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

[Vlastimil Babka (SUSE)]

On 9/17/26 18:25, Tim Menninger wrote:
> It reproduces with slab_nomerge.
> 
> I booted the same kernel with:
> 
>     $ cat /proc/cmdline
>     ... slab_nomerge ...
> 
> Uninstrumented throughput remains in the high-45 to low-46 GB/s
> range, as before. During perf lock it was around 30 GB/s in these
> runs, versus roughly 25 GB/s previously.
> 
> The dominant lock measurements are essentially unchanged:
> 
>                          default             slab_nomerge
>     unpinned/node0
>       contentions        4,782,839             4,858,759
>       total wait          14.39 min             14.26 min
>       average wait       180.47 us             176.13 us
> 
>     node0/node0
>       contentions        4,582,012             4,686,729
>       total wait          14.13 min             14.34 min
>       average wait       185.05 us             183.53 us
> 
>     unpinned/balanced
>       contentions        4,572,196             4,544,677
>       total wait          12.81 min             12.56 min
>       average wait       168.14 us             165.84 us
> 
> The uninstrumented cmpxchg_double_fail deltas were:
> 
>                          default     slab_nomerge
>     unpinned/node0          7,471          8,892
>     node0/node0               713            457
>     unpinned/balanced       1,503          1,504
> 
> The CPU-utilization change while perf lock is active also remains.
> System-wide %idle for uninstrumented/instrumented windows was:
> 
>                            default                  slab_nomerge
>                        uninstr.   instr.         uninstr.   instr.
>     unpinned/node0      31.46%    3.60%           23.02%    2.15%
>     node0/node0         84.26%   12.07%           84.62%   11.85%
>     unpinned/balanced   66.86%   14.83%           66.58%   14.78%

I believe the weirdest part in the original report was how slub stat
counters such as free_fastpath etc got order of magnitude higher with
instrumented vs non-instrumented.
Did any of that change with slab_nomerge?

> Thanks,
> Tim