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

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Date:   Thu, 9 Jun 2022 08:07:34 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Neal Cardwell <ncardwell@...gle.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        "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>,
        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 7:46 AM Neal Cardwell <ncardwell@...gle.com> wrote:
>
> /
>
>
> 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!)

local_reserve is negative in that case and adding a negative number is
subtraction.