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SubjectRe: [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

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
>

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