Re: [PATCH v2 3/4] sock: Consider memcg pressure when raising sockmem

[Shakeel Butt]

On Mon, May 22, 2023 at 03:01:21PM +0800, Abel Wu wrote:
> For now __sk_mem_raise_allocated() mainly considers global socket
> memory pressure and allows to raise if no global pressure observed,
> including the sockets whose memcgs are in pressure, which might
> result in longer memcg memstall.
> 
> So take net-memcg's pressure into consideration when allocating
> socket memory to alleviate long tail latencies.
> 
> Signed-off-by: Abel Wu <wuyun.abel@xxxxxxxxxxxxx>

Hi Abel,

Have you seen any real world production issue which is fixed by this
patch or is it more of a fix after reading code?

This code is quite subtle and small changes can cause unintended
behavior changes. At the moment the tcp memory accounting and memcg
accounting is intermingled and I think we should decouple them.

> ---
>  net/core/sock.c | 23 ++++++++++++++++-------
>  1 file changed, 16 insertions(+), 7 deletions(-)
> 
> diff --git a/net/core/sock.c b/net/core/sock.c
> index 801df091e37a..7641d64293af 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -2977,21 +2977,30 @@ int __sk_mem_raise_allocated(struct sock *sk, int size, int amt, int kind)
>  {
>  	bool memcg_charge = mem_cgroup_sockets_enabled && sk->sk_memcg;
>  	struct proto *prot = sk->sk_prot;
> -	bool charged = true;
> +	bool charged = true, pressured = false;
>  	long allocated;
>  
>  	sk_memory_allocated_add(sk, amt);
>  	allocated = sk_memory_allocated(sk);
> -	if (memcg_charge &&
> -	    !(charged = mem_cgroup_charge_skmem(sk->sk_memcg, amt,
> -						gfp_memcg_charge())))
> -		goto suppress_allocation;
> +
> +	if (memcg_charge) {
> +		charged = mem_cgroup_charge_skmem(sk->sk_memcg, amt,
> +						  gfp_memcg_charge());
> +		if (!charged)
> +			goto suppress_allocation;
> +		if (mem_cgroup_under_socket_pressure(sk->sk_memcg))

The memcg under pressure callback does a upward memcg tree walk, do
please make sure you have tested the performance impact of this.

> +			pressured = true;
> +	}
>  
>  	/* Under limit. */
> -	if (allocated <= sk_prot_mem_limits(sk, 0)) {
> +	if (allocated <= sk_prot_mem_limits(sk, 0))
>  		sk_leave_memory_pressure(sk);
> +	else
> +		pressured = true;
> +
> +	/* No pressure observed in global/memcg. */
> +	if (!pressured)
>  		return 1;
> -	}
>  
>  	/* Under pressure. */
>  	if (allocated > sk_prot_mem_limits(sk, 1))
> -- 
> 2.37.3
>