Re: [PATCH v2 00/20] slab: replace cpu (partial) slabs with sheaves

[Vlastimil Babka <vbabka@suse.cz>]

On 1/12/26 16:16, Vlastimil Babka wrote:
> Percpu sheaves caching was introduced as opt-in but the goal was to
> eventually move all caches to them. This is the next step, enabling
> sheaves for all caches (except the two bootstrap ones) and then removing
> the per cpu (partial) slabs and lots of associated code.
> 
> Besides (hopefully) improved performance, this removes the rather
> complicated code related to the lockless fastpaths (using
> this_cpu_try_cmpxchg128/64) and its complications with PREEMPT_RT or
> kmalloc_nolock().
> 
> The lockless slab freelist+counters update operation using
> try_cmpxchg128/64 remains and is crucial for freeing remote NUMA objects
> without repeating the "alien" array flushing of SLUB, and to allow
> flushing objects from sheaves to slabs mostly without the node
> list_lock.
> 
> This v2 is the first non-RFC. I would consider exposing the series to
> linux-next at this point.

Well if only I didn't forget to remove the RFC prefix before sending...

> Git branch for the v2:
>   https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/linux.git/log/?h=sheaves-for-all-v2
> 
> Based on:
>   https://git.kernel.org/pub/scm/linux/kernel/git/vbabka/slab.git/log/?h=slab/for-7.0/sheaves
>   - includes a sheaves optimization that seemed minor but there was lkp
>     test robot result with significant improvements:
>     https://lore.kernel.org/all/202512291555.56ce2e53-lkp@intel.com/
>     (could be an uncommon corner case workload though)
> 
> Significant (but not critical) remaining TODOs:
> - Integration of rcu sheaves handling with kfree_rcu batching.
>   - Currently the kfree_rcu batching is almost completely bypassed. I'm
>     thinking it could be adjusted to handle rcu sheaves in addition to
>     individual objects, to get the best of both.
> - Performance evaluation. Petr Tesarik has been doing that on the RFC
>   with some promising results (thanks!) and also found a memory leak.
> 
> Note that as many things, this caching scheme change is a tradeoff, as
> summarized by Christoph:
> 
>   https://lore.kernel.org/all/f7c33974-e520-387e-9e2f-1e523bfe1545@gentwo.org/
> 
> - Objects allocated from sheaves should have better temporal locality
>   (likely recently freed, thus cache hot) but worse spatial locality
>   (likely from many different slabs, increasing memory usage and
>   possibly TLB pressure on kernel's direct map).
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
> Changes in v2:
> - Rebased to v6.19-rc1+slab.git slab/for-7.0/sheaves
>   - Some of the preliminary patches from the RFC went in there.
> - Incorporate feedback/reports from many people (thanks!), including:
>   - Make caches with sheaves mergeable.
>   - Fix a major memory leak.
> - Cleanup of stat items.
> - Link to v1: https://patch.msgid.link/20251023-sheaves-for-all-v1-0-6ffa2c9941c0@suse.cz
> 
> ---
> Vlastimil Babka (20):
>       mm/slab: add rcu_barrier() to kvfree_rcu_barrier_on_cache()
>       mm/slab: move and refactor __kmem_cache_alias()
>       mm/slab: make caches with sheaves mergeable
>       slab: add sheaves to most caches
>       slab: introduce percpu sheaves bootstrap
>       slab: make percpu sheaves compatible with kmalloc_nolock()/kfree_nolock()
>       slab: handle kmalloc sheaves bootstrap
>       slab: add optimized sheaf refill from partial list
>       slab: remove cpu (partial) slabs usage from allocation paths
>       slab: remove SLUB_CPU_PARTIAL
>       slab: remove the do_slab_free() fastpath
>       slab: remove defer_deactivate_slab()
>       slab: simplify kmalloc_nolock()
>       slab: remove struct kmem_cache_cpu
>       slab: remove unused PREEMPT_RT specific macros
>       slab: refill sheaves from all nodes
>       slab: update overview comments
>       slab: remove frozen slab checks from __slab_free()
>       mm/slub: remove DEACTIVATE_TO_* stat items
>       mm/slub: cleanup and repurpose some stat items
> 
>  include/linux/slab.h |    6 -
>  mm/Kconfig           |   11 -
>  mm/internal.h        |    1 +
>  mm/page_alloc.c      |    5 +
>  mm/slab.h            |   53 +-
>  mm/slab_common.c     |   56 +-
>  mm/slub.c            | 2591 +++++++++++++++++---------------------------------
>  7 files changed, 950 insertions(+), 1773 deletions(-)
> ---
> base-commit: aff9fb2fffa1175bd5ae3b4630f3d4ae53af450b
> change-id: 20251002-sheaves-for-all-86ac13dc47a5
> 
> Best regards,