[Crash-utility] [PATCH 1/2] Fix cpu_slab freelist handling on SLUB

[OGAWA Hirofumi]

Dave Anderson <anderson at redhat.com> writes:

> Here's an example.  I ran your patch on a live 3.10-based kernel, and 
> see these counts on the xfs-based caches:

[...]

> ffff880035d54300 xfs_efd_item             400          0       300     15     8k
> ffff880035d54200 xfs_da_state             480          0       272      8    16k
> ffff880035d54100 xfs_btree_cur            208          0       312      8     8k
> ffff880035d54000 xfs_log_ticket           184          3       682     31     4k

[...]

> xfs_efd_item         180    300    400   20    2 : tunables    0    0    0 : slabdata     15     15      0
> xfs_da_state         272    272    480   34    4 : tunables    0    0    0 : slabdata      8      8      0
> xfs_btree_cur        312    312    208   39    2 : tunables    0    0    0 : slabdata      8      8      0
> xfs_log_ticket       682    682    184   22    1 : tunables    0    0    0 : slabdata     31     31      0
> crash> 
>
> which show 180, 272, 312 and 682 active counts.
>
> Can you explain the discrepancy?

The active counts means just a non percpu slab, not the allocated.
The algorithm of active counts is

At v4.5/mm/slub.c:get_slabinfo():

	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;

count_partial() make sum of node->parital's "page.objects - page.inuse".

IOW, all percpu slabs are included as active_objs (i.e. "active objects"
== "all objects on node full slabs" + "all objects on percpu slabs").


Another example (not confirmed whether accurate way though) is

# pwd
/sys/kernel/slab/UDPv6
# cat alloc_fastpath alloc_slowpath free_fastpath free_slowpath; grep ^UDPv6 /proc/slabinfo 
16173 C0=1087 C1=10007 C2=2101 C3=1364 C4=232 C5=725 C6=323 C7=334
8 C0=1 C1=1 C2=1 C3=1 C4=1 C5=1 C6=1 C7=1
16160 C0=1082 C1=10007 C2=2102 C3=1358 C4=228 C5=726 C6=322 C7=335
3 C0=1 C2=2
UDPv6                240    240   1088   30    8 : tunables    0    0    0 : slabdata      8      8      0

slabinfo says 240 actives. But sysfs's stats (need CONFIG_SLUB_STATS)
says, (16173 + 8) - (16160 + 3) == 18.

Thanks.
-- 
OGAWA Hirofumi <hirofumi at mail.parknet.co.jp>