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

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Message-ID: <CAKEwX=OBHe12R6fTbRn_dNGrz+T4ekE4MSo5w+7i_NNoprmnkw@mail.gmail.com>
Date:   Mon, 4 Dec 2023 09:48:52 -0800
From:   Nhat Pham <nphamcs@...il.com>
To:     Chengming Zhou <chengming.zhou@...ux.dev>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Matthew Wilcox <willy@...radead.org>,
        akpm@...ux-foundation.org, cerasuolodomenico@...il.com,
        yosryahmed@...gle.com, sjenning@...hat.com, ddstreet@...e.org,
        vitaly.wool@...sulko.com, mhocko@...nel.org,
        roman.gushchin@...ux.dev, shakeelb@...gle.com,
        muchun.song@...ux.dev, chrisl@...nel.org, linux-mm@...ck.org,
        kernel-team@...a.com, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kselftest@...r.kernel.org, shuah@...nel.org
Subject: Re: [PATCH v8 1/6] list_lru: allows explicit memcg and NUMA node selection

On Mon, Dec 4, 2023 at 12:30 AM Chengming Zhou <chengming.zhou@...ux.dev> wrote:
>
> 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@...radead.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!

That idea was considered in earlier iterations/discussions of the
patch series as well. Charging things is not free - there is an
overhead associated with it, which is why we are usually selective
about whether to charge something. We were not super keen to do this
for zswap_entry just to plumb around the list_lru's restriction. Might
as well pay the price of extending the list_lru interface now.

If in the future, not charging the zswap entry causes a separate
isolation issue, we could revisit this decision and charge it.
Otherwise, IMHO we should just stick with this for now.

>
> >
> > 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.
> >
> >