOCK_SIZE to PAGE_SIZE
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(-)
Thanks,
Dennis