Re: [RFC v1 2/5] mm, slub: add opt-in slub_percpu_array

[Pedro Falcato]

On Tue, Aug 8, 2023 at 10:54 AM Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
> kmem_cache_setup_percpu_array() will allocate a per-cpu array for
> caching alloc/free objects of given size for the cache. The cache
> has to be created with SLAB_NO_MERGE flag.
>
> The array is filled by freeing. When empty for alloc or full for
> freeing, it's simply bypassed by the operation, there's currently no
> batch freeing/allocations.
>
> The locking is copied from the page allocator's pcplists, based on
> embedded spin locks. Interrupts are not disabled, only preemption (cpu
> migration on RT). Trylock is attempted to avoid deadlock due to
> an intnerrupt, trylock failure means the array is bypassed.
>
> Sysfs stat counters alloc_cpu_cache and free_cpu_cache count operations
> that used the percpu array.
>
> Bulk allocation bypasses the array, bulk freeing does not.
>
> kmem_cache_prefill_percpu_array() can be called to ensure the array on
> the current cpu to at least the given number of objects. However this is
> only opportunistic as there's no cpu pinning and the trylocks may always
> fail. Therefore allocations cannot rely on the array for success even
> after the prefill. But misses should be rare enough that e.g. GFP_ATOMIC
> allocations should be acceptable after the refill.
> The operation is currently not optimized.

As I asked on IRC, I'm curious about three questions:

1) How does this affect SLUB's anti-queueing ideas?

2) Since this is so similar to SLAB's caching, is it realistic to make
this opt-out instead?

3) What performance difference do you expect/see from benchmarks?

> More TODO/FIXMEs:
>
> - NUMA awareness - preferred node currently ignored, __GFP_THISNODE not
>   honored
> - slub_debug - will not work for allocations from the array. Normally in
>   SLUB implementation the slub_debug kills all fast paths, but that
>   could lead to depleting the reserves if we ignore the prefill and use
>   GFP_ATOMIC. Needs more thought.
> ---
>  include/linux/slab.h     |   4 +
>  include/linux/slub_def.h |  10 ++
>  mm/slub.c                | 210 ++++++++++++++++++++++++++++++++++++++-
>  3 files changed, 223 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 848c7c82ad5a..f6c91cbc1544 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -196,6 +196,8 @@ struct kmem_cache *kmem_cache_create_usercopy(const char *name,
>  void kmem_cache_destroy(struct kmem_cache *s);
>  int kmem_cache_shrink(struct kmem_cache *s);
>
> +int kmem_cache_setup_percpu_array(struct kmem_cache *s, unsigned int count);
> +
>  /*
>   * Please use this macro to create slab caches. Simply specify the
>   * name of the structure and maybe some flags that are listed above.
> @@ -494,6 +496,8 @@ void kmem_cache_free(struct kmem_cache *s, void *objp);
>  void kmem_cache_free_bulk(struct kmem_cache *s, size_t size, void **p);
>  int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size, void **p);
>
> +int kmem_cache_prefill_percpu_array(struct kmem_cache *s, unsigned int count, gfp_t gfp);
> +
>  static __always_inline void kfree_bulk(size_t size, void **p)
>  {
>         kmem_cache_free_bulk(NULL, size, p);
> diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h
> index deb90cf4bffb..c85434668419 100644
> --- a/include/linux/slub_def.h
> +++ b/include/linux/slub_def.h
> @@ -13,8 +13,10 @@
>  #include <linux/local_lock.h>
>
>  enum stat_item {
> +       ALLOC_PERCPU_CACHE,     /* Allocation from percpu array cache */
>         ALLOC_FASTPATH,         /* Allocation from cpu slab */
>         ALLOC_SLOWPATH,         /* Allocation by getting a new cpu slab */
> +       FREE_PERCPU_CACHE,      /* Free to percpu array cache */
>         FREE_FASTPATH,          /* Free to cpu slab */
>         FREE_SLOWPATH,          /* Freeing not to cpu slab */
>         FREE_FROZEN,            /* Freeing to frozen slab */
> @@ -66,6 +68,13 @@ struct kmem_cache_cpu {
>  };
>  #endif /* CONFIG_SLUB_TINY */
>
> +struct slub_percpu_array {
> +       spinlock_t lock;

Since this is a percpu array, you probably want to avoid a lock here.
An idea would be to have some sort of bool accessing;
and then doing:

preempt_disable();
WRITE_ONCE(accessing, 1);

/* doing pcpu array stuff */
WRITE_ONCE(accessing, 0);
preempt_enable();

which would avoid the atomic in a fast path while still giving you
safety on IRQ paths. Although reclamation gets harder as you stop
being able to reclaim these pcpu arrays from other CPUs.

-- 
Pedro