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SubjectRe: [PATCH 2/2] mm: memcontrol: flush percpu slab vmstats on kmem offlining
On Mon 12-08-19 15:29:11, Roman Gushchin wrote:
> 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.

So the difference with other counters is that slab ones are reparented
and that's why we have treat them specially? I guess that is what the
comment in the code suggest but being explicit in the changelog would be

> -static void memcg_flush_percpu_vmstats(struct mem_cgroup *memcg)
> +static void memcg_flush_percpu_vmstats(struct mem_cgroup *memcg, bool slab_only)
> {
> unsigned long stat[MEMCG_NR_STAT];
> struct mem_cgroup *mi;
> int node, cpu, i;
> + int min_idx, max_idx;
> - for (i = 0; i < MEMCG_NR_STAT; i++)
> + if (slab_only) {
> + min_idx = NR_SLAB_RECLAIMABLE;
> + } else {
> + min_idx = 0;
> + max_idx = MEMCG_NR_STAT;
> + }

This is just ugly has hell! I really detest how this implicitly makes
counters value very special without any note in the node_stat_item
definition. Is it such a big deal to have a per counter flush and do
the loop over all counters resp. specific counters around it so much
worse? This should be really a slow path to safe few instructions or
cache misses, no?
Michal Hocko

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