Re: [PATCH v3] memcg: add charging of already allocated slab objects

[Vlastimil Babka]

On 8/29/24 19:53, Shakeel Butt 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@xxxxxxxxx>

Thanks, pushed to slab/for-next for test coverage, hopefully net people will
ack.

Also one thing:

We should add some kernel doc for this, no? Explaining when people are
supposed to use this, that objects from KMALLOC_NORMAL will be ignored, and
what the return value means (including where it's faked to be true).

> +bool kmem_cache_charge(void *objp, gfp_t gfpflags)
> +{
> +	if (!memcg_kmem_online())
> +		return true;
> +
> +	return memcg_slab_post_charge(objp, gfpflags);
> +}
> +EXPORT_SYMBOL(kmem_cache_charge);
> +
>  /**
>   * kmem_cache_alloc_node - Allocate an object on the specified node
>   * @s: The cache to allocate from.
> diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
> index 64d07b842e73..3c13ca8c11fb 100644
> --- a/net/ipv4/inet_connection_sock.c
> +++ b/net/ipv4/inet_connection_sock.c
> @@ -715,6 +715,7 @@ struct sock *inet_csk_accept(struct sock *sk, struct proto_accept_arg *arg)
>  	release_sock(sk);
>  	if (newsk && mem_cgroup_sockets_enabled) {
>  		int amt = 0;
> +		gfp_t gfp = GFP_KERNEL | __GFP_NOFAIL;
>  
>  		/* atomically get the memory usage, set and charge the
>  		 * newsk->sk_memcg.
> @@ -731,8 +732,8 @@ struct sock *inet_csk_accept(struct sock *sk, struct proto_accept_arg *arg)
>  		}
>  
>  		if (amt)
> -			mem_cgroup_charge_skmem(newsk->sk_memcg, amt,
> -						GFP_KERNEL | __GFP_NOFAIL);
> +			mem_cgroup_charge_skmem(newsk->sk_memcg, amt, gfp);
> +		kmem_cache_charge(newsk, gfp);
>  
>  		release_sock(newsk);
>  	}