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

[Harry Yoo]

... now I realize we need to add PERFORMANCE EVENTS SUBSYSTEM folks 
as well ;-)

Hmm, sounds like perf lock is somehow triggering slab allocations
and interfering the workload.

It could be because SLUB is merging nfs_page cache with some other
cache that perf uses.

Could you please check if it reproduces with slab_nomerge kernel
parameter?

On Thu, Sep 17, 2026 at 02:46:24PM +0100, Harry Yoo wrote:
> Hi Tim and Chuck, thanks for reporting this to linux-mm.
> Will take a look at this but let me Cc SLAB ALLOCATOR folks here first.
> 
> -- 
> Cheers,
> Harry / Hyeonggon
> 
> On Wed, Sep 16, 2026 at 11:22:27PM +0000, Tim Menninger wrote:
> > Chuck Lever suggested I bring this to linux-mm after we found some SLUB
> > measurements we could not account for while investigating an NFS/RDMA
> > throughput regression.
> > 
> > The original NFS discussion is here for context:
> > 
> >     https://lore.kernel.org/linux-nfs/0e2f9688-097c-4bb9-a7ec-82b41bdb3653@slotpi15m67/
> > 
> > The NFS regression itself has been separated from this issue. What
> > remains interesting here is the behavior of the nfs_page slab cache on
> > this machine, particularly the measurements with and without perf lock.
> > 
> > The system is:
> > 
> >     Intel Xeon Silver 4516Y+
> >     2 sockets
> >     24 cores/socket
> >     2 threads/core
> >     96 logical CPUs
> > 
> >     NUMA node0 CPUs: 0-23,48-71
> >     NUMA node1 CPUs: 24-47,72-95
> >
> > The workload is a high-throughput NFS/RDMA direct-read workload using
> > 1 MiB I/O, 160 threads, and iodepth 64.
> > 
> > The relevant debug options are all disabled:
> > 
> >     # CONFIG_KASAN is not set
> >     # CONFIG_PROVE_LOCKING is not set
> >     # CONFIG_LOCK_STAT is not set
> >     # CONFIG_DEBUG_SPINLOCK is not set
> >     # CONFIG_DEBUG_LIST is not set
> > 
> > All measurements below were collected on the unpatched base kernel:
> > 
> >     $ git rev-parse HEAD
> >     940de590b839f71d6dc846160534bf202401b8b7
> > 
> >     $ uname -r
> >     7.3.0-rc1-mainline+
> > 
> > The initial observation was a high apparent contention rate on the
> > nfs_page slab's list_lock. In 10-second perf-lock captures I was seeing
> > roughly 4.5M contended acquisitions and about 170-185 us of average
> > reported wait.
> > 
> > Chuck reproduced a similar acquisition rate on a single-node EPYC
> > system, but saw only about 7 ns average wait and fewer than 100
> > cmpxchg_double_fail events over a corresponding interval. He suggested
> > checking cmpxchg_double_fail because __slab_free() drops list_lock and
> > retries when the freelist cmpxchg fails.
> >
> > I repeated the measurements in three placement configurations:
> > 
> >     A. workload unpinned, CQs all on node0
> >     B. workload pinned to node0, CQs all on node0
> >     C. workload unpinned, CQs balanced across the nodes
> > 
> > Without perf lock, throughput is similar in all three:
> > 
> >     A. unpinned / CQs node0:       ~45.5 GB/s
> >     B. node0 pinned / CQs node0:   ~45.7 GB/s
> >     C. unpinned / balanced CQs:    ~45.5 GB/s
> > 
> > During the perf-lock captures, throughput is approximately 25 GB/s.
> > 
> > For each instrumented 10-second window I ran:
> > 
> >     sudo perf lock record -a -g -o "$D/perf-locks.data" -- sleep 10
> >     mpstat -P ALL 1 10
> > 
> > Before and after the same window I sampled the counters under:
> > 
> >     /sys/kernel/slab/nfs_page/
> > 
> > I also collected separate 10-second counter and mpstat windows under
> > the same workload configurations without perf lock.
> > 
> > The resulting slab counter deltas were:
> > 
> >                             A               B                 C
> >                      unpinned/node0    node0/node0   unpinned/balanced
> > 
> >     free_fastpath
> >     instrumented        655,917,745    457,936,140       328,670,030
> >     uninstrumented       34,906,233    119,984,226        59,469,931
> > 
> >     free_slowpath
> >     instrumented        236,735,724      7,720,917       270,716,506
> >     uninstrumented       85,039,753          2,814        60,145,870
> > 
> >     sheaf_flush
> >     instrumented         39,842,700     42,706,800         8,341,440
> >     uninstrumented        1,286,400     13,487,700           887,700
> > 
> >     barn_put_fail
