Re: [PATCH RFC 4/4] slab: don't batch kvfree_rcu() with SLUB_TINY

[Uladzislau Rezki <urezki@gmail.com>]

On Thu, Jan 23, 2025 at 11:37:21AM +0100, Vlastimil Babka wrote:
> kvfree_rcu() is batched for better performance except on TINY_RCU, which
> is a simple implementation for small UP systems. Similarly SLUB_TINY is
> an option intended for small systems, whether or not used together with
> TINY_RCU. In case SLUB_TINY is used with !TINY_RCU, it makes arguably
> sense to not do the batching and limit the memory footprint. It's also
> suboptimal to have RCU-specific #ifdefs in slab code.
> 
> With that, add CONFIG_KFREE_RCU_BATCHED to determine whether batching
> kvfree_rcu() implementation is used. It is not set by a user prompt, but
> enabled by default and disabled in case TINY_RCU or SLUB_TINY are
> enabled.
> 
> Use the new config for #ifdef's in slab code and extend their scope to
> cover all code used by the batched kvfree_rcu(). For example there's no
> need to perform kvfree_rcu_init() if the batching is disabled.
> 
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
>  include/linux/slab.h |  2 +-
>  mm/Kconfig           |  4 ++++
>  mm/slab_common.c     | 45 +++++++++++++++++++++++++--------------------
>  3 files changed, 30 insertions(+), 21 deletions(-)
> 
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index bcc62e5656c35c6a3f4caf26fb33d7447dead39a..9faf33734a8eee2425b90e679c0457ab459422a3 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -1083,7 +1083,7 @@ extern void kvfree_sensitive(const void *addr, size_t len);
>  
>  unsigned int kmem_cache_size(struct kmem_cache *s);
>  
> -#ifdef CONFIG_TINY_RCU
> +#ifndef CONFIG_KFREE_RCU_BATCHED
>  static inline void kvfree_rcu_barrier(void)
>  {
>  	rcu_barrier();
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 84000b01680869801a10f56f06d0c43d6521a8d2..e513308a4aed640ee556ecb5793c7f3f195bbcae 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -242,6 +242,10 @@ menu "Slab allocator options"
>  config SLUB
>  	def_bool y
>  
> +config KFREE_RCU_BATCHED
> +	def_bool y
> +	depends on !SLUB_TINY && !TINY_RCU
> +
>  config SLUB_TINY
>  	bool "Configure for minimal memory footprint"
>  	depends on EXPERT
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index f13d2c901daf1419993620459fbd5845eecb85f1..9f6d66313afc6684bdc0f32908fe01c83c60f283 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -1284,6 +1284,28 @@ EXPORT_TRACEPOINT_SYMBOL(kmem_cache_alloc);
>  EXPORT_TRACEPOINT_SYMBOL(kfree);
>  EXPORT_TRACEPOINT_SYMBOL(kmem_cache_free);
>  
> +#ifndef CONFIG_KFREE_RCU_BATCHED
> +
> +void kvfree_call_rcu(struct rcu_head *head, void *ptr)
> +{
> +	if (head) {
> +		kasan_record_aux_stack_noalloc(ptr);
> +		call_rcu(head, kvfree_rcu_cb);
> +		return;
> +	}
> +
> +	// kvfree_rcu(one_arg) call.
> +	might_sleep();
> +	synchronize_rcu();
> +	kvfree(ptr);
> +}
> +
> +void __init kvfree_rcu_init(void)
> +{
> +}
> +
> +#else /* CONFIG_KFREE_RCU_BATCHED */
> +
>  /*
>   * This rcu parameter is runtime-read-only. It reflects
>   * a minimum allowed number of objects which can be cached
> @@ -1858,24 +1880,6 @@ add_ptr_to_bulk_krc_lock(struct kfree_rcu_cpu **krcp,
>  	return true;
>  }
>  
> -#ifdef CONFIG_TINY_RCU
> -
> -void kvfree_call_rcu(struct rcu_head *head, void *ptr)
> -{
> -	if (head) {
> -		kasan_record_aux_stack_noalloc(ptr);
> -		call_rcu(head, kvfree_rcu_cb);
> -		return;
> -	}
> -
> -	// kvfree_rcu(one_arg) call.
> -	might_sleep();
> -	synchronize_rcu();
> -	kvfree(ptr);
> -}
> -
> -#else /* !CONFIG_TINY_RCU */
> -
>  static enum hrtimer_restart
>  schedule_page_work_fn(struct hrtimer *t)
>  {
> @@ -2084,8 +2088,6 @@ void kvfree_rcu_barrier(void)
>  }
>  EXPORT_SYMBOL_GPL(kvfree_rcu_barrier);
>  
> -#endif /* !CONFIG_TINY_RCU */
> -
>  static unsigned long
>  kfree_rcu_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
>  {
> @@ -2175,3 +2177,6 @@ void __init kvfree_rcu_init(void)
>  
>  	shrinker_register(kfree_rcu_shrinker);
>  }
> +
> +#endif /* CONFIG_KFREE_RCU_BATCHED */
> +
> 
> -- 
> 2.48.1
> 
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

A small nit: CONFIG_KVFREE_RCU_BATCHED?

--
Uladzislau Rezki