I've noticed that the "slab" value in memory.stat is sometimes 0, even if some children memory cgroups have a non-zero "slab" value. The following investigation showed that this is the result of the kmem_cache reparenting in combination with the per-cpu batching of slab vmstats. At the offlining some vmstat value may leave in the percpu cache, not being propagated upwards by the cgroup hierarchy. It means that stats on ancestor levels are lower than actual. Later when slab pages are released, the precise number of pages is substracted on the parent level, making the value negative. We don't show negative values, 0 is printed instead. To fix this issue, let's flush percpu slab memcg and lruvec stats on memcg offlining. This guarantees that numbers on all ancestor levels are accurate and match the actual number of outstanding slab pages.
Showing the changes to include/linux (1 of 2 files). Show all
Unified · Split
| @@ -215,8 +215,9 @@ enum node_stat_item { · 214 unmodified lines | ||
| 215 | 215 | NR_INACTIVE_FILE, /* " " " " " */ |
| 216 | 216 | NR_ACTIVE_FILE, /* " " " " " */ |
| 217 | 217 | NR_UNEVICTABLE, /* " " " " " */ |
| 218 | − NR_SLAB_RECLAIMABLE, | |
| 219 | − NR_SLAB_UNRECLAIMABLE, | |
| 218 | + NR_SLAB_RECLAIMABLE, /* Please do not reorder this item */ | |
| 219 | + NR_SLAB_UNRECLAIMABLE, /* and this one without looking at | |
| 220 | + * memcg_flush_percpu_vmstats() first. */ | |
| 220 | 221 | NR_ISOLATED_ANON, /* Temporary isolated pages from anon lru */ |
| 221 | 222 | NR_ISOLATED_FILE, /* Temporary isolated pages from file lru */ |
| 222 | 223 | WORKINGSET_NODES, |