On Mon, 29 Mar 2021 13:06:42 +0100 Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> wrote: > This series requires patches in Andrew's tree so the series is also > available at > > git://git.kernel.org/pub/scm/linux/kernel/git/mel/linux.git mm-percpu-local_lock-v1r15 > > tldr: Jesper and Chuck, it would be nice to verify if this series helps > the allocation rate of the bulk page allocator. RT people, this > *partially* addresses some problems PREEMPT_RT has with the page > allocator but it needs review. I've run a new micro-benchmark[1] which shows: (CPU: Intel(R) Xeon(R) CPU E5-1650 v4 @ 3.60GHz) BASELINE single_page alloc+put: 194 cycles(tsc) 54.106 ns LIST variant: time_bulk_page_alloc_free_list: step=bulk size Per elem: 200 cycles(tsc) 55.667 ns (step:1) Per elem: 143 cycles(tsc) 39.755 ns (step:2) Per elem: 132 cycles(tsc) 36.758 ns (step:3) Per elem: 128 cycles(tsc) 35.795 ns (step:4) Per elem: 123 cycles(tsc) 34.339 ns (step:8) Per elem: 120 cycles(tsc) 33.396 ns (step:16) Per elem: 118 cycles(tsc) 32.806 ns (step:32) Per elem: 115 cycles(tsc) 32.169 ns (step:64) Per elem: 116 cycles(tsc) 32.258 ns (step:128) ARRAY variant: time_bulk_page_alloc_free_array: step=bulk size Per elem: 195 cycles(tsc) 54.225 ns (step:1) Per elem: 127 cycles(tsc) 35.492 ns (step:2) Per elem: 117 cycles(tsc) 32.643 ns (step:3) Per elem: 111 cycles(tsc) 30.992 ns (step:4) Per elem: 106 cycles(tsc) 29.606 ns (step:8) Per elem: 102 cycles(tsc) 28.532 ns (step:16) Per elem: 99 cycles(tsc) 27.728 ns (step:32) Per elem: 98 cycles(tsc) 27.252 ns (step:64) Per elem: 97 cycles(tsc) 27.090 ns (step:128) [1] https://github.com/xdp-project/xdp-project/blob/master/areas/mem/page_pool06_alloc_pages_bulk.org#micro-benchmark-page_bench04_bulk-local_lock-v1r15 This should be seen in comparison with the older micro-benchmark[2] done on branch mm-bulk-rebase-v5r9. BASELINE single_page alloc+put: Per elem: 199 cycles(tsc) 55.472 ns LIST variant: time_bulk_page_alloc_free_list: step=bulk size Per elem: 206 cycles(tsc) 57.478 ns (step:1) Per elem: 154 cycles(tsc) 42.861 ns (step:2) Per elem: 145 cycles(tsc) 40.536 ns (step:3) Per elem: 142 cycles(tsc) 39.477 ns (step:4) Per elem: 142 cycles(tsc) 39.610 ns (step:8) Per elem: 137 cycles(tsc) 38.155 ns (step:16) Per elem: 135 cycles(tsc) 37.739 ns (step:32) Per elem: 134 cycles(tsc) 37.282 ns (step:64) Per elem: 133 cycles(tsc) 36.993 ns (step:128) ARRAY variant: time_bulk_page_alloc_free_array: step=bulk size Per elem: 202 cycles(tsc) 56.383 ns (step:1) Per elem: 144 cycles(tsc) 40.047 ns (step:2) Per elem: 134 cycles(tsc) 37.339 ns (step:3) Per elem: 128 cycles(tsc) 35.578 ns (step:4) Per elem: 120 cycles(tsc) 33.592 ns (step:8) Per elem: 116 cycles(tsc) 32.362 ns (step:16) Per elem: 113 cycles(tsc) 31.476 ns (step:32) Per elem: 110 cycles(tsc) 30.633 ns (step:64) Per elem: 110 cycles(tsc) 30.596 ns (step:128) [2] https://github.com/xdp-project/xdp-project/blob/master/areas/mem/page_pool06_alloc_pages_bulk.org#micro-benchmark-page_bench04_bulk This new patchset does show some improvements in the micro-benchmark. > The PCP (per-cpu page allocator in page_alloc.c) share locking requirements > with vmstat which is inconvenient and causes some issues. Possibly because > of that, the PCP list and vmstat share the same per-cpu space meaning that > it's possible that vmstat updates dirty cache lines holding per-cpu lists > across CPUs unless padding is used. The series splits that structure and > separates the locking. > > Second, PREEMPT_RT considers the following sequence to be unsafe > as documented in Documentation/locking/locktypes.rst > > local_irq_disable(); > spin_lock(&lock); > > The pcp allocator has this sequence for rmqueue_pcplist (local_irq_save) > -> __rmqueue_pcplist -> rmqueue_bulk (spin_lock). This series explicitly > separates the locking requirements for the PCP list (local_lock) and stat > updates (irqs disabled). Once that is done, the length of time IRQs are > disabled can be reduced and in some cases, IRQ disabling can be replaced > with preempt_disable. > > After that, it was very obvious that zone_statistics in particular has way > too much overhead and leaves IRQs disabled for longer than necessary. It > has perfectly accurate counters requiring IRQs be disabled for parallel > RMW sequences when inaccurate ones like vm_events would do. The series > makes the NUMA statistics (NUMA_HIT and friends) inaccurate counters that > only require preempt be disabled. > > Finally the bulk page allocator can then do all the stat updates in bulk > with IRQs enabled which should improve the efficiency of the bulk page > allocator. Technically, this could have been done without the local_lock > and vmstat conversion work and the order simply reflects the timing of > when different series were implemented. > > No performance data is included because despite the overhead of the > stats, it's within the noise for most workloads but Jesper and Chuck may > observe a significant different with the same tests used for the bulk > page allocator. The series is more likely to be interesting to the RT > folk in terms of slowing getting the PREEMPT tree into mainline. I've try to run some longer packet benchmarks later. A quick test showed performance was within same range, and slightly better. The perf report and objdump, did reveal that code layout prefers the label "failed:" as the primary code path, which should only be used for single page allocs, which is kind of weird. (But as performance is the same or slightly better, I will not complain). > drivers/base/node.c | 18 +-- > include/linux/mmzone.h | 29 +++-- > include/linux/vmstat.h | 65 ++++++----- > mm/mempolicy.c | 2 +- > mm/page_alloc.c | 173 ++++++++++++++++------------ > mm/vmstat.c | 254 +++++++++++++++-------------------------- > 6 files changed, 254 insertions(+), 287 deletions(-) -- Best regards, Jesper Dangaard Brouer MSc.CS, Principal Kernel Engineer at Red Hat LinkedIn: http://www.linkedin.com/in/brouer
