netdev - Re: [PATCH net-next 4/7] net: implement per-cpu reserves for memory

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Date:   Thu, 9 Jun 2022 10:46:26 -0400
From:   Neal Cardwell <ncardwell@...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>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Wei Wang <weiwan@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next 4/7] net: implement per-cpu reserves for memory_allocated

/


On Thu, Jun 9, 2022 at 2:34 AM Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> From: Eric Dumazet <edumazet@...gle.com>
>
> We plan keeping sk->sk_forward_alloc as small as possible
> in future patches.
>
> This means we are going to call sk_memory_allocated_add()
> and sk_memory_allocated_sub() more often.
>
> Implement a per-cpu cache of +1/-1 MB, to reduce number
> of changes to sk->sk_prot->memory_allocated, which
> would otherwise be cause of false sharing.
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---
>  include/net/sock.h | 38 +++++++++++++++++++++++++++++---------
>  1 file changed, 29 insertions(+), 9 deletions(-)
>
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 825f8cbf791f02d798f17dd4f7a2659cebb0e98a..59040fee74e7de8d63fbf719f46e172906c134bb 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -1397,22 +1397,48 @@ static inline bool sk_under_memory_pressure(const struct sock *sk)
>         return !!*sk->sk_prot->memory_pressure;
>  }
>
> +static inline long
> +proto_memory_allocated(const struct proto *prot)
> +{
> +       return max(0L, atomic_long_read(prot->memory_allocated));
> +}
> +
>  static inline long
>  sk_memory_allocated(const struct sock *sk)
>  {
> -       return atomic_long_read(sk->sk_prot->memory_allocated);
> +       return proto_memory_allocated(sk->sk_prot);
>  }
>
> +/* 1 MB per cpu, in page units */
> +#define SK_MEMORY_PCPU_RESERVE (1 << (20 - PAGE_SHIFT))
> +
>  static inline long
>  sk_memory_allocated_add(struct sock *sk, int amt)
>  {
> -       return atomic_long_add_return(amt, sk->sk_prot->memory_allocated);
> +       int local_reserve;
> +
> +       preempt_disable();
> +       local_reserve = __this_cpu_add_return(*sk->sk_prot->per_cpu_fw_alloc, amt);
> +       if (local_reserve >= SK_MEMORY_PCPU_RESERVE) {
> +               __this_cpu_sub(*sk->sk_prot->per_cpu_fw_alloc, local_reserve);
> +               atomic_long_add(local_reserve, sk->sk_prot->memory_allocated);
> +       }
> +       preempt_enable();
> +       return sk_memory_allocated(sk);
>  }
>
>  static inline void
>  sk_memory_allocated_sub(struct sock *sk, int amt)
>  {
> -       atomic_long_sub(amt, sk->sk_prot->memory_allocated);
> +       int local_reserve;
> +
> +       preempt_disable();
> +       local_reserve = __this_cpu_sub_return(*sk->sk_prot->per_cpu_fw_alloc, amt);
> +       if (local_reserve <= -SK_MEMORY_PCPU_RESERVE) {
> +               __this_cpu_sub(*sk->sk_prot->per_cpu_fw_alloc, local_reserve);
> +               atomic_long_add(local_reserve, sk->sk_prot->memory_allocated);

I would have thought these last two lines would be:

               __this_cpu_add(*sk->sk_prot->per_cpu_fw_alloc, local_reserve);
               atomic_long_sub(local_reserve, sk->sk_prot->memory_allocated);

Otherwise I don't see how sk->sk_prot->memory_allocated) ever
decreases in these sk_memory_allocated_add/sk_memory_allocated_sub
functions?

That is, is there a copy-and-paste/typo issue in these two lines? Or
is my understanding backwards? (In which case I apologize for the
noise!)

thanks,
neal





> +       }
> +       preempt_enable();
>  }
>
>  #define SK_ALLOC_PERCPU_COUNTER_BATCH 16
> @@ -1441,12 +1467,6 @@ proto_sockets_allocated_sum_positive(struct proto *prot)
>         return percpu_counter_sum_positive(prot->sockets_allocated);
>  }
>
> -static inline long
> -proto_memory_allocated(struct proto *prot)
> -{
> -       return atomic_long_read(prot->memory_allocated);
> -}
> -
>  static inline bool
>  proto_memory_pressure(struct proto *prot)
>  {
> --
> 2.36.1.255.ge46751e96f-goog
>