Re: [PATCH v3 1/2] slub: introduce count_partial_free_approx()

[David Rientjes]

On Fri, 19 Apr 2024, Jianfeng Wang wrote:

> diff --git a/mm/slub.c b/mm/slub.c
> index 1bb2a93cf7b6..993cbbdd2b6c 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3213,6 +3213,43 @@ static inline bool free_debug_processing(struct kmem_cache *s,
>  #endif /* CONFIG_SLUB_DEBUG */
>  
>  #if defined(CONFIG_SLUB_DEBUG) || defined(SLAB_SUPPORTS_SYSFS)
> +#define MAX_PARTIAL_TO_SCAN 10000
> +
> +static unsigned long count_partial_free_approx(struct kmem_cache_node *n)
> +{
> +	unsigned long flags;
> +	unsigned long x = 0;
> +	struct slab *slab;
> +
> +	spin_lock_irqsave(&n->list_lock, flags);
> +	if (n->nr_partial <= MAX_PARTIAL_TO_SCAN) {
> +		list_for_each_entry(slab, &n->partial, slab_list)
> +			x += slab->objects - slab->inuse;
> +	} else {
> +		/*
> +		 * For a long list, approximate the total count of objects in
> +		 * it to meet the limit on the number of slabs to scan.
> +		 * Scan from both the list's head and tail for better accuracy.
> +		 */
> +		unsigned long scanned = 0;
> +
> +		list_for_each_entry(slab, &n->partial, slab_list) {
> +			x += slab->objects - slab->inuse;
> +			if (++scanned == MAX_PARTIAL_TO_SCAN / 2)
> +				break;
> +		}
> +		list_for_each_entry_reverse(slab, &n->partial, slab_list) {
> +			x += slab->objects - slab->inuse;
> +			if (++scanned == MAX_PARTIAL_TO_SCAN)
> +				break;
> +		}
> +		x = mult_frac(x, n->nr_partial, scanned);
> +		x = min(x, node_nr_objs(n));
> +	}
> +	spin_unlock_irqrestore(&n->list_lock, flags);
> +	return x;
> +}

Creative :)

The default value of MAX_PARTIAL_TO_SCAN seems to work well in practice 
while being large enough to bias for actual values?

I can't think of a better way to avoid the disruption that very long 
partial lists cause.  If the actual value is needed, it will need to be 
read from the sysfs file for that slab cache.

It does beg the question of whether we want to extend slabinfo to indicate 
that some fields are approximations, however.  Adding a suffix such as 
" : approx" to a slab cache line may be helpful if the disparity in the 
estimates would actually make a difference in practice.

I have a hard time believing that this approximation will not be "close 
enough" for all practical purposes, given that the value could very well 
substantially change the instant after the iteration is done anyway.

So for that reason, this sounds good to me!

Acked-by: David Rientjes <rientjes@xxxxxxxxxx>

> +
>  static unsigned long count_partial(struct kmem_cache_node *n,
>  					int (*get_count)(struct slab *))
>  {
> @@ -7089,7 +7126,7 @@ void get_slabinfo(struct kmem_cache *s, struct slabinfo *sinfo)
>  	for_each_kmem_cache_node(s, node, n) {
>  		nr_slabs += node_nr_slabs(n);
>  		nr_objs += node_nr_objs(n);
> -		nr_free += count_partial(n, count_free);
> +		nr_free += count_partial_free_approx(n);
>  	}
>  
>  	sinfo->active_objs = nr_objs - nr_free;