Re: [Regression] mm:slab/sheaves: severe performance regression in cross-CPU slab allocation

[Hao Li]

On Tue, Feb 24, 2026 at 10:52:28AM +0800, Ming Lei wrote:
> Hello Vlastimil and MM guys,
> 
> The SLUB "sheaves" series merged via 815c8e35511d ("Merge branch
> 'slab/for-7.0/sheaves' into slab/for-next") introduces a severe
> performance regression for workloads with persistent cross-CPU
> alloc/free patterns. ublk null target benchmark IOPS drops
> significantly compared to v6.19: from ~36M IOPS to ~13M IOPS (~64%
> drop).
> 
> Bisecting within the sheaves series is blocked by a kernel panic at
> 17c38c88294d ("slab: remove cpu (partial) slabs usage from allocation
> paths"), so the exact first bad commit could not be identified.
> 
> Reproducer
> ==========
> 
> Hardware: NUMA machine with >= 32 CPUs
> Kernel:   v7.0-rc (with slab/for-7.0/sheaves merged)
> 
>     # build kublk selftest
>     make -C tools/testing/selftests/ublk/
> 
>     # create ublk null target device with 16 queues
>     tools/testing/selftests/ublk/kublk add -t null -q 16
> 
>     # run fio/t/io_uring benchmark: 16 jobs, 20 seconds, non-polled
>     taskset -c 0-31 fio/t/io_uring -p0 -n 16 -r 20 /dev/ublkb0
> 
>     # cleanup
>     tools/testing/selftests/ublk/kublk del -n 0
> 
> Good: v6.19 (and 41f1a08645ab, the mainline parent of the slab merge)
> Bad:  815c8e35511d (Merge branch 'slab/for-7.0/sheaves' into slab/for-next)
> 

Hi Ming,

I also have a similar machine, but my test results show that the IOPS is below
1M, only around 900K. That seems quite strange to me.

My test commands are:

```bash
tools/testing/selftests/ublk/kublk add -t null -q 16
taskset -c 24-47 /home/haolee/fio/t/io_uring -p0 -n 16 -r 20 /dev/ublkb0
```

Below are my machine numa info. Could there be something configured incorrectly
on my side?

available: 8 nodes (0-7)
node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
node 0 size: 193175 MB
node 0 free: 164227 MB
node 1 cpus: 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
node 1 size: 0 MB
node 1 free: 0 MB
node 2 cpus: 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71
node 2 size: 0 MB
node 2 free: 0 MB
node 3 cpus: 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95
node 3 size: 0 MB
node 3 free: 0 MB
node 4 cpus: 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119
node 4 size: 193434 MB
node 4 free: 189559 MB
node 5 cpus: 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143
node 5 size: 0 MB
node 5 free: 0 MB
node 6 cpus: 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167
node 6 size: 0 MB
node 6 free: 0 MB
node 7 cpus: 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191
node 7 size: 0 MB
node 7 free: 0 MB
node distances:
node   0   1   2   3   4   5   6   7
  0:  10  12  12  12  32  32  32  32
  1:  12  10  12  12  32  32  32  32
  2:  12  12  10  12  32  32  32  32
  3:  12  12  12  10  32  32  32  32
  4:  32  32  32  32  10  12  12  12
  5:  32  32  32  32  12  10  12  12
  6:  32  32  32  32  12  12  10  12
  7:  32  32  32  32  12  12  12  10


-- 
Thanks,
Hao