linux-kernel - Re: [PATCH 2/2] mm: memcontrol: flush percpu slab vmstats on kmem offlining

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Date:   Wed, 14 Aug 2019 13:32:42 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Roman Gushchin <guro@...com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Johannes Weiner <hannes@...xchg.org>,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [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
nice.

[...]
> -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;
> +		max_idx = NR_SLAB_UNRECLAIMABLE;
> +	} 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
SUSE Labs