netdev - Re: [PATCH net-next] tcp: sk_forced_mem

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Date:   Fri, 10 Jun 2022 23:44:52 -0400
From:   Soheil Hassas Yeganeh <soheil@...gle.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        netdev <netdev@...r.kernel.org>, Wei Wang <weiwan@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next] tcp: sk_forced_mem_schedule() optimization

On Fri, Jun 10, 2022 at 11:30 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> From: Eric Dumazet <edumazet@...gle.com>
>
> sk_memory_allocated_add() has three callers, and returns
> to them @memory_allocated.
>
> sk_forced_mem_schedule() is one of them, and ignores
> the returned value.
>
> Change sk_memory_allocated_add() to return void.
>
> Change sock_reserve_memory() and __sk_mem_raise_allocated()
> to call sk_memory_allocated().
>
> This removes one cache line miss [1] for RPC workloads,
> as first skbs in TCP write queue and receive queue go through
> sk_forced_mem_schedule().
>
> [1] Cache line holding tcp_memory_allocated.
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Acked-by: Soheil Hassas Yeganeh <soheil@...gle.com>

Nice find!

> ---
>  include/net/sock.h | 3 +--
>  net/core/sock.c    | 9 ++++++---
>  2 files changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 0063e8410a4e3ed91aef9cf34eb1127f7ce33b93..304a5e39d41e27105148058066e8fa23490cf9fa 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -1412,7 +1412,7 @@ sk_memory_allocated(const struct sock *sk)
>  /* 1 MB per cpu, in page units */
>  #define SK_MEMORY_PCPU_RESERVE (1 << (20 - PAGE_SHIFT))
>
> -static inline long
> +static inline void
>  sk_memory_allocated_add(struct sock *sk, int amt)
>  {
>         int local_reserve;
> @@ -1424,7 +1424,6 @@ sk_memory_allocated_add(struct sock *sk, int amt)
>                 atomic_long_add(local_reserve, sk->sk_prot->memory_allocated);
>         }
>         preempt_enable();
> -       return sk_memory_allocated(sk);
>  }
>
>  static inline void
> diff --git a/net/core/sock.c b/net/core/sock.c
> index 697d5c8e2f0def49351a7d38ec59ab5e875d3b10..92a0296ccb1842f11fb8dd4b2f768885d05daa8f 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -1019,7 +1019,8 @@ static int sock_reserve_memory(struct sock *sk, int bytes)
>                 return -ENOMEM;
>
>         /* pre-charge to forward_alloc */
> -       allocated = sk_memory_allocated_add(sk, pages);
> +       sk_memory_allocated_add(sk, pages);
> +       allocated = sk_memory_allocated(sk);
>         /* If the system goes into memory pressure with this
>          * precharge, give up and return error.
>          */
> @@ -2906,11 +2907,13 @@ EXPORT_SYMBOL(sk_wait_data);
>   */
>  int __sk_mem_raise_allocated(struct sock *sk, int size, int amt, int kind)
>  {
> -       struct proto *prot = sk->sk_prot;
> -       long allocated = sk_memory_allocated_add(sk, amt);
>         bool memcg_charge = mem_cgroup_sockets_enabled && sk->sk_memcg;
> +       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())))
> --
> 2.36.1.476.g0c4daa206d-goog
>