Re: [PATCH v8 1/6] list_lru: allows explicit memcg and NUMA node selection

[Chengming Zhou <chengming.zhou@linux.dev>]

On 2023/12/1 04:35, Johannes Weiner wrote:
> On Thu, Nov 30, 2023 at 12:07:41PM -0800, Nhat Pham wrote:
>> On Thu, Nov 30, 2023 at 11:57 AM Matthew Wilcox <willy@infradead.org> wrote:
>>>
>>> On Thu, Nov 30, 2023 at 11:40:18AM -0800, Nhat Pham wrote:
>>>> This patch changes list_lru interface so that the caller must explicitly
>>>> specify numa node and memcg when adding and removing objects. The old
>>>> list_lru_add() and list_lru_del() are renamed to list_lru_add_obj() and
>>>> list_lru_del_obj(), respectively.
>>>
>>> Wouldn't it be better to add list_lru_add_memcg() and
>>> list_lru_del_memcg() and have:
>>>
>>> +bool list_lru_del(struct list_lru *lru, struct list_head *item)
>>> +{
>>> +       int nid = page_to_nid(virt_to_page(item));
>>> +       struct mem_cgroup *memcg = list_lru_memcg_aware(lru) ?
>>> +               mem_cgroup_from_slab_obj(item) : NULL;
>>> +
>>> +       return list_lru_del_memcg(lru, item, nid, memcg);
>>> +}
>>>
>>> Seems like _most_ callers will want the original versions and only
>>> a few will want the explicit memcg/nid versions.  No?
>>>
>>
>> I actually did something along that line in earlier iterations of this
>> patch series (albeit with poorer naming - __list_lru_add() instead of
>> list_lru_add_memcg()). The consensus after some back and forth was
>> that the original list_lru_add() was not a very good design (the
>> better one was this new version that allows for explicit numa/memcg
>> selection). So I agreed to fix it everywhere as a prep patch.
>>
>> I don't have strong opinions here to be completely honest, but I do
>> think this new API makes more sense (at the cost of quite a bit of
>> elbow grease to fix every callsites and extra reviewing).
> 
> Maybe I can shed some light since I was pushing for doing it this way.
> 
> The quiet assumption that 'struct list_head *item' is (embedded in) a
> slab object that is also charged to a cgroup is a bit much, given that
> nothing in the name or documentation of the function points to that.
> 
> It bit us in the THP shrinker where that list head is embedded in a
> tailpage (virt_to_page(page) is fun to debug). And it caused some
> confusion in this case as well, where the zswap entry is a slab object
> but not charged (the entry descriptor is not attractive for cgroup
> accounting, only the backing memory it points to.)

Hi,

I have a question, maybe I missed something since I haven't read all
the earlier versions.

IIUC, the problem here is that "zswap_entry" has different memcg and node
than the "page", so I wonder if we can just charge "zswap_entry" to the
same memcg of the "page".

Like we can do these when allocating the "zswap_entry":

	old_memcg = set_active_memcg(memcg)
	kmem_cache_alloc_lru(zswap_entry_cache, lru, gfp)
	set_active_memcg(old_memcg)

The good points are:

1. "zswap_entry" is charged to the memcg of "page", which is more sensible?

2. We can reuse the kmem_cache_alloc_lru() interface, which makes code simpler
   since we don't need to manage list_lru_memcg by ourselves.

3. Maybe the new list_lru_add() and list_lru_del() are not needed anymore?
   Since the "zswap_entry" is of the same memcg and node with the "page".
   But don't know if THP shrinker still need it.

Thanks!

> 
> Yes, for most users - at least right now - the current assumption is
> accurate. The thinking was just that if we do have to differentiate
> callers now anyway, we might as well make the interface a bit more
> self-documenting and harder to misuse going forward, even if it's a
> bit more churn now.
> 
>