Re: [PATCH] mm, slab: eagerly delete inactive offlined SLABs

[Michal Hocko]

[Cc Vladimir]

On Wed 21-03-18 15:43:01, Shakeel Butt wrote:
> With kmem cgroup support, high memcgs churn can leave behind a lot of
> empty kmem_caches. Usually such kmem_caches will be destroyed when the
> corresponding memcg gets released but the memcg release can be
> arbitrarily delayed. These empty kmem_caches wastes cache_reaper's time.
> So, the reaper should destroy such empty offlined kmem_caches.
> 
> Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> ---
>  mm/slab.c        | 18 ++++++++++++++++--
>  mm/slab.h        | 15 +++++++++++++++
>  mm/slab_common.c |  2 +-
>  3 files changed, 32 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/slab.c b/mm/slab.c
> index 66f2db98f026..9c174a799ffb 100644
> --- a/mm/slab.c
> +++ b/mm/slab.c
> @@ -4004,6 +4004,16 @@ static void drain_array(struct kmem_cache *cachep, struct kmem_cache_node *n,
>  	slabs_destroy(cachep, &list);
>  }
>  
> +static bool is_slab_active(struct kmem_cache *cachep)
> +{
> +	int node;
> +	struct kmem_cache_node *n;
> +
> +	for_each_kmem_cache_node(cachep, node, n)
> +		if (READ_ONCE(n->total_slabs) - n->free_slabs)
> +			return true;
> +	return false;
> +}
>  /**
>   * cache_reap - Reclaim memory from caches.
>   * @w: work descriptor
> @@ -4018,7 +4028,7 @@ static void drain_array(struct kmem_cache *cachep, struct kmem_cache_node *n,
>   */
>  static void cache_reap(struct work_struct *w)
>  {
> -	struct kmem_cache *searchp;
> +	struct kmem_cache *searchp, *tmp;
>  	struct kmem_cache_node *n;
>  	int node = numa_mem_id();
>  	struct delayed_work *work = to_delayed_work(w);
> @@ -4027,7 +4037,7 @@ static void cache_reap(struct work_struct *w)
>  		/* Give up. Setup the next iteration. */
>  		goto out;
>  
> -	list_for_each_entry(searchp, &slab_caches, list) {
> +	list_for_each_entry_safe(searchp, tmp, &slab_caches, list) {
>  		check_irq_on();
>  
>  		/*
> @@ -4061,6 +4071,10 @@ static void cache_reap(struct work_struct *w)
>  				5 * searchp->num - 1) / (5 * searchp->num));
>  			STATS_ADD_REAPED(searchp, freed);
>  		}
> +
> +		/* Eagerly delete inactive kmem_cache of an offlined memcg. */
> +		if (!is_memcg_online(searchp) && !is_slab_active(searchp))
> +			shutdown_cache(searchp);
>  next:
>  		cond_resched();
>  	}
> diff --git a/mm/slab.h b/mm/slab.h
> index e8981e811c45..e911b10efae7 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -166,6 +166,7 @@ static inline slab_flags_t kmem_cache_flags(unsigned int object_size,
>  			      SLAB_TEMPORARY | \
>  			      SLAB_ACCOUNT)
>  
> +int shutdown_cache(struct kmem_cache *s);
>  int __kmem_cache_shutdown(struct kmem_cache *);
>  void __kmem_cache_release(struct kmem_cache *);
>  int __kmem_cache_shrink(struct kmem_cache *);
> @@ -290,6 +291,15 @@ static __always_inline void memcg_uncharge_slab(struct page *page, int order,
>  	memcg_kmem_uncharge(page, order);
>  }
>  
> +static __always_inline bool is_memcg_online(struct kmem_cache *s)
> +{
> +	if (!memcg_kmem_enabled())
> +		return true;
> +	if (is_root_cache(s))
> +		return true;
> +	return mem_cgroup_online(s->memcg_params.memcg);
> +}
> +
>  extern void slab_init_memcg_params(struct kmem_cache *);
>  extern void memcg_link_cache(struct kmem_cache *s);
>  extern void slab_deactivate_memcg_cache_rcu_sched(struct kmem_cache *s,
> @@ -342,6 +352,11 @@ static inline void memcg_uncharge_slab(struct page *page, int order,
>  {
>  }
>  
> +static inline bool is_memcg_online(struct kmem_cache *s)
> +{
> +	return true;
> +}
> +
>  static inline void slab_init_memcg_params(struct kmem_cache *s)
>  {
>  }
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 61ab2ca8bea7..d197e878636b 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -573,7 +573,7 @@ static void slab_caches_to_rcu_destroy_workfn(struct work_struct *work)
>  	}
>  }
>  
> -static int shutdown_cache(struct kmem_cache *s)
> +int shutdown_cache(struct kmem_cache *s)
>  {
>  	/* free asan quarantined objects */
>  	kasan_cache_shutdown(s);
> -- 
> 2.17.0.rc0.231.g781580f067-goog

-- 
Michal Hocko
SUSE Labs