linux-kernel - Re: [PATCH v6 17/19] mm: memcg/slab: use a single set of kmem

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Message-ID: <CALvZod5NCCpt2rkyXXr69OnVXb9ew7875vAV=iWZdqJhXcKEWQ@mail.gmail.com>
Date:   Mon, 22 Jun 2020 12:21:28 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Roman Gushchin <guro@...com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Lameter <cl@...ux.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Linux MM <linux-mm@...ck.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Kernel Team <kernel-team@...com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 17/19] mm: memcg/slab: use a single set of kmem_caches
 for all allocations

On Mon, Jun 8, 2020 at 4:07 PM Roman Gushchin <guro@...com> wrote:
>
> Instead of having two sets of kmem_caches: one for system-wide and
> non-accounted allocations and the second one shared by all accounted
> allocations, we can use just one.
>
> The idea is simple: space for obj_cgroup metadata can be allocated
> on demand and filled only for accounted allocations.
>
> It allows to remove a bunch of code which is required to handle
> kmem_cache clones for accounted allocations. There is no more need
> to create them, accumulate statistics, propagate attributes, etc.
> It's a quite significant simplification.
>
> Also, because the total number of slab_caches is reduced almost twice
> (not all kmem_caches have a memcg clone), some additional memory
> savings are expected. On my devvm it additionally saves about 3.5%
> of slab memory.
>
> Suggested-by: Johannes Weiner <hannes@...xchg.org>
> Signed-off-by: Roman Gushchin <guro@...com>
> Reviewed-by: Vlastimil Babka <vbabka@...e.cz>
> ---
[snip]
>  static inline void memcg_slab_post_alloc_hook(struct kmem_cache *s,
>                                               struct obj_cgroup *objcg,
> -                                             size_t size, void **p)
> +                                             gfp_t flags, size_t size,
> +                                             void **p)
>  {
>         struct page *page;
>         unsigned long off;
>         size_t i;
>
> +       if (!objcg)
> +               return;
> +
> +       flags &= ~__GFP_ACCOUNT;
>         for (i = 0; i < size; i++) {
>                 if (likely(p[i])) {
>                         page = virt_to_head_page(p[i]);
> +
> +                       if (!page_has_obj_cgroups(page) &&

The page is already linked into the kmem_cache, don't you need
synchronization for memcg_alloc_page_obj_cgroups(). What's the reason
to remove this from charge_slab_page()?

> +                           memcg_alloc_page_obj_cgroups(page, s, flags)) {
> +                               obj_cgroup_uncharge(objcg, obj_full_size(s));
> +                               continue;
> +                       }
> +
>                         off = obj_to_index(s, page, p[i]);
>                         obj_cgroup_get(objcg);
>                         page_obj_cgroups(page)[off] = objcg;