Re: slabinfo shows incorrect active_objs ???

[Vlastimil Babka <vbabka@suse.cz>]

On 2/22/22 10:22, 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.
> 
> for example:
>  slabinfo shows that all 20 objects are in use.
> 
> [root@fc34-vvs linux]# uname -a
> Linux fc34-vvs.sw.ru 5.17.0-rc3+ #42 SMP PREEMPT Mon Feb 21 20:14:54 UTC
> 2022 x86_64 x86_64 x86_64 GNU/Linux
> 
> [root@fc34-vvs linux]# cat /proc/slabinfo
> slabinfo - version: 2.1
> # name            <active_objs> <num_objs> <objsize> <objperslab>
> <pagesperslab> : tunables <limit> <batchcount> <sharedfactor> : slabdata
> <active_slabs> <num_slabs> <sharedavail>
> ...
> kmalloc-cg-8k         20     20   8192    4    8 : tunables    0    0    0 :
> slabdata      5      5      0
> 
> At the same time crash said that only 2 objects are in use.
> 
> crash> kmem -s kmalloc-cg-8k
> CACHE             OBJSIZE  ALLOCATED     TOTAL  SLABS  SSIZE  NAME
> ffff8f4840043b00     8192          2        20      5    32k  kmalloc-cg-8k
> 
> And this looks like true, see kmem -S output below.
> 
> Is it a bug or perhaps a well-known feature that I missed?

It's a known tradeoff. It would affect the allocation/free hot paths to
account the per-cpu slabs more accurately and thus some precision of
slabinfo is sacrificed.

> Numbers are counted in mm/slub.c, see below,
> but count_partial() doe not includes free objects of cpu caches
> 
> Moreover adequate statistic is not showed in any other interfaces too
> /sys/kerenl/slab/ read cpu slab caches but does not output these numbers.
> 
> Thank you,
>     Vasily Averin
> 
> #ifdef CONFIG_SLUB_DEBUG
> void get_slabinfo(struct kmem_cache *s, struct slabinfo *sinfo)
> {
>         unsigned long nr_slabs = 0;
>         unsigned long nr_objs = 0;
>         unsigned long nr_free = 0;
>         int node;
>         struct kmem_cache_node *n;
> 
>         for_each_kmem_cache_node(s, node, n) {
>                 nr_slabs += node_nr_slabs(n);
>                 nr_objs += node_nr_objs(n);
>                 nr_free += count_partial(n, count_free);
>         }
> 
>         sinfo->active_objs = nr_objs - nr_free;
>         sinfo->num_objs = nr_objs;
> 
> 
> 
> crash> kmem -S kmalloc-cg-8k
> CACHE             OBJSIZE  ALLOCATED     TOTAL  SLABS  SSIZE  NAME
> ffff8f4840043b00     8192          2        20      5    32k  kmalloc-cg-8k
> CPU 0 KMEM_CACHE_CPU:
>   ffff8f4b58236360
> CPU 0 SLAB:
>   (empty)
> CPU 0 PARTIAL:
>   (empty)
> CPU 1 KMEM_CACHE_CPU:
>   ffff8f4b58276360
> CPU 1 SLAB:
>   SLAB              MEMORY            NODE  TOTAL  ALLOCATED  FREE
>   ffffed3f842af400  ffff8f484abd0000     0      4          1     3
>   FREE / [ALLOCATED]
>    ffff8f484abd0000  (cpu 1 cache)
>    ffff8f484abd2000  (cpu 1 cache)
>    ffff8f484abd4000  (cpu 1 cache)
>   [ffff8f484abd6000]
> CPU 1 PARTIAL:
>   (empty)
> CPU 2 KMEM_CACHE_CPU:
>   ffff8f4b582b6360
> CPU 2 SLAB:
>   (empty)
> CPU 2 PARTIAL:
>   (empty)
> CPU 3 KMEM_CACHE_CPU:
>   ffff8f4b582f6360
> CPU 3 SLAB:
>   SLAB              MEMORY            NODE  TOTAL  ALLOCATED  FREE
>   ffffed3f842ce600  ffff8f484b398000     0      4          0     4
>   FREE / [ALLOCATED]
>    ffff8f484b398000  (cpu 3 cache)
>    ffff8f484b39a000  (cpu 3 cache)
>    ffff8f484b39c000  (cpu 3 cache)
>    ffff8f484b39e000  (cpu 3 cache)
> CPU 3 PARTIAL:
>   (empty)
> CPU 4 KMEM_CACHE_CPU:
>   ffff8f4b58336360
> CPU 4 SLAB:
>   SLAB              MEMORY            NODE  TOTAL  ALLOCATED  FREE
>   ffffed3f8418c200  ffff8f4846308000     0      4          0     4
>   FREE / [ALLOCATED]
>    ffff8f4846308000  (cpu 4 cache)
>    ffff8f484630a000  (cpu 4 cache)
>    ffff8f484630c000  (cpu 4 cache)
>    ffff8f484630e000  (cpu 4 cache)
> CPU 4 PARTIAL:
>   (empty)
> CPU 5 KMEM_CACHE_CPU:
>   ffff8f4b58376360
> CPU 5 SLAB:
>   (empty)
> CPU 5 PARTIAL:
>   (empty)
> CPU 6 KMEM_CACHE_CPU:
>   ffff8f4b583b6360
> CPU 6 SLAB:
>   SLAB              MEMORY            NODE  TOTAL  ALLOCATED  FREE
>   ffffed3f8412d000  ffff8f4844b40000     0      4          0     4
>   FREE / [ALLOCATED]
>    ffff8f4844b40000  (cpu 6 cache)
>    ffff8f4844b42000  (cpu 6 cache)
>    ffff8f4844b44000  (cpu 6 cache)
>    ffff8f4844b46000  (cpu 6 cache)
> CPU 6 PARTIAL:
>   (empty)
> CPU 7 KMEM_CACHE_CPU:
>   ffff8f4b583f6360
> CPU 7 SLAB:
>   SLAB              MEMORY            NODE  TOTAL  ALLOCATED  FREE
>   ffffed3f84124000  ffff8f4844900000     0      4          1     3
>   FREE / [ALLOCATED]
>    ffff8f4844900000  (cpu 7 cache)
>    ffff8f4844902000  (cpu 7 cache)
>   [ffff8f4844904000]
>    ffff8f4844906000  (cpu 7 cache)
> CPU 7 PARTIAL:
>   (empty)
> KMEM_CACHE_NODE   NODE  SLABS  PARTIAL  PER-CPU
> ffff8f48400416c0     0      5        0        5
> NODE 0 PARTIAL:
>   (empty)
> NODE 0 FULL:
>   (not tracked)
> 
> 
>