Re: [PATCH v3 4/5] sock: Consider memcg pressure when raising sockmem

[Paolo Abeni]

On Tue, 2023-05-23 at 17:46 +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>
> ---
>  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..b899e0b9feda 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -2976,22 +2976,31 @@ EXPORT_SYMBOL(sk_wait_data);
>  int __sk_mem_raise_allocated(struct sock *sk, int size, int amt, int kind)
>  {
>  	bool memcg_charge = mem_cgroup_sockets_enabled && sk->sk_memcg;
> +	bool charged = true, pressured = false;
>  	struct proto *prot = sk->sk_prot;
> -	bool charged = true;
>  	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))
> +			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;

The above looks not correct to me. 

	allocated > sk_prot_mem_limits(sk, 0) 

does not mean the protocol has memory pressure. Such condition is
checked later with:

	if (allocated > sk_prot_mem_limits(sk, 1))

Here an allocation could fail even if memcg charge is successful and
the protocol is not under pressure, which in turn sounds quite (too
much?) conservative.

Cheers,

Paolo