Re: [PATCH net-next v2] packet: packet fanout rollover during socket overload

[Eric Dumazet <eric.dumazet@gmail.com>]

On Mon, 2013-03-18 at 19:07 -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. Maintain flow
> affinity during normal load using an rxhash fanout policy, while
> dispersing unexpected traffic storms that hit a single cpu, such
> as spoofed-source DoS flows. Rollover 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 absorb bursts, 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.
> 
> 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@google.com>
> ---

Reviewed-by: Eric Dumazet <edumazet@google.com>