rps: selective flow shedding during softnet overflow

A cpu executing the network receive path sheds packets when its input
queue grows to netdev_max_backlog. A single high rate flow (such as a
spoofed source DoS) can exceed a single cpu processing rate and will
degrade throughput of other flows hashed onto the same cpu.

This patch adds a more fine grained hashtable. If the netdev backlog
is above a threshold, IRQ cpus track the ratio of total traffic of
each flow (using 4096 buckets, configurable). The ratio is measured
by counting the number of packets per flow over the last 256 packets
from the source cpu. Any flow that occupies a large fraction of this
(set at 50%) will see packet drop while above the threshold.

Tested:
Setup is a muli-threaded UDP echo server with network rx IRQ on cpu0,
kernel receive (RPS) on cpu0 and application threads on cpus 2--7
each handling 20k req/s. Throughput halves when hit with a 400 kpps
antagonist storm. With this patch applied, antagonist overload is
dropped and the server processes its complete load.

The patch is effective when kernel receive processing is the
bottleneck. The above RPS scenario is a extreme, but the same is
reached with RFS and sufficient kernel processing (iptables, packet
socket tap, ..).

Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 12 insertions, 0 deletions

diff --git a/net/Kconfig b/net/Kconfig
index 2ddc9046868e..08de901415ee 100644
--- a/net/Kconfig
+++ b/net/Kconfig
@@ -259,6 +259,18 @@ config BPF_JIT
           packet sniffing (libpcap/tcpdump). Note : Admin should enable
           this feature changing /proc/sys/net/core/bpf_jit_enable
 
+config NET_FLOW_LIMIT
+        boolean
+        depends on RPS
+        default y
+        ---help---
+          The network stack has to drop packets when a receive processing CPU's
+          backlog reaches netdev_max_backlog. If a few out of many active flows
+          generate the vast majority of load, drop their traffic earlier to
+          maintain capacity for the other flows. This feature provides servers
+          with many clients some protection against DoS by a single (spoofed)
+          flow that greatly exceeds average workload.
+
 menu "Network testing"
 
 config NET_PKTGEN