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From: Michal Hocko <mhocko@kernel.org>
To: Roman Gushchin <guro@fb.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	linux-mm@kvack.org, Johannes Weiner <hannes@cmpxchg.org>,
	linux-kernel@vger.kernel.org, kernel-team@fb.com
Subject: Re: [PATCH 2/2] mm: memcontrol: flush percpu slab vmstats on kmem offlining
Date: Wed, 14 Aug 2019 13:32:42 +0200
Message-ID: <20190814113242.GV17933@dhcp22.suse.cz> (raw)
In-Reply-To: <20190812222911.2364802-3-guro@fb.com>

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


  reply index

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-12 22:29 [PATCH 0/2] flush percpu vmstats Roman Gushchin
2019-08-12 22:29 ` [PATCH 1/2] mm: memcontrol: flush percpu vmstats before releasing memcg Roman Gushchin
2019-08-13 21:27   ` Andrew Morton
2019-08-13 21:46     ` Roman Gushchin
2019-08-14 11:26   ` Michal Hocko
2019-08-12 22:29 ` [PATCH 2/2] mm: memcontrol: flush percpu slab vmstats on kmem offlining Roman Gushchin
2019-08-14 11:32   ` Michal Hocko [this message]
2019-08-14 21:54     ` Roman Gushchin
2019-08-15  8:35       ` Michal Hocko

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