Re: [PATCH v5 02/14] slab: add sheaf support for batching kfree_rcu() operations

[Uladzislau Rezki <urezki@gmail.com>]

On Wed, Jul 23, 2025 at 03:34:35PM +0200, Vlastimil Babka wrote:
> Extend the sheaf infrastructure for more efficient kfree_rcu() handling.
> For caches with sheaves, on each cpu maintain a rcu_free sheaf in
> addition to main and spare sheaves.
> 
> kfree_rcu() operations will try to put objects on this sheaf. Once full,
> the sheaf is detached and submitted to call_rcu() with a handler that
> will try to put it in the barn, or flush to slab pages using bulk free,
> when the barn is full. Then a new empty sheaf must be obtained to put
> more objects there.
> 
> It's possible that no free sheaves are available to use for a new
> rcu_free sheaf, and the allocation in kfree_rcu() context can only use
> GFP_NOWAIT and thus may fail. In that case, fall back to the existing
> kfree_rcu() implementation.
> 
> Expected advantages:
> - batching the kfree_rcu() operations, that could eventually replace the
>   existing batching
> - sheaves can be reused for allocations via barn instead of being
>   flushed to slabs, which is more efficient
>   - this includes cases where only some cpus are allowed to process rcu
>     callbacks (Android)
> 
> Possible disadvantage:
> - objects might be waiting for more than their grace period (it is
>   determined by the last object freed into the sheaf), increasing memory
>   usage - but the existing batching does that too.
> 
> Only implement this for CONFIG_KVFREE_RCU_BATCHED as the tiny
> implementation favors smaller memory footprint over performance.
> 
> Add CONFIG_SLUB_STATS counters free_rcu_sheaf and free_rcu_sheaf_fail to
> count how many kfree_rcu() used the rcu_free sheaf successfully and how
> many had to fall back to the existing implementation.
> 
> Reviewed-by: Harry Yoo <harry.yoo@oracle.com>
> Reviewed-by: Suren Baghdasaryan <surenb@google.com>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
>  mm/slab.h        |   2 +
>  mm/slab_common.c |  24 +++++++
>  mm/slub.c        | 193 +++++++++++++++++++++++++++++++++++++++++++++++++++++--
>  3 files changed, 214 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/slab.h b/mm/slab.h
> index 1980330c2fcb4a4613a7e4f7efc78b349993fd89..44c9b70eaabbd87c06fb39b79dfb791d515acbde 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -459,6 +459,8 @@ static inline bool is_kmalloc_normal(struct kmem_cache *s)
>  	return !(s->flags & (SLAB_CACHE_DMA|SLAB_ACCOUNT|SLAB_RECLAIM_ACCOUNT));
>  }
>  
> +bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj);
> +
>  #define SLAB_CORE_FLAGS (SLAB_HWCACHE_ALIGN | SLAB_CACHE_DMA | \
>  			 SLAB_CACHE_DMA32 | SLAB_PANIC | \
>  			 SLAB_TYPESAFE_BY_RCU | SLAB_DEBUG_OBJECTS | \
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index e2b197e47866c30acdbd1fee4159f262a751c5a7..2d806e02568532a1000fd3912db6978e945dcfa8 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -1608,6 +1608,27 @@ static void kfree_rcu_work(struct work_struct *work)
>  		kvfree_rcu_list(head);
>  }
>  
> +static bool kfree_rcu_sheaf(void *obj)
> +{
> +	struct kmem_cache *s;
> +	struct folio *folio;
> +	struct slab *slab;
> +
> +	if (is_vmalloc_addr(obj))
> +		return false;
> +
> +	folio = virt_to_folio(obj);
> +	if (unlikely(!folio_test_slab(folio)))
> +		return false;
> +
> +	slab = folio_slab(folio);
> +	s = slab->slab_cache;
> +	if (s->cpu_sheaves)
> +		return __kfree_rcu_sheaf(s, obj);
> +
> +	return false;
> +}
> +
>  static bool
>  need_offload_krc(struct kfree_rcu_cpu *krcp)
>  {
> @@ -1952,6 +1973,9 @@ void kvfree_call_rcu(struct rcu_head *head, void *ptr)
>  	if (!head)
>  		might_sleep();
>  
> +	if (kfree_rcu_sheaf(ptr))
> +		return;
> +
>
I have a question here. kfree_rcu_sheaf(ptr) tries to revert freeing
an object over one more newly introduced path. This patch adds infra
for such purpose whereas we already have a main path over which we
free memory.

Why do not we use existing logic? As i see you can do:

   if (unlikely(!slab_free_hook(s, p[i], init, true))) {
        p[i] = p[--sheaf->size];
        continue;
   }

in the kfree_rcu_work() function where we process all ready to free objects.
I mean, for slab objects we can replace kfree_bulk() and scan all pointers
and free them over slab_free_hook().

Also we do use a pooled API and other improvements to speed up freeing.

Thanks!

--
Uladzislau Rezki