Re: [PATCH net-next 2/2] sock: Fix improper heuristic on raising memory

[Shakeel Butt <shakeelb@google.com>]

On Wed, Sep 20, 2023 at 09:25:41PM +0800, Abel Wu wrote:
> Before sockets became aware of net-memcg's memory pressure since
> commit e1aab161e013 ("socket: initial cgroup code."), the memory
> usage would be granted to raise if below average even when under
> protocol's pressure. This provides fairness among the sockets of
> same protocol.
> 
> That commit changes this because the heuristic will also be
> effective when only memcg is under pressure which makes no sense.
> Fix this by skipping this heuristic when under memcg pressure.
> 
> Fixes: e1aab161e013 ("socket: initial cgroup code.")
> Signed-off-by: Abel Wu <wuyun.abel@bytedance.com>
> ---
>  net/core/sock.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/sock.c b/net/core/sock.c
> index 379eb8b65562..ef5cf6250f17 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -3093,8 +3093,16 @@ int __sk_mem_raise_allocated(struct sock *sk, int size, int amt, int kind)
>  	if (sk_has_memory_pressure(sk)) {
>  		u64 alloc;
>  
> -		if (!sk_under_memory_pressure(sk))
> +		if (memcg && mem_cgroup_under_socket_pressure(memcg))
> +			goto suppress_allocation;
> +
> +		if (!sk_under_global_memory_pressure(sk))
>  			return 1;

I am onboard with replacing sk_under_memory_pressure() with
sk_under_global_memory_pressure(). However suppressing on memcg pressure
is a behavior change from status quo and need more thought and testing.

I think there are three options for this hunk:

1. proposed patch
2. Consider memcg pressure only for !in_softirq().
3. Don't consider memcg pressure at all.

All three options are behavior change from the status quo but with
different risk levels. (1) may reintroduce the regression fixed by
720ca52bcef22 ("net-memcg: avoid stalls when under memory pressure").
(2) is more inlined with 720ca52bcef22. (3) has the risk to making memcg
limits ineffective.

IMHO we should go with (2) as there is already a precedence in
720ca52bcef22.

thanks,
Shakeel