linux-kernel - Re: [PATCH -mm v3 8/8] slab: do not keep free objects/slabs on dead memcg caches

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Date:	Tue, 24 Jun 2014 16:38:41 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Vladimir Davydov <vdavydov@...allels.com>
Cc:	akpm@...ux-foundation.org, cl@...ux.com, rientjes@...gle.com,
	penberg@...nel.org, hannes@...xchg.org, mhocko@...e.cz,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH -mm v3 8/8] slab: do not keep free objects/slabs on dead
 memcg caches

On Fri, Jun 13, 2014 at 12:38:22AM +0400, Vladimir Davydov wrote:
> Since a dead memcg cache is destroyed only after the last slab allocated
> to it is freed, we must disable caching of free objects/slabs for such
> caches, otherwise they will be hanging around forever.
> 
> For SLAB that means we must disable per cpu free object arrays and make
> free_block always discard empty slabs irrespective of node's free_limit.
> 
> To disable per cpu arrays, we free them on kmem_cache_shrink (see
> drain_cpu_caches -> do_drain) and make __cache_free fall back to
> free_block if there is no per cpu array. Also, we have to disable
> allocation of per cpu arrays on cpu hotplug for dead caches (see
> cpuup_prepare, __do_tune_cpucache).
> 
> After we disabled free objects/slabs caching, there is no need to reap
> those caches periodically. Moreover, it will only result in slowdown. So
> we also make cache_reap skip then.
> 
> Signed-off-by: Vladimir Davydov <vdavydov@...allels.com>
> ---
>  mm/slab.c |   31 ++++++++++++++++++++++++++++++-
>  1 file changed, 30 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/slab.c b/mm/slab.c
> index b3af82419251..7e91f5f1341d 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -1210,6 +1210,9 @@ static int cpuup_prepare(long cpu)
>  		struct array_cache *shared = NULL;
>  		struct array_cache **alien = NULL;
>  
> +		if (memcg_cache_dead(cachep))
> +			continue;
> +
>  		nc = alloc_arraycache(node, cachep->limit,
>  					cachep->batchcount, GFP_KERNEL);
>  		if (!nc)
> @@ -2411,10 +2414,18 @@ static void do_drain(void *arg)
>  
>  	check_irq_off();
>  	ac = cpu_cache_get(cachep);
> +	if (!ac)
> +		return;
> +
>  	spin_lock(&cachep->node[node]->list_lock);
>  	free_block(cachep, ac->entry, ac->avail, node);
>  	spin_unlock(&cachep->node[node]->list_lock);
>  	ac->avail = 0;
> +
> +	if (memcg_cache_dead(cachep)) {
> +		cachep->array[smp_processor_id()] = NULL;
> +		kfree(ac);
> +	}
>  }
>  
>  static void drain_cpu_caches(struct kmem_cache *cachep)
> @@ -3368,7 +3379,8 @@ static void free_block(struct kmem_cache *cachep, void **objpp, int nr_objects,
>  
>  		/* fixup slab chains */
>  		if (page->active == 0) {
> -			if (n->free_objects > n->free_limit) {
> +			if (n->free_objects > n->free_limit ||
> +			    memcg_cache_dead(cachep)) {
>  				n->free_objects -= cachep->num;
>  				/* No need to drop any previously held
>  				 * lock here, even if we have a off-slab slab
> @@ -3462,6 +3474,17 @@ static inline void __cache_free(struct kmem_cache *cachep, void *objp,
>  
>  	kmemcheck_slab_free(cachep, objp, cachep->object_size);
>  
> +#ifdef CONFIG_MEMCG_KMEM
> +	if (unlikely(!ac)) {
> +		int nodeid = page_to_nid(virt_to_page(objp));
> +
> +		spin_lock(&cachep->node[nodeid]->list_lock);
> +		free_block(cachep, &objp, 1, nodeid);
> +		spin_unlock(&cachep->node[nodeid]->list_lock);
> +		return;
> +	}
> +#endif
> +

And, please document intention of this code. :)

And, you said that this way of implementation would be slow because
there could be many object in dead caches and this implementation
needs node spin_lock on each object freeing. Is it no problem now?

If you have any performance data about this implementation and
alternative one, could you share it?

Thanks.
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