Re: [SLUB] nfs_page cmpxchg_double_fail and perf lock perturbation on dual-socket NFS/RDMA

[Tim Menninger]

> Did any of that change with slab_nomerge?

No. The large change in the SLUB counters still reproduced with
slab_nomerge when using perf lock record. For example:

                         uninstrumented       perf lock record
    unpinned/node0
      free_fastpath          75,366,487          646,378,908
      free_slowpath          41,594,255          259,704,037

    node0/node0
      free_fastpath         119,705,541          440,896,371
      free_slowpath               6,344            6,679,339

    unpinned/balanced
      free_fastpath          59,404,047          384,715,890
      free_slowpath          60,602,379          324,936,304

> ... unless perf lock was writing data to an NFS filesystem?

It was not. The perf data file was on the local root filesystem:

    $ df -T linux-mm/
    Filesystem                        Type
    /dev/mapper/ubuntu--vg-ubuntu--lv ext4

> You can use the BPF version of perf lock to check lock contention like
> below. (it only work with 'contention' subcommand.)
>
> $ sudo perf lock con -ab sleep 10
>
> or
>
> $ sudo perf lock con -ab -E 5 sleep 10

I reran the three placement cases using the BPF version, aggregating
by lock address:

    sudo perf lock con -abl -E 5 -- sleep 10

I ran only perf lock and mpstat concurrently.

The BPF version is substantially less disruptive. Comparing adjacent
10-second uninstrumented and instrumented windows:

                         uninstrumented       BPF perf lock

    unpinned/node0
      throughput            45.5 GB/s             43-44 GB/s
      free_fastpath          76,217,260        118,511,381
      free_slowpath          39,962,005         22,717,811
      cmpxchg_double_fail         9,299              9,587
      system idle               22.74%             10.61%

    node0/node0
      throughput            46.5 GB/s             46.5 GB/s
      free_fastpath         119,739,790        148,686,356
      free_slowpath               5,686              8,791
      cmpxchg_double_fail         1,483              3,170
      system idle               83.33%             82.42%

    unpinned/balanced
      throughput            46.5 GB/s             46.5 GB/s
      free_fastpath          59,341,738         92,824,030
      free_slowpath          60,000,396         60,658,896
      cmpxchg_double_fail         1,742             23,772
      system idle               68.27%             50.03%

The per-lock-address BPF results are also quite different from the
perf lock record results:

    unpinned/node0:
      contended    total wait    avg wait     address
      8,345,025     12.27 min     88.23 us    ff3ad60e50e85100

    node0/node0:
      contended    total wait    avg wait     address
      6,461,976     30.70 sec      4.75 us    ff3ad58ecf36ac80

    unpinned/balanced:
      contended    total wait    avg wait     address
      6,592,568      6.50 min     59.18 us    ff3ad60e50e85100
        143,397    666.98 ms       4.65 us    ff3ad58ecf36ac80

perf identifies these as kmem_cache_node spinlocks.