On Wed, Feb 27, 2019 at 09:18:27PM -0500, Dennis Zhou wrote: > 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 > scan_hint. > > [1] https://lore.kernel.org/netdev/CANn89iKb_vW+LA-91RV=zuAqbNycPFUYW54w_S=KZ3HdcWPw6Q@xxxxxxxxxxxxxx/ > [2] https://lore.kernel.org/netdev/20181116154329.247947-1-edumazet@xxxxxxxxxx/ > [3] https://lore.kernel.org/netdev/vbfzhrj9smb.fsf@xxxxxxxxxxxx/ > > 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 Applied to percpu/for-5.2. Thanks, Dennis
