Re: slabinfo shows incorrect active_objs ???

[Roman Gushchin <roman.gushchin@linux.dev>]

> On Feb 23, 2022, at 9:31 AM, Vlastimil Babka <vbabka@suse.cz> 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.

Right, something like this.

A bigger picture here: since we’ve moved to a per-object accounting, accounted allocations became a bit slower. It seems to be a non-issue in the real life (at least I haven’t seen any reports, maybe because it counter-weighted by memory savings and lower fragmentation), however benchmarks clearly show it and it’s something nice to fix at some point. The obvious solution is to maintain some sort of a per-memcg cache of pre-charged objects for particularly hot slab caches. But to do this we need to understand dynamically which slab caches are hot (and which cgroups are using them). This will require a per-memcg per-slab accounting mechanism. So maybe it will replace the drgn script in the long run.

Thanks!