[Devel] Re: [PATCH 13/23] slub: create duplicate cache

[Glauber Costa]

On 04/24/2012 11:18 AM, Frederic Weisbecker wrote:
> On Sun, Apr 22, 2012 at 08:53:30PM -0300, Glauber Costa wrote:
>> This patch provides kmem_cache_dup(), that duplicates
>> a cache for a memcg, preserving its creation properties.
>> Object size, alignment and flags are all respected.
>>
>> When a duplicate cache is created, the parent cache cannot
>> be destructed during the child lifetime. To assure this,
>> its reference count is increased if the cache creation
>> succeeds.
>>
>> Signed-off-by: Glauber Costa<glommer at parallels.com>
>> CC: Christoph Lameter<cl at linux.com>
>> CC: Pekka Enberg<penberg at cs.helsinki.fi>
>> CC: Michal Hocko<mhocko at suse.cz>
>> CC: Kamezawa Hiroyuki<kamezawa.hiroyu at jp.fujitsu.com>
>> CC: Johannes Weiner<hannes at cmpxchg.org>
>> CC: Suleiman Souhlal<suleiman at google.com>
>> ---
>>   include/linux/memcontrol.h |    3 +++
>>   include/linux/slab.h       |    3 +++
>>   mm/memcontrol.c            |   44 ++++++++++++++++++++++++++++++++++++++++++++
>>   mm/slub.c                  |   37 +++++++++++++++++++++++++++++++++++++
>>   4 files changed, 87 insertions(+), 0 deletions(-)
>>
>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>> index 99e14b9..493ecdd 100644
>> --- a/include/linux/memcontrol.h
>> +++ b/include/linux/memcontrol.h
>> @@ -445,6 +445,9 @@ int memcg_css_id(struct mem_cgroup *memcg);
>>   void mem_cgroup_register_cache(struct mem_cgroup *memcg,
>>   				      struct kmem_cache *s);
>>   void mem_cgroup_release_cache(struct kmem_cache *cachep);
>> +extern char *mem_cgroup_cache_name(struct mem_cgroup *memcg,
>> +				   struct kmem_cache *cachep);
>> +
>>   #else
>>   static inline void mem_cgroup_register_cache(struct mem_cgroup *memcg,
>>   					     struct kmem_cache *s)
>> diff --git a/include/linux/slab.h b/include/linux/slab.h
>> index c7a7e05..909b508 100644
>> --- a/include/linux/slab.h
>> +++ b/include/linux/slab.h
>> @@ -323,6 +323,9 @@ extern void *__kmalloc_track_caller(size_t, gfp_t, unsigned long);
>>
>>   #ifdef CONFIG_CGROUP_MEM_RES_CTLR_KMEM
>>   #define MAX_KMEM_CACHE_TYPES 400
>> +extern struct kmem_cache *kmem_cache_dup(struct mem_cgroup *memcg,
>> +					 struct kmem_cache *cachep);
>> +void kmem_cache_drop_ref(struct kmem_cache *cachep);
>>   #else
>>   #define MAX_KMEM_CACHE_TYPES 0
>>   #endif /* CONFIG_CGROUP_MEM_RES_CTLR_KMEM */
>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>> index 0015ed0..e881d83 100644
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -467,6 +467,50 @@ struct cg_proto *tcp_proto_cgroup(struct mem_cgroup *memcg)
>>   EXPORT_SYMBOL(tcp_proto_cgroup);
>>   #endif /* CONFIG_INET */
>>
>> +/*
>> + * This is to prevent races againt the kmalloc cache creations.
>> + * Should never be used outside the core memcg code. Therefore,
>> + * copy it here, instead of letting it in lib/
>> + */
>> +static char *kasprintf_no_account(gfp_t gfp, const char *fmt, ...)
>> +{
>> +	unsigned int len;
>> +	char *p = NULL;
>> +	va_list ap, aq;
>> +
>> +	va_start(ap, fmt);
>> +	va_copy(aq, ap);
>> +	len = vsnprintf(NULL, 0, fmt, aq);
>> +	va_end(aq);
>> +
>> +	p = kmalloc_no_account(len+1, gfp);
>
> I can't seem to find kmalloc_no_account() in this patch or may be
> I missed it in a previous one?

It is in a previous one (actually two, one for the slab, one for the 
slub). They are bundled in the cache creation, but I could separate it
for clarity, if you prefer.


>> +	if (!p)
>> +		goto out;
>> +
>> +	vsnprintf(p, len+1, fmt, ap);
>> +
>> +out:
>> +	va_end(ap);
>> +	return p;
>> +}
>> +
>> +char *mem_cgroup_cache_name(struct mem_cgroup *memcg, struct kmem_cache *cachep)
>> +{
>> +	char *name;
>> +	struct dentry *dentry = memcg->css.cgroup->dentry;
>> +
>> +	BUG_ON(dentry == NULL);
>> +
>> +	/* Preallocate the space for "dead" at the end */
>> +	name = kasprintf_no_account(GFP_KERNEL, "%s(%d:%s)dead",
>> +	    cachep->name, css_id(&memcg->css), dentry->d_name.name);
>> +
>> +	if (name)
>> +		/* Remove "dead" */
>> +		name[strlen(name) - 4] = '\0';
>
> Why this space for "dead" ?

Ok, sorry. Since I didn't include the destruction part, it got too easy 
for whoever wasn't following the last discussion on this to get lost - 
My bad. So here it is:

When we destroy the memcg, some objects may still hold the cache in 
memory. It is like a reference count, in a sense, which each object 
being a reference.

In typical cases, like non-shrinkable caches that has create - destroy 
patterns, the caches will go away as soon as the tasks using them.

But in cache-like structure like the dentry cache, the objects may hang 
around until a shrinker pass takes them out. And even then, some of them 
will live on.

In this case, we will display them with "dead" in the name.

We could hide them, but then it gets weirder because it would be hard to 
understand where is your used memory when you need to inspect your system.

Creating another file, slabinfo_deadcaches, and keeping the names, is 
also a possibility, if people think that the string append is way too ugly.