Re: [PATCH v10 28/35] list_lru: per-memcg walks

Andrew Morton
2013-06-05T23:08:37Z

On Mon, 3 Jun 2013 23:29:57 +0400 Glauber Costa <glommer@openvz.org> wrote:

> This patch extend the list_lru interfaces to allow for a memcg

"extends"

> parameter. Because most of its users won't need it, instead of
> modifying the function signatures we create a new set of _memcg()
> functions and write the old API ontop of that.

"on top"

> Signed-off-by: Glauber Costa <glommer@openvz.org>
> Cc: Dave Chinner <dchinner@redhat.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: Michal Hocko <mhocko@suse.cz>
> c: Hugh Dickins <hughd@google.com>

I'd rate him a d, personally.

> Cc: Kamezawa Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
>
> ...
>
> --- a/lib/list_lru.c
> +++ b/lib/list_lru.c
> @@ -50,13 +50,16 @@ lru_node_of_index(struct list_lru *lru, int index, int nid)
> rcu_read_lock();
> rmb();
> /*
> - * The array exist, but the particular memcg does not. That is an
> - * impossible situation: it would mean we are trying to add to a list
> - * belonging to a memcg that does not exist. Either wasn't created or
> - * has been already freed. In both cases it should no longer have
> - * objects. BUG_ON to avoid a NULL dereference.
> + * The array exist, but the particular memcg does not. This cannot
> + * happen when we are called from memcg_kmem_lru_of_page with a
> + * definite memcg, but it can happen when we are iterating over all
> + * memcgs (for instance, when disposing all lists.
> */
> - BUG_ON(!lru->memcg_lrus[index]);
> + if (!lru->memcg_lrus[index]) {
> + rcu_read_unlock();
> + return NULL;
> + }

It took 28 patches, but my head is now spinning and my vision is fading
in and out.

> nlru = &lru->memcg_lrus[index]->node[nid];
> rcu_read_unlock();
> return nlru;
> @@ -80,6 +83,23 @@ memcg_kmem_lru_of_page(struct list_lru *lru, struct page *page)
> return lru_node_of_index(lru, memcg_id, nid);
> }
>
> +/*
> + * This helper will loop through all node-data in the LRU, either global or
> + * per-memcg. If memcg is either not present or not used,
> + * memcg_limited_groups_array_size will be 0. _idx starts at -1, and it will
> + * still be allowed to execute once.
> + *
> + * We convention that for _idx = -1, the global node info should be used.

I don't think that "convention" is a verb, but I rather like the way
it is used here.

> + * After that, we will go through each of the memcgs, starting at 0.
> + *
> + * We don't need any kind of locking for the loop because
> + * memcg_limited_groups_array_size can only grow, gaining new fields at the
> + * end. The old ones are just copied, and any interesting manipulation happen
> + * in the node list itself, and we already lock the list.

Might be worth mentioning what type _idx should have. Although I suspect
the code will work OK if _idx has unsigned type.

> + */
> +#define for_each_memcg_lru_index(_idx) \
> + for ((_idx) = -1; ((_idx) < memcg_limited_groups_array_size); (_idx)++)
> +
> int
> list_lru_add(
> struct list_lru *lru,
> @@ -139,10 +159,19 @@ list_lru_del(
> EXPORT_SYMBOL_GPL(list_lru_del);
>
> unsigned long
> -list_lru_count_node(struct list_lru *lru, int nid)
> +list_lru_count_node_memcg(struct list_lru *lru, int nid,
> + struct mem_cgroup *memcg)
> {
> long count = 0;

But this function returns unsigned long.

> - struct list_lru_node *nlru = &lru->node[nid];
> + int memcg_id = -1;
> + struct list_lru_node *nlru;
> +
> + if (memcg && memcg_kmem_is_active(memcg))
> + memcg_id = memcg_cache_id(memcg);
> +
> + nlru = lru_node_of_index(lru, memcg_id, nid);
> + if (!nlru)
> + return 0;
>
> spin_lock(&nlru->lock);
> BUG_ON(nlru->nr_items < 0);
> @@ -151,19 +180,28 @@ list_lru_count_node(struct list_lru *lru, int nid)
>
> return count;
> }
> -EXPORT_SYMBOL_GPL(list_lru_count_node);
> +EXPORT_SYMBOL_GPL(list_lru_count_node_memcg);
>
> unsigned long
> -list_lru_walk_node(
> +list_lru_walk_node_memcg(
> struct list_lru *lru,
> int nid,
> list_lru_walk_cb isolate,
> void *cb_arg,
> - unsigned long *nr_to_walk)
> + unsigned long *nr_to_walk,
> + struct mem_cgroup *memcg)
> {
> - struct list_lru_node *nlru = &lru->node[nid];
> struct list_head *item, *n;
> unsigned long isolated = 0;
> + struct list_lru_node *nlru;
> + int memcg_id = -1;
> +
> + if (memcg && memcg_kmem_is_active(memcg))
> + memcg_id = memcg_cache_id(memcg);

Could use a helper function for this I guess. The nice thing about
this is that it gives one a logical place at which to describe what's
going on.

> + nlru = lru_node_of_index(lru, memcg_id, nid);
> + if (!nlru)
> + return 0;
>
> spin_lock(&nlru->lock);
> list_for_each_safe(item, n, &nlru->list) {
> @@ -200,7 +238,7 @@ restart:
> spin_unlock(&nlru->lock);
> return isolated;
> }
> -EXPORT_SYMBOL_GPL(list_lru_walk_node);
> +EXPORT_SYMBOL_GPL(list_lru_walk_node_memcg);
>
> static unsigned long
> list_lru_dispose_all_node(
>
> ...
>

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