netdev - Re: [PATCH net-next] packet: packet fanout rollover during socket overload

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Date:	Wed, 13 Mar 2013 11:51:50 -0400
From:	Willem de Bruijn <willemb@...gle.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org, David Miller <davem@...emloft.net>
Subject: Re: [PATCH net-next] packet: packet fanout rollover during socket overload

On Wed, Mar 13, 2013 at 10:25 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> On Tue, 2013-03-12 at 11:37 -0400, Willem de Bruijn wrote:
> > Minimize packet drop in a fanout group. If one socket is full,
> > roll over packets to another from the group. The intended use is
> > to maintain flow affinity during normal load using an rxhash or
> > cpu fanout policy, while dispersing unexpected traffic storms that
> > hit a single cpu, such as spoofed-source DoS flows. This mechanism
> > breaks affinity for flows arriving at saturated sockets during
> > those conditions.
> >
> > The patch adds a fanout policy ROLLOVER that rotates between sockets,
> > filling each socket before moving to the next. It also adds a fanout
> > flag ROLLOVER. If passed along with any other fanout policy, the
> > primary policy is applied until the chosen socket is full. Then,
> > rollover selects another socket, to delay packet drop until the
> > entire system is saturated.
> >
> > Probing sockets is not free. Selecting the last used socket, as
> > rollover does, is a greedy approach that maximizes chance of
> > success, at the cost of extreme load imbalance. In practice, with
> > sufficiently long queues to handle rate, sockets are drained in
> > parallel and load balance looks uniform in `top`.
> >
> > To avoid contention, scales counters with number of sockets and
> > accesses them lockfree. Values are bounds checked to ensure
> > correctness. An alternative would be to use atomic rr_cur.
> >
> > Tested using an application with 9 threads pinned to CPUs, one socket
> > per thread and sufficient busywork per packet operation to limits each
> > thread to handling 32 Kpps. When sent 500 Kpps single UDP stream
> > packets, a FANOUT_CPU setup processes 32 Kpps in total without this
> > patch, 270 Kpps with the patch. Tested with read() and with a packet
> > ring (V1).
> >
> > Signed-off-by: Willem de Bruijn <willemb@...gle.com>
> > ---
> >  include/linux/if_packet.h |   2 +
> >  net/packet/af_packet.c    | 112 ++++++++++++++++++++++++++++++++++++----------
> >  2 files changed, 90 insertions(+), 24 deletions(-)
>
>
> > -static struct sock *fanout_demux_cpu(struct packet_fanout *f, struct sk_buff *skb, unsigned int num)
> > +static unsigned int fanout_demux_rollover(struct packet_fanout *f,
> > +                                       struct sk_buff *skb,
> > +                                       unsigned int idx, unsigned int skip,
> > +                                       unsigned int num)
> >  {
> > -     unsigned int cpu = smp_processor_id();
> > +     unsigned int i, j;
> >
> > -     return f->arr[cpu % num];
> > +     i = j = min(f->next[idx], (int) f->num_members - 1);
>
> min_t(int, f->next[idx], f->num_members - 1);
>
> BTW, num_members can be 0
>
> You really should do
>
> int members = ACCESS_ONCE(f->num_members) - 1;
>
> if (members < 0)
>         return idx;
>
> and only use members in your loop.

Thanks for catching that. I'll revise as mentioned.

>
> > +     do {
> > +             if (i != skip && packet_rcv_has_room(pkt_sk(f->arr[i]), skb)) {
> > +                     if (i != j)
> > +                             f->next[idx] = i;
> > +                     return i;
> > +             }
> > +             if (++i >= f->num_members)
> > +                     i = 0;
> > +     } while (i != j && idx < f->num_members);
> > +
> > +     return idx;
> > +}
> >
+

It is probably also better if the rollover flag always leaves some
room in the socket chosen with f->next. This to ensure that that
socket can still enqueue its own packets, avoiding sending it into
rollover mode, too.
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