Re: slabinfo shows incorrect active_objs ???

[Vlastimil Babka <vbabka@suse.cz>]

On 2/22/22 21:59, Roman Gushchin wrote:
> 
>> On Feb 22, 2022, at 4:10 AM, Vasily Averin <vvs@virtuozzo.com> wrote:

BTW please To/Cc directly all slab maintainers on future slab related
threads (added now).

>> On 22.02.2022 13:23, Hyeonggon Yoo wrote:
>>>> On Tue, Feb 22, 2022 at 12:22:02PM +0300, Vasily Averin wrote:
>>>> Dear all,
>>>> 
>>>> I've found that /proc/slabinfo shows inadequate numbers of in-use slab objects.
>>>> it assumes that all objects stored in cpu caches are always 100% in use.
>> 
>>>> Is it a bug or perhaps a well-known feature that I missed?
>>> This is not a bug..
>> 
>> Thank you for explanation,
>> I think it would be useful to document this somewhere. (Documnetation? man slabinfo ?)
>> Also I would like to know is it some (fast) way to get real numbers in userspace ?
>> crash is too fat for this task.
>> Do you know perhaps some other userspace utility or may be systemtap/drgn script?
> 
> Btw, implementing fast slab counters independent from the sl*b implementation and the physical layout of data might be an interesting idea.

Interesting idea, but merging will be an issue if we ever manage to
officially allow kfree() on object allocated by kmem_cache_alloc() - which
is now blocked by SLOB (there was a recent thread that stalled).

> Currently /proc/slabinfo is often confusing because of the slab merging. It’s particularly true when someone tries to compare memory usage on two different kernel versions, for example: the set of slab caches might look very different depending on subtle changes in object sizes and the caches merging outcome.
>