Re: [PATCH] mm/list_lru: replace spinlock with RCU in __list_lru_count_one

[Vladimir Davydov]

[Cc Kirill]

AFAIU this has already been fixed in exactly the same fashion by Kirill
(mmotm commit 8e7d1201ec71 "mm: make counting of list_lru_one::nr_items
lockless"). Kirill is working on further optimizations right now, see

  https://lkml.kernel.org/r/152163840790.21546.980703278415599202.stgit@xxxxxxxxxxxxxxxxxxxxx

On Tue, Mar 27, 2018 at 10:15:46AM +0200, Michal Hocko wrote:
> [CC Dave]
> 
> On Tue 27-03-18 15:59:04, Li RongQing wrote:
> > when reclaim memory, shink_slab will take lots of time even if
> > no memory is reclaimed, since list_lru_count_one called by it
> > needs to take a spinlock
> >
> > try to optimize it by replacing spinlock with RCU in
> > __list_lru_count_one
> 
> Isn't the RCU overkill here? Why cannot we simply do an optimistic
> lockless check for nr_items? It would be racy but does it actually
> matter? We should be able to tolerate occasional 0 to non-zero and vice
> versa transitions AFAICS.
> 
> > 
> >     $dd if=aaa  of=bbb  bs=1k count=3886080
> >     $rm -f bbb
> >     $time echo 100000000 >/cgroup/memory/test/memory.limit_in_bytes
> > 
> > Before: 0m0.415s ===> after: 0m0.395s
> > 
> > Signed-off-by: Li RongQing <lirongqing@xxxxxxxxx>
> > ---
> >  include/linux/list_lru.h |  2 ++
> >  mm/list_lru.c            | 69 ++++++++++++++++++++++++++++++++++--------------
> >  2 files changed, 51 insertions(+), 20 deletions(-)
> > 
> > diff --git a/include/linux/list_lru.h b/include/linux/list_lru.h
> > index bb8129a3474d..ae472538038e 100644
> > --- a/include/linux/list_lru.h
> > +++ b/include/linux/list_lru.h
> > @@ -29,6 +29,7 @@ struct list_lru_one {
> >  	struct list_head	list;
> >  	/* may become negative during memcg reparenting */
> >  	long			nr_items;
> > +	struct rcu_head		rcu;
> >  };
> >  
> >  struct list_lru_memcg {
> > @@ -46,6 +47,7 @@ struct list_lru_node {
> >  	struct list_lru_memcg	*memcg_lrus;
> >  #endif
> >  	long nr_items;
> > +	struct rcu_head		rcu;
> >  } ____cacheline_aligned_in_smp;
> >  
> >  struct list_lru {
> > diff --git a/mm/list_lru.c b/mm/list_lru.c
> > index fd41e969ede5..4c58ed861729 100644
> > --- a/mm/list_lru.c
> > +++ b/mm/list_lru.c
> > @@ -52,13 +52,13 @@ static inline bool list_lru_memcg_aware(struct list_lru *lru)
> >  static inline struct list_lru_one *
> >  list_lru_from_memcg_idx(struct list_lru_node *nlru, int idx)
> >  {
> > -	/*
> > -	 * The lock protects the array of per cgroup lists from relocation
> > -	 * (see memcg_update_list_lru_node).
> > -	 */
> > -	lockdep_assert_held(&nlru->lock);
> > -	if (nlru->memcg_lrus && idx >= 0)
> > -		return nlru->memcg_lrus->lru[idx];
> > +	struct list_lru_memcg *tmp;
> > +
> > +	WARN_ON_ONCE(!rcu_read_lock_held());
> > +
> > +	tmp = rcu_dereference(nlru->memcg_lrus);
> > +	if (tmp && idx >= 0)
> > +		return rcu_dereference(tmp->lru[idx]);
> >  
> >  	return &nlru->lru;
> >  }
> > @@ -113,14 +113,17 @@ bool list_lru_add(struct list_lru *lru, struct list_head *item)
> >  	struct list_lru_one *l;
> >  
> >  	spin_lock(&nlru->lock);
> > +	rcu_read_lock();
> >  	if (list_empty(item)) {
> >  		l = list_lru_from_kmem(nlru, item);
> >  		list_add_tail(item, &l->list);
> >  		l->nr_items++;
> >  		nlru->nr_items++;
> > +		rcu_read_unlock();
> >  		spin_unlock(&nlru->lock);
> >  		return true;
> >  	}
> > +	rcu_read_unlock();
> >  	spin_unlock(&nlru->lock);
> >  	return false;
> >  }
> > @@ -133,14 +136,17 @@ bool list_lru_del(struct list_lru *lru, struct list_head *item)
> >  	struct list_lru_one *l;
> >  
> >  	spin_lock(&nlru->lock);
> > +	rcu_read_lock();
> >  	if (!list_empty(item)) {
> >  		l = list_lru_from_kmem(nlru, item);
> >  		list_del_init(item);
> >  		l->nr_items--;
> >  		nlru->nr_items--;
> > +		rcu_read_unlock();
> >  		spin_unlock(&nlru->lock);
> >  		return true;
> >  	}
> > +	rcu_read_unlock();
> >  	spin_unlock(&nlru->lock);
> >  	return false;
> >  }
> > @@ -166,12 +172,13 @@ static unsigned long __list_lru_count_one(struct list_lru *lru,
> >  {
> >  	struct list_lru_node *nlru = &lru->node[nid];
