Re: [External] Re: [PATCH v2 17/21] mm: list_lru: replace linear array with xarray

[Muchun Song <songmuchun@bytedance.com>]

On Thu, May 27, 2021 at 8:08 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Thu, May 27, 2021 at 02:21:44PM +0800, Muchun Song wrote:
> > If we run 10k containers in the system, the size of the
> > list_lru_memcg->lrus can be ~96KB per list_lru. When we decrease the
> > number containers, the size of the array will not be shrinked. It is
> > not scalable. The xarray is a good choice for this case. We can save
> > a lot of memory when there are tens of thousands continers in the
> > system. If we use xarray, we also can remove the logic code of
> > resizing array, which can simplify the code.
>
> I am all for this, in concept.  Some thoughts below ...
>
> > @@ -56,10 +51,8 @@ struct list_lru {
> >  #ifdef CONFIG_MEMCG_KMEM
> >       struct list_head        list;
> >       int                     shrinker_id;
> > -     /* protects ->memcg_lrus->lrus[i] */
> > -     spinlock_t              lock;
> >       /* for cgroup aware lrus points to per cgroup lists, otherwise NULL */
> > -     struct list_lru_memcg   __rcu *memcg_lrus;
> > +     struct xarray           *xa;
> >  #endif
>
> Normally, we embed an xarray in its containing structure instead of
> allocating it.  It's only a pointer, int and spinlock, so generally
> 16 bytes, as opposed to the 8 bytes for the pointer and a 16 byte
> allocation.  There is a minor wrinkle in that currently 'NULL' is
> used to indicate "is not cgroup aware".  Maybe there's another way
> to indicate that?

Sure. I can drop patch 8 in this series. In that case, we can use
->memcg_aware to indicate that.


>
> > @@ -51,22 +51,12 @@ static int lru_shrinker_id(struct list_lru *lru)
> >  static inline struct list_lru_one *
> >  list_lru_from_memcg_idx(struct list_lru *lru, int nid, int idx)
> >  {
> > -     struct list_lru_memcg *memcg_lrus;
> > -     struct list_lru_node *nlru = &lru->node[nid];
> > +     if (list_lru_memcg_aware(lru) && idx >= 0) {
> > +             struct list_lru_per_memcg *mlru = xa_load(lru->xa, idx);
> >
> > -     /*
> > -      * Either lock or RCU protects the array of per cgroup lists
> > -      * from relocation (see memcg_update_list_lru).
> > -      */
> > -     memcg_lrus = rcu_dereference_check(lru->memcg_lrus,
> > -                                        lockdep_is_held(&nlru->lock));
> > -     if (memcg_lrus && idx >= 0) {
> > -             struct list_lru_per_memcg *mlru;
> > -
> > -             mlru = rcu_dereference_check(memcg_lrus->lrus[idx], true);
> >               return mlru ? &mlru->nodes[nid] : NULL;
> >       }
> > -     return &nlru->lru;
> > +     return &lru->node[nid].lru;
> >  }
>
> ... perhaps we move the xarray out from under the #ifdef and use index 0
> for non-memcg-aware lrus?  The XArray is specially optimised for arrays
> which only have one entry at 0.

Sounds like a good idea. I can do a try.

>
> >  int list_lru_memcg_alloc(struct list_lru *lru, struct mem_cgroup *memcg, gfp_t gfp)
> >  {
> > +     XA_STATE(xas, lru->xa, 0);
> >       unsigned long flags;
> > -     struct list_lru_memcg *memcg_lrus;
> > -     int i;
> > +     int i, ret = 0;
> >
> >       struct list_lru_memcg_table {
> >               struct list_lru_per_memcg *mlru;
> > @@ -601,22 +522,45 @@ int list_lru_memcg_alloc(struct list_lru *lru, struct mem_cgroup *memcg, gfp_t g
> >               }
> >       }
> >
> > -     spin_lock_irqsave(&lru->lock, flags);
> > -     memcg_lrus = rcu_dereference_protected(lru->memcg_lrus, true);
> > +     xas_lock_irqsave(&xas, flags);
> >       while (i--) {
> >               int index = memcg_cache_id(table[i].memcg);
> >               struct list_lru_per_memcg *mlru = table[i].mlru;
> >
> > -             if (index < 0 || rcu_dereference_protected(memcg_lrus->lrus[index], true))
> > +             xas_set(&xas, index);
> > +retry:
> > +             if (unlikely(index < 0 || ret || xas_load(&xas))) {
> >                       kfree(mlru);
> > -             else
> > -                     rcu_assign_pointer(memcg_lrus->lrus[index], mlru);
> > +             } else {
> > +                     ret = xa_err(xas_store(&xas, mlru));
>
> This is mixing advanced and normal XArray concepts ... sorry to have
> confused you.  I think what you meant to do here was:
>
>                         xas_store(&xas, mlru);
>                         ret = xas_error(&xas);

Sure. Thanks for pointing it out. It's my bad usage.

>
> Or you can avoid introducing 'ret' at all, and keep your errors in the
> xa_state.  You're kind of mirroring the xa_state errors into 'ret'
> anyway, so that seems easier to understand?

Make sense. I will do this in the next version. Thanks for your
all suggestions.

>
> > -     memcg_id = memcg_alloc_cache_id();
> > +     memcg_id = ida_simple_get(&memcg_cache_ida, 0, MEMCG_CACHES_MAX_SIZE,
> > +                               GFP_KERNEL);
>
>         memcg_id = ida_alloc_max(&memcg_cache_ida,
>                         MEMCG_CACHES_MAX_SIZE - 1, GFP_KERNEL);
>
> ... although i think there's actually a fencepost error, and this really
> should be MEMCG_CACHES_MAX_SIZE.

Totally agree. I have fixed this issue in patch 19.

>
> >       objcg = obj_cgroup_alloc();
> >       if (!objcg) {
> > -             memcg_free_cache_id(memcg_id);
> > +             ida_simple_remove(&memcg_cache_ida, memcg_id);
>
>                 ida_free(&memcg_cache_ida, memcg_id);

I Will update to this new API.

>