Re: [PATCH v6 12/17] mm: Set bit in memcg shrinker bitmap on first list_lru item apearance

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

On Mon, May 21, 2018 at 12:31:34PM +0300, Kirill Tkhai wrote:
> 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

I'd prefer them to think through all corner cases before touching this
code :-)

> 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.

The reason why I'd rather move those checks from memcg_set_shrinker_bit
to call sites is that now looking at the function code makes me wonder
why this function has to turn into a no-op if shrinker_id < 0 or memcg
is NULL, why these corner cases are even possible. To understand that, I
have to look at all places where this function is called, which are
located in a different source file. This is rather inconvenient IMO. But
I guess it's bikesheding so I don't insist.