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

["Li,Rongqing" <lirongqing@baidu.com>]

> -----邮件原件-----
> 发件人: Vladimir Davydov [mailto:vdavydov.dev@gmail.com]
> 发送时间: 2018年3月27日 17:09
> 收件人: Michal Hocko <mhocko@kernel.org>
> 抄送: Li,Rongqing <lirongqing@baidu.com>; linux-kernel@vger.kernel.org;
> linux-mm@kvack.org; Andrew Morton <akpm@linux-foundation.org>;
> Johannes Weiner <hannes@cmpxchg.org>; Dave Chinner
> <david@fromorbit.com>; Kirill Tkhai <ktkhai@virtuozzo.com>
> 主题: Re: [PATCH] mm/list_lru: replace spinlock with RCU in
> __list_lru_count_one
> 
> [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
> 
> 

Ok, thanks

-Rong


> https://lkml.kernel.org/r/152163840790.21546.980703278415599202.stgit
> @localhost.localdomain
> 
> 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@baidu.com>
> > > ---
> > >  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
> >