Resend: Report: Performance regression from ib_umem_get on zone device pages

[jane.chu@oracle.com]

Resend due to a serious typo.

On 4/23/2025 12:21 PM, jane.chu@oracle.com wrote:
> Hi,
> 
> I recently looked at an mr cache registration regression issue that 
> follows device-dax backed mr memory, not system RAM backed mr memory.
> 
> It boils down to
>    1567b49d1a40 lib/scatterlist: add check when merging zone device pages
>    [PATCH v11 5/9] lib/scatterlist: add check when merging zone device 
> pages
>    https://lore.kernel.org/all/20221021174116.7200-6-logang@deltatee.com/
> 
> that went into v6.2-rc1.
> 
> The line that introduced the regression is
>    ib_uverbs_reg_mr
>      mlx5_ib_reg_user_mr
>        ib_umem_get
>          sg_alloc_append_table_from_pages
>            pages_are_mergeable
>              zone_device_pages_have_same_pgmap(a,b)
>                return a->pgmap == b->pgmap               <-------
> 
> Sub "return a->pgmap == b->pgmap" with "return true" purely as an 
> experiment and the regression reliably went away.
> 
> So this looks like a case of CPU cache thrashing, but I don't know how to 
> fix it. Could someone help address the issue?  I'd be happy to help 
> verifying.
> 
> My test system is a two-socket bare metal Intel(R) Xeon(R) Platinum 
> 8352Y with with 12 Intel NVDIMMs installed.
> 
> # lscpu
> Architecture:        x86_64
> CPU op-mode(s):      32-bit, 64-bit
> Model name:          Intel(R) Xeon(R) Platinum 8352Y CPU @ 2.20GHz
> L1d cache:           48K        <----
> L1i cache:           32K
> L2 cache:            1280K
> L3 cache:            49152K
> NUMA node0 CPU(s):   0-31,64-95
> NUMA node1 CPU(s):   32-63,96-127
> 
> # cat /proc/meminfo
> MemTotal:       263744088 kB
> MemFree:        252151828 kB
> MemAvailable:   251806008 kB
> 
> There are 12 device-dax instances configured exactly the same -
> # ndctl list -m devdax | egrep -m 1 'map'
>      "map":"mem",
> # ndctl list -m devdax | egrep -c 'map'
> 12
> # ndctl list -m devdax
> [
>    {
>      "dev":"namespace1.0",
>      "mode":"devdax",
>      "map":"mem",
>      "size":135289372672,
>      "uuid":"a67deda8-e5b3-4a6e-bea2-c1ebdc0fd996",
>      "chardev":"dax1.0",
>      "align":2097152
>    },
> [..]
> 
> The system is idle unless running mr registration test. The test 
> attempts to register 61440 mrs by 64 threads in parallel, each mr is 2MB 
> and is backed by device-dax memory.
> 
> The flow of a single test run:
>    1. reserve virtual address space for (61440 * 2MB) via mmap with 
> PROT_NONE and MAP_ANONYMOUS | MAP_NORESERVE| MAP_PRIVATE
>    2. mmap ((61440 * 2MB) / 12) from each of the 12 device-dax to the 
> reserved virtual address space sequentially to form a continual VA space
>    3. touch the entire mapped memory page by page
>    4. take timestamp,
>       create 40 pthreads, each thread registers (61440 / 40) mrs via 
> ibv_reg_mr(),
>       take another timestamp after pthread_join
>    5. wait 10 seconds
>    6. repeat step 4 except for deregistration via ibv_dereg_mr()
>    7. tear down everything
> 
> I hope the above description is helpful as I am not at liberty to share 
> the test code.
> 
> Here is the highlight from perfdiff comparing the culprit(PATCH 5/9) 
> against the baseline(PATCH 4/9).
> 
> baseline = 49580e690755 block: add check when merging zone device pages
> culprit  = 1567b49d1a40 lib/scatterlist: add check when merging zone 
> device pages
> 
> # Baseline  Delta Abs  Shared Object              Symbol
> # ........  .........  ......................... ............................................................
> #
>      26.53%    -19.46%  [kernel.kallsyms]          [k] follow_page_mask
>      49.15%    +11.56%  [kernel.kallsyms]          [k] 
> native_queued_spin_lock_slowpath
>                 +1.38%  [kernel.kallsyms]          [k] 
> pages_are_mergeable       <----
>                 +0.82%  [kernel.kallsyms]          [k] 
> __rdma_block_iter_next
>       0.74%     +0.68%  [kernel.kallsyms]          [k] osq_lock
>                 +0.56%  [kernel.kallsyms]          [k] 
> mlx5r_umr_update_mr_pas
>       2.25%     +0.49%  [kernel.kallsyms]          [k] 
> follow_pmd_mask.isra.0
>       1.92%     +0.37%  [kernel.kallsyms]          [k] _raw_spin_lock
>       1.13%     +0.35%  [kernel.kallsyms]          [k] __get_user_pages
> 
> With baseline, per mr registration takes ~2950 nanoseconds, +- 50ns,
> with culprit, per mr registration takes ~6850 nanoseconds, +- 50ns.
> 
> Regards,
> -jane