Re: [PATCH] slub: limit count of partial slabs scanned to gather statistics

[David Rientjes <rientjes@google.com>]

On Mon, 4 May 2020, Konstantin Khlebnikov wrote:

> To get exact count of free and used objects slub have to scan list of
> partial slabs. This may take at long time. Scanning holds spinlock and
> blocks allocations which move partial slabs to per-cpu lists and back.
> 
> Example found in the wild:
> 
> # cat /sys/kernel/slab/dentry/partial
> 14478538 N0=7329569 N1=7148969
> # time cat /sys/kernel/slab/dentry/objects
> 286225471 N0=136967768 N1=149257703
> 
> real	0m1.722s
> user	0m0.001s
> sys	0m1.721s
> 
> The same problem in slab was addressed in commit f728b0a5d72a ("mm, slab:
> faster active and free stats") by adding more kmem cache statistics.
> For slub same approach requires atomic op on fast path when object frees.
> 
> Let's simply limit count of scanned slabs and print warning.
> Limit set in /sys/module/slub/parameters/max_partial_to_count.
> Default is 10000 which should be enough for most sane cases.
> 
> Return linear approximation if list of partials is longer than limit.
> Nobody should notice difference.
> 

Hi Konstantin,

Do you only exhibit this on slub for SO_ALL|SO_OBJECTS?  I notice the 
timing in the changelog is only looking at "objects" and not "partial".

If so, it seems this is also a problem for get_slabinfo() since it also 
uses the count_free() callback for count_partial().

Concern would be that the kernel has now drastically changed a statistic 
that it exports to userspace.  There was some discussion about this back 
in 2016[*] and one idea was that slabinfo would truncate its scanning and 
append a '+' to the end of the value to indicate it exceeds the max, i.e. 
10000+.  I think that '+' actually caused the problem itself for userspace 
processes.

I think the patch is too far reaching, however, since it impacts all 
count_partial() counting and not only for the case cited in the changelog.  
Are there examples for things other than the count_free() callback?

 [*] https://lore.kernel.org/patchwork/patch/708427/

> Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
> ---
>  mm/slub.c |   15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 9bf44955c4f1..86a366f7acb6 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2407,16 +2407,29 @@ static inline unsigned long node_nr_objs(struct kmem_cache_node *n)
>  #endif /* CONFIG_SLUB_DEBUG */
>  
>  #if defined(CONFIG_SLUB_DEBUG) || defined(CONFIG_SYSFS)
> +
> +static unsigned long max_partial_to_count __read_mostly = 10000;
> +module_param(max_partial_to_count, ulong, 0644);
> +
>  static unsigned long count_partial(struct kmem_cache_node *n,
>  					int (*get_count)(struct page *))
>  {
> +	unsigned long counted = 0;
>  	unsigned long flags;
>  	unsigned long x = 0;
>  	struct page *page;
>  
>  	spin_lock_irqsave(&n->list_lock, flags);
> -	list_for_each_entry(page, &n->partial, slab_list)
> +	list_for_each_entry(page, &n->partial, slab_list) {
>  		x += get_count(page);
> +
> +		if (++counted > max_partial_to_count) {
> +			pr_warn_once("SLUB: too much partial slabs to count all objects, increase max_partial_to_count.\n");
> +			/* Approximate total count of objects */
> +			x = mult_frac(x, n->nr_partial, counted);
> +			break;
> +		}
> +	}
>  	spin_unlock_irqrestore(&n->list_lock, flags);
>  	return x;
>  }
> 
>