> >  	struct list_lru_one *l;
> > -	unsigned long count;
> > +	unsigned long count = 0;
> >  
> > -	spin_lock(&nlru->lock);
> > +	rcu_read_lock();
> >  	l = list_lru_from_memcg_idx(nlru, memcg_idx);
> > -	count = l->nr_items;
> > -	spin_unlock(&nlru->lock);
> > +	if (l)
> > +		count = l->nr_items;
> > +	rcu_read_unlock();
> >  
> >  	return count;
> >  }
> > @@ -204,6 +211,7 @@ __list_lru_walk_one(struct list_lru *lru, int nid, int memcg_idx,
> >  	unsigned long isolated = 0;
> >  
> >  	spin_lock(&nlru->lock);
> > +	rcu_read_lock();
> >  	l = list_lru_from_memcg_idx(nlru, memcg_idx);
> >  restart:
> >  	list_for_each_safe(item, n, &l->list) {
> > @@ -250,6 +258,7 @@ __list_lru_walk_one(struct list_lru *lru, int nid, int memcg_idx,
> >  		}
> >  	}
> >  
> > +	rcu_read_unlock();
> >  	spin_unlock(&nlru->lock);
> >  	return isolated;
> >  }
> > @@ -296,9 +305,14 @@ static void __memcg_destroy_list_lru_node(struct list_lru_memcg *memcg_lrus,
> >  					  int begin, int end)
> >  {
> >  	int i;
> > +	struct list_lru_one *tmp;
> >  
> > -	for (i = begin; i < end; i++)
> > -		kfree(memcg_lrus->lru[i]);
> > +	for (i = begin; i < end; i++) {
> > +		tmp = memcg_lrus->lru[i];
> > +		rcu_assign_pointer(memcg_lrus->lru[i], NULL);
> > +		if (tmp)
> > +			kfree_rcu(tmp, rcu);
> > +	}
> >  }
> >  
> >  static int __memcg_init_list_lru_node(struct list_lru_memcg *memcg_lrus,
> > @@ -314,7 +328,7 @@ static int __memcg_init_list_lru_node(struct list_lru_memcg *memcg_lrus,
> >  			goto fail;
> >  
> >  		init_one_lru(l);
> > -		memcg_lrus->lru[i] = l;
> > +		rcu_assign_pointer(memcg_lrus->lru[i], l);
> >  	}
> >  	return 0;
> >  fail:
> > @@ -325,25 +339,37 @@ static int __memcg_init_list_lru_node(struct list_lru_memcg *memcg_lrus,
> >  static int memcg_init_list_lru_node(struct list_lru_node *nlru)
> >  {
> >  	int size = memcg_nr_cache_ids;
> > +	struct list_lru_memcg *tmp;
> >  
> > -	nlru->memcg_lrus = kvmalloc(size * sizeof(void *), GFP_KERNEL);
> > -	if (!nlru->memcg_lrus)
> > +	tmp = kvmalloc(size * sizeof(void *), GFP_KERNEL);
> > +	if (!tmp)
> >  		return -ENOMEM;
> >  
> > -	if (__memcg_init_list_lru_node(nlru->memcg_lrus, 0, size)) {
> > -		kvfree(nlru->memcg_lrus);
> > +	if (__memcg_init_list_lru_node(tmp, 0, size)) {
> > +		kvfree(tmp);
> >  		return -ENOMEM;
> >  	}
> >  
> > +	rcu_assign_pointer(nlru->memcg_lrus, tmp);
> > +
> >  	return 0;
> >  }
> >  
> > -static void memcg_destroy_list_lru_node(struct list_lru_node *nlru)
> > +static void memcg_destroy_list_lru_node_rcu(struct rcu_head *rcu)
> >  {
> > +	struct list_lru_node *nlru;
> > +
> > +	nlru = container_of(rcu, struct list_lru_node, rcu);
> > +
> >  	__memcg_destroy_list_lru_node(nlru->memcg_lrus, 0, memcg_nr_cache_ids);
> >  	kvfree(nlru->memcg_lrus);
> >  }
> >  
> > +static void memcg_destroy_list_lru_node(struct list_lru_node *nlru)
> > +{
> > +	call_rcu(&nlru->rcu, memcg_destroy_list_lru_node_rcu);
> > +}
> > +
> >  static int memcg_update_list_lru_node(struct list_lru_node *nlru,
> >  				      int old_size, int new_size)
> >  {
> > @@ -371,9 +397,10 @@ static int memcg_update_list_lru_node(struct list_lru_node *nlru,
> >  	 * we have to use IRQ-safe primitives here to avoid deadlock.
> >  	 */
> >  	spin_lock_irq(&nlru->lock);
> > -	nlru->memcg_lrus = new;
> > +	rcu_assign_pointer(nlru->memcg_lrus, new);
> >  	spin_unlock_irq(&nlru->lock);
> >  
> > +	synchronize_rcu();
> >  	kvfree(old);
> >  	return 0;
> >  }
> > @@ -487,6 +514,7 @@ static void memcg_drain_list_lru_node(struct list_lru_node *nlru,
> >  	 * we have to use IRQ-safe primitives here to avoid deadlock.
> >  	 */
> >  	spin_lock_irq(&nlru->lock);
> > +	rcu_read_lock();
> >  
> >  	src = list_lru_from_memcg_idx(nlru, src_idx);
> >  	dst = list_lru_from_memcg_idx(nlru, dst_idx);
> > @@ -495,6 +523,7 @@ static void memcg_drain_list_lru_node(struct list_lru_node *nlru,
> >  	dst->nr_items += src->nr_items;
> >  	src->nr_items = 0;
> >  
> > +	rcu_read_unlock();
> >  	spin_unlock_irq(&nlru->lock);
> >  }
> >  
> > -- 
> > 2.11.0
> 
> -- 
> Michal Hocko
> SUSE Labs
>