Re: slabinfo shows incorrect active_objs ???

[Vasily Averin <vvs@virtuozzo.com>]

On 25.02.2022 03:08, Roman Gushchin wrote:
> 
>> On Feb 24, 2022, at 5:17 AM, Vasily Averin <vvs@virtuozzo.com> wrote:
>>
>> On 22.02.2022 19:32, Shakeel Butt wrote:
>>> If you are just interested in the stats, you can use SLAB for your experiments.
>>
>> Unfortunately memcg_slabino.py does not support SLAB right now.
>>
>>> On 23.02.2022 20:31, Vlastimil Babka wrote:
>>>> On 2/23/22 04:45, Hyeonggon Yoo wrote:
>>>> On Wed, Feb 23, 2022 at 01:32:36AM +0100, Vlastimil Babka wrote:
>>>>> Hm it would be easier just to disable merging when the precise counters are
>>>>> enabled. Assume it would be a config option (possibly boot-time option with
>>>>> static keys) anyway so those who don't need them can avoid the overhead.
>>>>
>>>> Is it possible to accurately account objects in SLUB? I think it's not
>>>> easy because a CPU can free objects to remote cpu's partial slabs using
>>>> cmpxchg_double()...
>>> AFAIU Roman's idea would be that each alloc/free would simply inc/dec an
>>> object counter that's disconnected from physical handling of particular sl*b
>>> implementation. It would provide exact count of objects from the perspective
>>> of slab users.
>>> I assume for reduced overhead the counters would be implemented in a percpu
>>> fashion as e.g. vmstats. Slabinfo gathering would thus have to e.g. sum up
>>> those percpu counters.
>>
>> I like this idea too and I'm going to spend some time for its implementation.
> 
> Sounds good!
> 
> Unfortunately it’s quite tricky: the problem is that there is potentially a large and dynamic set of cgroups and also large and dynamic set of slab caches. Given the performance considerations, it’s also unlikely to avoid using percpu variables.
> So we come to the (nr_slab_caches * nr_cgroups * nr_cpus) number of “objects”. If we create them proactively, we’re likely wasting lot of memory. Creating them on demand is tricky too (especially without losing some accounting accuracy).

I told about global (i.e. non-memcg) precise slab counters only.
I'm expect it can done under new config option and/or static key, and if present use them in /proc/slabinfo output.

At present I'm still going to extract memcg counters via your memcg_slabinfo script.

Thank you,
	Vasily Averin