linux-kernel - Re: [PATCH 3/3] mm: oom: show unreclaimable slab info when unreclaimable slabs

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Message-ID: <20171004142736.u4z7zdar6g7bqgrj@dhcp22.suse.cz>
Date:   Wed, 4 Oct 2017 16:27:36 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Yang Shi <yang.s@...baba-inc.com>
Cc:     cl@...ux.com, penberg@...nel.org, rientjes@...gle.com,
        iamjoonsoo.kim@....com, akpm@...ux-foundation.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] mm: oom: show unreclaimable slab info when
 unreclaimable slabs > user memory

On Wed 04-10-17 02:06:17, Yang Shi wrote:
> +static bool is_dump_unreclaim_slabs(void)
> +{
> +	unsigned long nr_lru;
> +
> +	nr_lru = global_node_page_state(NR_ACTIVE_ANON) +
> +		 global_node_page_state(NR_INACTIVE_ANON) +
> +		 global_node_page_state(NR_ACTIVE_FILE) +
> +		 global_node_page_state(NR_INACTIVE_FILE) +
> +		 global_node_page_state(NR_ISOLATED_ANON) +
> +		 global_node_page_state(NR_ISOLATED_FILE) +
> +		 global_node_page_state(NR_UNEVICTABLE);
> +
> +	return (global_node_page_state(NR_SLAB_UNRECLAIMABLE) > nr_lru);
> +}

I am sorry I haven't pointed this earlier (I was following only half
way) but this should really be memcg aware. You are checking only global
counters. I do not think it is an absolute must to provide per-memcg
data but you should at least check !is_memcg_oom(oc).

[...]
> +void dump_unreclaimable_slab(void)
> +{
> +	struct kmem_cache *s, *s2;
> +	struct slabinfo sinfo;
> +
> +	pr_info("Unreclaimable slab info:\n");
> +	pr_info("Name                      Used          Total\n");
> +
> +	/*
> +	 * Here acquiring slab_mutex is risky since we don't prefer to get
> +	 * sleep in oom path. But, without mutex hold, it may introduce a
> +	 * risk of crash.
> +	 * Use mutex_trylock to protect the list traverse, dump nothing
> +	 * without acquiring the mutex.
> +	 */
> +	if (!mutex_trylock(&slab_mutex))
> +		return;

I would move the trylock up so that we do not get empty and confusing
Unreclaimable slab info: and add a note that we are not dumping anything
due to lock contention
	pr_warn("excessive unreclaimable slab memory but cannot dump stats to give you more details\n");

Other than that this looks sensible to me.

> +	list_for_each_entry_safe(s, s2, &slab_caches, list) {
> +		if (!is_root_cache(s) || (s->flags & SLAB_RECLAIM_ACCOUNT))
> +			continue;
> +
> +		memset(&sinfo, 0, sizeof(sinfo));
> +		get_slabinfo(s, &sinfo);
> +
> +		if (sinfo.num_objs > 0)
> +			pr_info("%-17s %10luKB %10luKB\n", cache_name(s),
> +				(sinfo.active_objs * s->size) / 1024,
> +				(sinfo.num_objs * s->size) / 1024);
> +	}
> +	mutex_unlock(&slab_mutex);
> +}
> +
>  #if defined(CONFIG_MEMCG) && !defined(CONFIG_SLOB)
>  void *memcg_slab_start(struct seq_file *m, loff_t *pos)
>  {
> -- 
> 1.8.3.1

-- 
Michal Hocko
SUSE Labs