Re: [PATCH v6 07/10] mm: synchronize access to kmem_cache dying flag using a spinlock

[Vladimir Davydov <vdavydov.dev@gmail.com>]

On Tue, Jun 04, 2019 at 07:44:51PM -0700, Roman Gushchin wrote:
> Currently the memcg_params.dying flag and the corresponding
> workqueue used for the asynchronous deactivation of kmem_caches
> is synchronized using the slab_mutex.
> 
> It makes impossible to check this flag from the irq context,
> which will be required in order to implement asynchronous release
> of kmem_caches.
> 
> So let's switch over to the irq-save flavor of the spinlock-based
> synchronization.
> 
> Signed-off-by: Roman Gushchin <guro@fb.com>
> ---
>  mm/slab_common.c | 19 +++++++++++++++----
>  1 file changed, 15 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index 09b26673b63f..2914a8f0aa85 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -130,6 +130,7 @@ int __kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t nr,
>  #ifdef CONFIG_MEMCG_KMEM
>  
>  LIST_HEAD(slab_root_caches);
> +static DEFINE_SPINLOCK(memcg_kmem_wq_lock);
>  
>  void slab_init_memcg_params(struct kmem_cache *s)
>  {
> @@ -629,6 +630,7 @@ void memcg_create_kmem_cache(struct mem_cgroup *memcg,
>  	struct memcg_cache_array *arr;
>  	struct kmem_cache *s = NULL;
>  	char *cache_name;
> +	bool dying;
>  	int idx;
>  
>  	get_online_cpus();
> @@ -640,7 +642,13 @@ void memcg_create_kmem_cache(struct mem_cgroup *memcg,
>  	 * The memory cgroup could have been offlined while the cache
>  	 * creation work was pending.
>  	 */
> -	if (memcg->kmem_state != KMEM_ONLINE || root_cache->memcg_params.dying)
> +	if (memcg->kmem_state != KMEM_ONLINE)
> +		goto out_unlock;
> +
> +	spin_lock_irq(&memcg_kmem_wq_lock);
> +	dying = root_cache->memcg_params.dying;
> +	spin_unlock_irq(&memcg_kmem_wq_lock);
> +	if (dying)
>  		goto out_unlock;

I do understand why we need to sync setting dying flag for a kmem cache
about to be destroyed in flush_memcg_workqueue vs checking the flag in
kmemcg_cache_deactivate: this is needed so that we don't schedule a new
deactivation work after we flush RCU/workqueue. However, I don't think
it's necessary to check the dying flag here, in memcg_create_kmem_cache:
we can't schedule a new cache creation work after kmem_cache_destroy has
started, because one mustn't allocate from a dead kmem cache; since we
flush the queue before getting to actual destruction, no cache creation
work can be pending. Yeah, it might happen that a cache creation work
starts execution while flush_memcg_workqueue is in progress, but I don't
see any point in optimizing this case - after all, cache destruction is
a very cold path. Since checking the flag in memcg_create_kmem_cache
raises question, I suggest to simply drop this check.

Anyway, it would be nice to see some comment in the code explaining why
we check dying flag under a spin lock in kmemcg_cache_deactivate.

>  
>  	idx = memcg_cache_id(memcg);
> @@ -735,14 +743,17 @@ static void kmemcg_cache_deactivate(struct kmem_cache *s)
>  
>  	__kmemcg_cache_deactivate(s);
>  
> +	spin_lock_irq(&memcg_kmem_wq_lock);
>  	if (s->memcg_params.root_cache->memcg_params.dying)
> -		return;
> +		goto unlock;
>  
>  	/* pin memcg so that @s doesn't get destroyed in the middle */
>  	css_get(&s->memcg_params.memcg->css);
>  
>  	s->memcg_params.work_fn = __kmemcg_cache_deactivate_after_rcu;
>  	call_rcu(&s->memcg_params.rcu_head, kmemcg_rcufn);
> +unlock:
> +	spin_unlock_irq(&memcg_kmem_wq_lock);
>  }
>  
>  void memcg_deactivate_kmem_caches(struct mem_cgroup *memcg)
> @@ -852,9 +863,9 @@ static int shutdown_memcg_caches(struct kmem_cache *s)
>  
>  static void flush_memcg_workqueue(struct kmem_cache *s)
>  {
> -	mutex_lock(&slab_mutex);
> +	spin_lock_irq(&memcg_kmem_wq_lock);
>  	s->memcg_params.dying = true;
> -	mutex_unlock(&slab_mutex);
> +	spin_unlock_irq(&memcg_kmem_wq_lock);
>  
>  	/*
>  	 * SLAB and SLUB deactivate the kmem_caches through call_rcu. Make