On 2/23/24 08:36, Jianfeng Wang wrote:
>
> On 2/22/24 7:02 PM, Christoph Lameter (Ampere) wrote:
>> On Thu, 22 Feb 2024, Chengming Zhou wrote:
>>
>>> Anyway, I put the code below for discussion...
>>
>> Can we guestimate the free objects based on the number of partial slabs. That number is available.
>>
>
> Yes.
> I've thought about calculating the average number of free objects in a
> partial slab (through sampling) and then estimating the total number of
> free objects as (avg * n->nr_partial).
>
> See the following.
>
> ---
> mm/slub.c | 20 ++++++++++++++++++--
> 1 file changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/mm/slub.c b/mm/slub.c
> index 63d281dfacdb..13385761049c 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2963,6 +2963,8 @@ 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(struct kmem_cache_node *n,
> int (*get_count)(struct slab *))
> {
> @@ -2971,8 +2973,22 @@ static unsigned long count_partial(struct kmem_cache_node *n,
> struct slab *slab;
>
> spin_lock_irqsave(&n->list_lock, flags);
> - list_for_each_entry(slab, &n->partial, slab_list)
> - x += get_count(slab);
> + if (n->nr_partial > MAX_PARTIAL_TO_SCAN) {
> + /* Estimate total count of objects via sampling */
> + unsigned long sample_rate = n->nr_partial / MAX_PARTIAL_TO_SCAN;
> + unsigned long scanned = 0;
> + unsigned long counted = 0;
> + list_for_each_entry(slab, &n->partial, slab_list) {
Unfortunately this is still going through the whole list.
> + if (++scanned % sample_rate == 0) {
> + x += get_count(slab);
The get_count() itself is trivial, so this won't help much.
(except saving a function call which can be noticeable with all the
retpolines and other unfortunate stuff these days; we could refactor the
code to be inline, but processing the whole long list would still be an issue).
So this would have to instead count first up to MAX_PARTIAL_TO_SCAN slabs
and then terminate the loop. This can make the sampling biased, unfortunately.
> + counted++;
> + }
> + }
> + x = mult_frac(x, n->nr_partial, counted);
> + } else {
> + list_for_each_entry(slab, &n->partial, slab_list)
> + x += get_count(slab);
> + }
> spin_unlock_irqrestore(&n->list_lock, flags);
> return x;
> }
