Re: [PATCH v2] memcg: add charging of already allocated slab objects

[Yosry Ahmed <yosryahmed@google.com>]

On Tue, Aug 27, 2024 at 4:52 PM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>
> At the moment, the slab objects are charged to the memcg at the
> allocation time. However there are cases where slab objects are
> allocated at the time where the right target memcg to charge it to is
> not known. One such case is the network sockets for the incoming
> connection which are allocated in the softirq context.
>
> Couple hundred thousand connections are very normal on large loaded
> server and almost all of those sockets underlying those connections get
> allocated in the softirq context and thus not charged to any memcg.
> However later at the accept() time we know the right target memcg to
> charge. Let's add new API to charge already allocated objects, so we can
> have better accounting of the memory usage.
>
> To measure the performance impact of this change, tcp_crr is used from
> the neper [1] performance suite. Basically it is a network ping pong
> test with new connection for each ping pong.
>
> The server and the client are run inside 3 level of cgroup hierarchy
> using the following commands:
>
> Server:
>  $ tcp_crr -6
>
> Client:
>  $ tcp_crr -6 -c -H ${server_ip}
>
> If the client and server run on different machines with 50 GBPS NIC,
> there is no visible impact of the change.
>
> For the same machine experiment with v6.11-rc5 as base.
>
>           base (throughput)     with-patch
> tcp_crr   14545 (+- 80)         14463 (+- 56)
>
> It seems like the performance impact is within the noise.
>
> Link: https://github.com/google/neper [1]
> Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
> ---
> v1: https://lore.kernel.org/all/20240826232908.4076417-1-shakeel.butt@linux.dev/
> Changes since v1:
> - Correctly handle large allocations which bypass slab
> - Rearrange code to avoid compilation errors for !CONFIG_MEMCG builds
>
> RFC: https://lore.kernel.org/all/20240824010139.1293051-1-shakeel.butt@linux.dev/
> Changes since the RFC:
> - Added check for already charged slab objects.
> - Added performance results from neper's tcp_crr
>
>  include/linux/slab.h            |  1 +
>  mm/slub.c                       | 51 +++++++++++++++++++++++++++++++++
>  net/ipv4/inet_connection_sock.c |  5 ++--
>  3 files changed, 55 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index eb2bf4629157..05cfab107c72 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -547,6 +547,7 @@ void *kmem_cache_alloc_lru_noprof(struct kmem_cache *s, struct list_lru *lru,
>                             gfp_t gfpflags) __assume_slab_alignment __malloc;
>  #define kmem_cache_alloc_lru(...)      alloc_hooks(kmem_cache_alloc_lru_noprof(__VA_ARGS__))
>
> +bool kmem_cache_charge(void *objp, gfp_t gfpflags);
>  void kmem_cache_free(struct kmem_cache *s, void *objp);
>
>  kmem_buckets *kmem_buckets_create(const char *name, slab_flags_t flags,
> diff --git a/mm/slub.c b/mm/slub.c
> index c9d8a2497fd6..8265ea5f25be 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2185,6 +2185,43 @@ void memcg_slab_free_hook(struct kmem_cache *s, struct slab *slab, void **p,
>
>         __memcg_slab_free_hook(s, slab, p, objects, obj_exts);
>  }
> +
> +#define KMALLOC_TYPE (SLAB_KMALLOC | SLAB_CACHE_DMA | \
> +                     SLAB_ACCOUNT | SLAB_RECLAIM_ACCOUNT)
> +
> +static __fastpath_inline
> +bool memcg_slab_post_charge(void *p, gfp_t flags)
> +{
> +       struct slabobj_ext *slab_exts;
> +       struct kmem_cache *s;
> +       struct folio *folio;
> +       struct slab *slab;
> +       unsigned long off;
> +
> +       folio = virt_to_folio(p);
> +       if (!folio_test_slab(folio)) {
> +               return __memcg_kmem_charge_page(folio_page(folio, 0), flags,
> +                                               folio_order(folio)) == 0;
> +       }
> +
> +       slab = folio_slab(folio);
> +       s = slab->slab_cache;
> +
> +       /* Ignore KMALLOC_NORMAL cache to avoid circular dependency. */
> +       if ((s->flags & KMALLOC_TYPE) == SLAB_KMALLOC)
> +               return true;

Taking a step back here, why do we need this? Which circular
dependency are we avoiding here?