Re: [PATCH v10 27/35] lru: add an element to a memcg list

Glauber Costa
2013-06-06T08:44:44Z

>> ...
>>
>> @@ -53,12 +53,23 @@ struct list_lru {
>> * structure, we may very well fail.
>> */
>> struct list_lru_node node[MAX_NUMNODES];
>> + atomic_long_t node_totals[MAX_NUMNODES];
>> nodemask_t active_nodes;
>> #ifdef CONFIG_MEMCG_KMEM
>> /* All memcg-aware LRUs will be chained in the lrus list */
>> struct list_head lrus;
>> /* M x N matrix as described above */
>> struct list_lru_array **memcg_lrus;
>> + /*
>> + * The memcg_lrus is RCU protected
>
> It is? I don't recall seeing that in the earlier patches. Is some
> description missing?
>

Yes, it is.

memcg_update_lrus will do synchronize_rcu(), and lru_node_of_index will
do the read locking.

>> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
>> index 3442eb9..50f199f 100644
>> --- a/include/linux/memcontrol.h
>> +++ b/include/linux/memcontrol.h
>> @@ -24,6 +24,7 @@
>> #include <linux/hardirq.h>
>> #include <linux/jump_label.h>
>> #include <linux/list_lru.h>
>> +#include <linux/mm.h>
>
> erk. There's a good chance that mm.h already includes memcontrol.h, or
> vice versa, by some circuitous path. Expect problems from this.
>
> afaict the include is only needed for struct page? If so, simply
> adding a forward declaration for that would be prudent.
>
In fact, Rothwell had just already complained about this.
Funny, I have been running this with multiple configs for a while on 2
machines + kbot.

>
>> + * 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
>
> "it wasn't created or it"
>
>> + * has been already freed. In both cases it should no longer have
>> + * objects. BUG_ON to avoid a NULL dereference.
>
> Well. We could jsut permit the NULL reference - that provides the same
> info. But an explicit BUG_ON does show that it has been thought through!
>
Actually this is one of the bugs I have to fix. *right now* this code is
correct, but later on is not. When we are unmounting for instance, we
loop through all indexes. I have updated this in the patch that does the
loop, but to avoid generating more mental effort than it is due, I can
just move it right here.

>> + */
>> + BUG_ON(!lru->memcg_lrus[index]);
>> + nlru = &lru->memcg_lrus[index]->node[nid];
>> + rcu_read_unlock();
>> + return nlru;
>> +#else
>> + BUG_ON(index >= 0); /* nobody should be passing index < 0 with !KMEM */
>> + return &lru->node[nid];
>> +#endif
>> +}
>> +
>>
>> ...
>>
>> int
>> 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 page *page = virt_to_page(item);
>> + struct list_lru_node *nlru;
>> + int nid = page_to_nid(page);
>> +
>> + nlru = memcg_kmem_lru_of_page(lru, page);
>>
>> spin_lock(&nlru->lock);
>> BUG_ON(nlru->nr_items < 0);
>> if (list_empty(item)) {
>> list_add_tail(item, &nlru->list);
>> - if (nlru->nr_items++ == 0)
>> + nlru->nr_items++;
>> + /*
>> + * We only consider a node active or inactive based on the
>> + * total figure for all involved children.
>
> Is "children" an appropriate term in this context? Where would one go
> to understand the overall object hierarchy here?
>
children is always an appropriate term. Every time one mentions it
people go sentimental and are more likely to be helpful.

But that aside, I believe this could be changed to something else.

>> + */
>> + if (atomic_long_add_return(1, &lru->node_totals[nid]) == 1)
>> node_set(nid, lru->active_nodes);
>> spin_unlock(&nlru->lock);
>> return 1;
>>
>> ...
>>
>> + /*
>> + * Even if we were to use call_rcu, we still have to keep the old array
>> + * pointer somewhere. It is easier for us to just synchronize rcu here
>> + * since we are in a fine context. Now we guarantee that there are no
>> + * more users of old_array, and proceed freeing it for all LRUs
>
> "a fine context" is a fine term, but it's unclear what is meant by it ;)
>
fine!

I mean a synchronize_rcu friendly context (can sleep, etc)

>> + */
>> + synchronize_rcu();
>> + list_for_each_entry(lru, &all_memcg_lrus, lrus) {
>> + kfree(lru->old_array);
>> + lru->old_array = NULL;
>> + }
>> mutex_unlock(&all_memcg_lrus_mutex);
>> return ret;
>> }
>>

Here is the answer to your "is this really RCU protected?? " btw.

>> ...
>>
>> --- a/mm/memcontrol.c
>> +++ b/mm/memcontrol.c
>> @@ -3162,19 +3162,22 @@ int memcg_update_cache_sizes(struct mem_cgroup *memcg)
>> */
>> memcg_kmem_set_activated(memcg);
>>
>> - ret = memcg_update_all_caches(num+1);
>> - if (ret)
>> - goto out;
>> -
>> /*
>> - * We should make sure that the array size is not updated until we are
>> - * done; otherwise we have no easy way to know whether or not we should
>> - * grow the array.
>> + * We have to make absolutely sure that we update the LRUs before we
>> + * update the caches. Once the caches are updated, they will be able to
>> + * start hosting objects. If a cache is created very quickly, and and
>
> s/and/an/
>
>> + * element is used and disposed to the LRU quickly as well, we may end
>> + * up with a NULL pointer in list_lru_add because the lists are not yet
>> + * ready.
>> */
>> ret = memcg_update_all_lrus(num + 1);
>> if (ret)
>> goto out;
>>
>> + ret = memcg_update_all_caches(num+1);
>> + if (ret)
>> + goto out;
>> +
>> memcg->kmemcg_id = num;
>>
>> memcg_update_array_size(num + 1);
>>
>> ...
>>
>

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