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

[Vlastimil Babka (SUSE)]

On 2/25/26 10:31, Ming Lei wrote:
> Hi Vlastimil,
> 
> On Wed, Feb 25, 2026 at 09:45:03AM +0100, Vlastimil Babka (SUSE) wrote:
>> On 2/24/26 21:27, Vlastimil Babka wrote:
>> > 
>> > It made sense to me not to refill sheaves when we can't reclaim, but I
>> > didn't anticipate this interaction with mempools. We could change them
>> > but there might be others using a similar pattern. Maybe it would be for
>> > the best to just drop that heuristic from __pcs_replace_empty_main()
>> > (but carefully as some deadlock avoidance depends on it, we might need
>> > to e.g. replace it with gfpflags_allow_spinning()). I'll send a patch
>> > tomorrow to test this theory, unless someone beats me to it (feel free to).
>> Could you try this then, please? Thanks!
> 
> Thanks for working on this issue!
> 
> Unfortunately the patch doesn't make a difference on IOPS in the perf test,
> follows the collected perf profile on linus tree(basically 7.0-rc1 with your patch):

Hm that's weird, still the slowpath is prominent in your profile.

I followed your reproducer instructions, although only with a small
virtme-ng based setup. What's the output of "numactl -H" on yours, btw?

Anyway what I saw is my patch raised the IOPS substantially, and with
CONFIG_SLUB_STATS=y enabled I could see that
/sys/kernel/slab/bio-248/alloc_slowpath had substantial values before the
patch and zero afterwards.

Maybe if you could also enable CONFIG_SLUB_STATS=y and see in which cache(s)
there's significant alloc_slowpath even after the patch, it could help.

Thanks!


> ```
> 04cb971e2d28 (HEAD -> master) mm:slab/sheaves: severe performance regression in cross-CPU slab allocation
> a5a9cf3f020f mm: fix NULL NODE_DATA dereference for memoryless nodes on boot
> 7dff99b35460 (origin/master) Remove WARN_ALL_UNSEEDED_RANDOM kernel config option
> 551d44200152 default_gfp(): avoid using the "newfangled" __VA_OPT__ trick
> 6de23f81a5e0 (tag: v7.0-rc1) Linux 7.0-rc1
> ```
> 
> +   49.03%     2.00%  io_uring         [kernel.kallsyms]       [k] __blkdev_direct_IO_async
> -   38.66%     1.16%  io_uring         [kernel.kallsyms]       [k] bio_alloc_bioset
>    - 37.51% bio_alloc_bioset
>       - 34.98% mempool_alloc_noprof
>          - 34.87% kmem_cache_alloc_noprof
>             - 33.82% ___slab_alloc
>                - 30.25% get_from_any_partial
>                   - 29.59% get_from_partial_node
>                      - 28.42% __raw_spin_lock_irqsave
>                           native_queued_spin_lock_slowpath
>                + 2.16% allocate_slab
>                + 0.60% alloc_from_new_slab
>               0.51% __pcs_replace_empty_main
>         1.58% bio_associate_blkg
>    + 1.16% submitter_uring_fn
> +   35.16%     0.30%  io_uring         [kernel.kallsyms]       [k] kmem_cache_alloc_noprof
> +   35.13%     0.12%  io_uring         [kernel.kallsyms]       [k] mempool_alloc_noprof
> 
> 
> Thanks,
> Ming
>