linux-kernel - Re: [PATCH 2/2] mm: oom: show unreclaimable slab info when kernel panic

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Date:   Wed, 20 Sep 2017 05:45:30 +0800
From:   "Yang Shi" <yang.s@...baba-inc.com>
To:     David Rientjes <rientjes@...gle.com>
Cc:     cl@...ux.com, penberg@...nel.org, iamjoonsoo.kim@....com,
        akpm@...ux-foundation.org, mhocko@...nel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: oom: show unreclaimable slab info when kernel
 panic



On 9/19/17 1:57 PM, David Rientjes wrote:
> On Tue, 19 Sep 2017, Yang Shi wrote:
> 
>> --- a/mm/slab_common.c
>> +++ b/mm/slab_common.c
>> @@ -35,6 +35,8 @@
>>   static DECLARE_WORK(slab_caches_to_rcu_destroy_work,
>>   		    slab_caches_to_rcu_destroy_workfn);
>>   
>> +#define K(x) ((x)/1024)
>> +
>>   /*
>>    * Set of flags that will prevent slab merging
>>    */
>> @@ -1272,6 +1274,34 @@ static int slab_show(struct seq_file *m, void *p)
>>   	return 0;
>>   }
>>   
>> +void show_unreclaimable_slab()
>> +{
>> +	struct kmem_cache *s = NULL;
>> +	struct slabinfo sinfo;
>> +
>> +	memset(&sinfo, 0, sizeof(sinfo));
>> +
>> +	printk("Unreclaimable slabs:\n");
>> +
>> +	/*
>> +	 * Here acquiring slab_mutex is unnecessary since we don't prefer to
>> +	 * get sleep in oom path right before kernel panic, and avoid race condition.
>> +	 * Since it is already oom, so there should be not any big allocation
>> +	 * which could change the statistics significantly.
>> +	 */
>> +	list_for_each_entry(s, &slab_caches, list) {
>> +		if (!is_root_cache(s))
>> +			continue;
>> +
>> +		get_slabinfo(s, &sinfo);
>> +
>> +		if (!is_reclaimable(s) && sinfo.num_objs > 0)
>> +			printk("%-17s %luKB\n", cache_name(s), K(sinfo.num_objs * s->size));
>> +	}
> 
> I like this, but could we be even more helpful by giving the user more
> information from sinfo beyond just the total size of objects allocated?

Sure, we definitely can. But, the question is what info is helpful to 
users to diagnose oom other than the size.

I think of the below:
	- the number of active objs, the number of total objs, the percentage 
of active objs per cache
	- the number of active slabs, the number of total slabs, the percentage 
of active slabs per cache

Anything else?

Thanks,
Yang

>