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

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Date:   Thu, 9 Jun 2022 09:48:03 -0400
From:   Soheil Hassas Yeganeh <soheil@...gle.com>
To:     Eric Dumazet <edumazet@...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>, Wei Wang <weiwan@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>
Subject: Re: [PATCH net-next 4/7] net: implement per-cpu reserves for memory_allocated

On Thu, Jun 9, 2022 at 9:47 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Thu, Jun 9, 2022 at 6:34 AM Soheil Hassas Yeganeh <soheil@...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>
> >
> > Acked-by: Soheil Hassas Yeganeh <soheil@...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);
> >
> > This is just a nitpick, but we could
> > __this_cpu_write(*sk->sk_prot->per_cpu_fw_alloc, 0) instead which
> > should be slightly faster.
>
> This would require us to block irqs, not only preempt_disable()/preempt_enable()
>
> Otherwise when doing the write, there is no guarantee we replace the
> intended value,
> as an interrupt could have changed this cpu per_cpu_fw_alloc.
>
> A __this_cpu_cmpxchg() would make sure of that, but would be more
> expensive than __this_cpu_sub() and would require a loop.
>
>  With my change, there is a tiny possibility that
> *sk->sk_prot->per_cpu_fw_alloc, is not in the -1/+1 1MB range,
> but no lasting consequences, next update will consolidate things, and
> tcp_memory_allocated will not drift.

Ah that makes sense. Thank you for the explanation!

> >
> > > +               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);
> > > +       }
> > > +       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
> > >