> >     instrumented            663,994        711,745           139,037
> >     uninstrumented           21,464        224,762            14,765
> > 
> >     barn_get_fail
> >     instrumented          4,609,925        840,344         4,651,225
> >     uninstrumented        1,438,643        224,787         1,017,006
> > 
> >     cmpxchg_double_fail
> >     instrumented             70,524          8,077            28,062
> >     uninstrumented            7,471            713             1,503
> > 
> >     alloc_slowpath
> >     all cases                     0              0                 0
> > 
> > The SLUB counter profile changes substantially with perf lock despite
> > the lower NFS throughput, and the exact mix depends strongly on
> > placement.
> > 
> > The uninstrumented node0/node0 run also reproduces the barn/sheaf
> > relationship Chuck pointed out earlier. The nfs_page sheaf capacity is
> > 60:
> > 
> >     sheaf_flush / 60 = 13,487,700 / 60 = 224,795
> >     barn_put_fail                      = 224,762
> > 
> > The placement dependence of free_slowpath is also large. It falls from
> > about 85M events per 10 seconds in the unpinned/node0 case to 2,814 in
> > the node0/node0 case.
> > 
> > The reported perf-lock result, however, is similar across all three
> > placements:
> > 
> >                          contentions    total wait    average wait
> >     unpinned/node0         4,782,839      14.39 min      180.47 us
> >     node0/node0            4,582,012      14.13 min      185.05 us
> >     unpinned/balanced      4,572,196      12.81 min      168.14 us
> > 
> > I also revisited an inconsistency Chuck noticed in my earlier
> > measurements. Previously I had compared aggregate perf-lock wait from
> > one 10-second capture with CPU utilization measured during a different
> > window.
> > 
> > I now have paired 10-second mpstat samples for each placement, with and
> > without perf lock. The node values below are averages of the per-CPU
> > %idle values for the CPUs in each NUMA node:
> > 
> >                                   system-wide       node0       node1
> >                                   %idle             %idle       %idle
> > 
> >     A. unpinned / CQs node0
> >        uninstrumented                 31.46           8.0        54.6
> >        instrumented                    3.60           0.06        7.1
> > 
> >     B. node0 pinned / CQs node0
> >        uninstrumented                 84.26          69.2        99.4
> >        instrumented                   12.07          10.4        13.8
> > 
> >     C. unpinned / balanced CQs
> >        uninstrumented                 66.86          66.7        66.9
> >        instrumented                   14.83          19.0        10.6
> > 
> > This resolves the accounting inconsistency in my earlier measurements.
> > The large aggregate perf-lock wait and high idle percentage had come
> > from different windows. In the aligned samples, the system is much
> > busier during the perf-lock capture than in the corresponding
> > uninstrumented run.
> > 
> > I am still unsure how representative the reported ~170-185 us average
> > wait is of the uninstrumented workload.
> > 
> > The remaining number I am less sure how to interpret is
> > cmpxchg_double_fail. In the uninstrumented windows I see:
> > 
> >     unpinned / CQs node0:       7,471 / 10 sec
> >     node0 pinned / CQs node0:     713 / 10 sec
> >     unpinned / balanced CQs:    1,503 / 10 sec
> > 
> > compared with fewer than 100 in Chuck's test.
> > 
> > I understand that cmpxchg_double_fail counts failed slab freelist
> > updates rather than failed logical frees, so I am not sure what the
> > appropriate denominator is here. In particular, the node0/node0 case
> > still has 713 failures while sustaining full throughput and only 2,814
> > free_slowpath events over the interval.
> > 
> > My questions are:
> > 
> > 1. Do the uninstrumented cmpxchg_double_fail rates above look abnormal
> >    for this workload/topology, or are they within the range one would
> >    expect from this degree of concurrency and NUMA placement?
> > 
> > 2. Is there a less invasive way you would recommend measuring the
> >    nfs_page list_lock/freelist contention? I would like to distinguish
> >    the steady-state behavior from what is observed during the perf-lock
> >    capture.
> > 
> > Thanks,
> > Tim

-- 
Cheers,
Harry / Hyeonggon