Re: slabinfo shows incorrect active_objs ???

[Vlastimil Babka <vbabka@suse.cz>]

On 2/22/22 13:10, Vasily Averin wrote:
> 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?

Oh I realized you can get much closer to real numbers via doing
echo 1 > /sys/kernel/slab/<cache>/shrink
and then reading slabinfo immediately

Although it will be racy if the flushed slabs are immediately refilled by
allocation activity, and the flush will affect performance. But maybe in
some situations it's useful.

> I'm preparing new set of memcg accounting patches, with reparired
> tools/cgroup/memcg_slapinfo.py
> I can get numbers of accounted resources, but I need to understand how may
> resources was NOT
> accounted to memcg but allocated on host. I expected get these numbers from
> host's slabinfo but
> it does not show correct numbers.
> 
> Thank you,
>     Vasily Averin
>