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SubjectRe: [PATCH v6 12/17] mm: Set bit in memcg shrinker bitmap on first list_lru item apearance
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On 20.05.2018 10:55, Vladimir Davydov wrote:
> On Fri, May 18, 2018 at 11:43:42AM +0300, Kirill Tkhai wrote:
>> Introduce set_shrinker_bit() function to set shrinker-related
>> bit in memcg shrinker bitmap, and set the bit after the first
>> item is added and in case of reparenting destroyed memcg's items.
>>
>> This will allow next patch to make shrinkers be called only,
>> in case of they have charged objects at the moment, and
>> to improve shrink_slab() performance.
>>
>> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
>> ---
>> include/linux/memcontrol.h | 14 ++++++++++++++
>> mm/list_lru.c | 22 ++++++++++++++++++++--
>> 2 files changed, 34 insertions(+), 2 deletions(-)
>>
>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>> index e51c6e953d7a..7ae1b94becf3 100644
>> --- a/include/linux/memcontrol.h
>> +++ b/include/linux/memcontrol.h
>> @@ -1275,6 +1275,18 @@ static inline int memcg_cache_id(struct mem_cgroup *memcg)
>>
>> extern int memcg_expand_shrinker_maps(int new_id);
>>
>> +static inline void memcg_set_shrinker_bit(struct mem_cgroup *memcg,
>> + int nid, int shrinker_id)
>> +{
>
>> + if (shrinker_id >= 0 && memcg && memcg != root_mem_cgroup) {
>
> Nit: I'd remove these checks from this function and require the caller
> to check that shrinker_id >= 0 and memcg != NULL or root_mem_cgroup.
> See below how the call sites would look then.
>
>> + struct memcg_shrinker_map *map;
>> +
>> + rcu_read_lock();
>> + map = rcu_dereference(memcg->nodeinfo[nid]->shrinker_map);
>> + set_bit(shrinker_id, map->map);
>> + rcu_read_unlock();
>> + }
>> +}
>> #else
>> #define for_each_memcg_cache_index(_idx) \
>> for (; NULL; )
>> @@ -1297,6 +1309,8 @@ static inline void memcg_put_cache_ids(void)
>> {
>> }
>>
>> +static inline void memcg_set_shrinker_bit(struct mem_cgroup *memcg,
>> + int nid, int shrinker_id) { }
>> #endif /* CONFIG_MEMCG_KMEM */
>>
>> #endif /* _LINUX_MEMCONTROL_H */
>> diff --git a/mm/list_lru.c b/mm/list_lru.c
>> index cab8fad7f7e2..7df71ab0de1c 100644
>> --- a/mm/list_lru.c
>> +++ b/mm/list_lru.c
>> @@ -31,6 +31,11 @@ static void list_lru_unregister(struct list_lru *lru)
>> mutex_unlock(&list_lrus_mutex);
>> }
>>
>> +static int lru_shrinker_id(struct list_lru *lru)
>> +{
>> + return lru->shrinker_id;
>> +}
>> +
>> static inline bool list_lru_memcg_aware(struct list_lru *lru)
>> {
>> /*
>> @@ -94,6 +99,11 @@ static void list_lru_unregister(struct list_lru *lru)
>> {
>> }
>>
>> +static int lru_shrinker_id(struct list_lru *lru)
>> +{
>> + return -1;
>> +}
>> +
>> static inline bool list_lru_memcg_aware(struct list_lru *lru)
>> {
>> return false;
>> @@ -119,13 +129,17 @@ bool list_lru_add(struct list_lru *lru, struct list_head *item)
>> {
>> int nid = page_to_nid(virt_to_page(item));
>> struct list_lru_node *nlru = &lru->node[nid];
>> + struct mem_cgroup *memcg;
>> struct list_lru_one *l;
>>
>> spin_lock(&nlru->lock);
>> if (list_empty(item)) {
>> - l = list_lru_from_kmem(nlru, item, NULL);
>> + l = list_lru_from_kmem(nlru, item, &memcg);
>> list_add_tail(item, &l->list);
>> - l->nr_items++;
>> + /* Set shrinker bit if the first element was added */
>> + if (!l->nr_items++)
>> + memcg_set_shrinker_bit(memcg, nid,
>> + lru_shrinker_id(lru));
>
> This would turn into
>
> if (!l->nr_items++ && memcg)
> memcg_set_shrinker_bit(memcg, nid, lru_shrinker_id(lru));
>
> Note, you don't need to check that lru_shrinker_id(lru) is >= 0 here as
> the fact that memcg != NULL guarantees that. Also, memcg can't be
> root_mem_cgroup here as kmem objects allocated for the root cgroup go
> unaccounted.
>
>> nlru->nr_items++;
>> spin_unlock(&nlru->lock);
>> return true;
>> @@ -520,6 +534,7 @@ static void memcg_drain_list_lru_node(struct list_lru *lru, int nid,
>> struct list_lru_node *nlru = &lru->node[nid];
>> int dst_idx = dst_memcg->kmemcg_id;
>> struct list_lru_one *src, *dst;
>> + bool set;
>>
>> /*
>> * Since list_lru_{add,del} may be called under an IRQ-safe lock,
>> @@ -531,7 +546,10 @@ static void memcg_drain_list_lru_node(struct list_lru *lru, int nid,
>> dst = list_lru_from_memcg_idx(nlru, dst_idx);
>>
>> list_splice_init(&src->list, &dst->list);
>> + set = (!dst->nr_items && src->nr_items);
>> dst->nr_items += src->nr_items;
>> + if (set)
>> + memcg_set_shrinker_bit(dst_memcg, nid, lru_shrinker_id(lru));
>
> This would turn into
>
> if (set && dst_idx >= 0)
> memcg_set_shrinker_bit(dst_memcg, nid, lru_shrinker_id(lru));
>
> Again, the shrinker is guaranteed to be memcg aware in this function and
> dst_memcg != NULL.
>
> IMHO such a change would make the code a bit more straightforward.

IMHO, this makes the code less readable. Using single generic function with
generic check is easier, then using two different checks for different places.
Next a person, who will modify the logic, does not have to think about particulars
of strange checks in list_lru_add() and memcg_drain_list_lru_node(), if he/she
does not involved in the change of maps logic. Memory cgroup is already fell
into many corner cases, let's do not introduce them in new places.

>> src->nr_items = 0;
>>
>> spin_unlock_irq(&nlru->lock);

Kirill

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