diff options
authorLawrence Brakmo <brakmo@fb.com>2017-10-20 11:05:39 -0700
committerDavid S. Miller <davem@davemloft.net>2017-10-22 03:12:05 +0100
commite6546ef6d86d0fc38e0e84ccae80e641f3fc0087 (patch)
parentnfp: use struct fields for 8 bit-wide access (diff)
bpf: add support for BPF_SOCK_OPS_BASE_RTT
A congestion control algorithm can make a call to the BPF socket_ops program to request the base RTT. The base RTT can be congestion control dependent and is meant to represent a congestion threshold such that RTTs above it indicate congestion. This is especially useful for flows within a DC where the base RTT is easy to obtain. Being provided a base RTT solves a basic problem in RTT based congestion avoidance algorithms (such as Vegas, NV and BBR). Although it is easy to get the base RTT when the network is not congested, it is very diffcult to do when it is very congested. Newer connections get an inflated value of the base RTT leading to unfariness (newer flows with a larger base RTT get more bandwidth). As a result, RTT based congestion avoidance algorithms tend to update their base RTTs to improve fairness. In very congested networks this can lead to base RTT inflation, reducing the ability of these RTT based congestion control algorithms to prevent congestion. Note that in my experiments with TCP-NV, the base RTT provided can be much larger than the actual hardware RTT. For example, experimenting with hosts within a rack where the hardware RTT is 16-20us, I've used base RTTs up to 150us. The effect of using a larger base RTT is that the congestion avoidance algorithm will allow more queueing. When there are only a few flows the main effect is larger measured RTTs and RPC latencies due to the increased queueing. When there are a lot of flows, a larger base RTT can lead to more congestion and more packet drops. For this case, where the hardware RTT is 20us, a base RTT of 80us produces good results. This patch only introduces BPF_SOCK_OPS_BASE_RTT, a later patch in this set adds support for using it in TCP-NV. Further study and testing is needed before support can be added to other delay based congestion avoidance algorithms. Signed-off-by: Lawrence Brakmo <brakmo@fb.com> Acked-by: Alexei Starovoitov <ast@fb.com> Acked-by: Daniel Borkmann <daniel@iogearbox.net> Signed-off-by: David S. Miller <davem@davemloft.net>
1 files changed, 7 insertions, 0 deletions
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index d83f95ea6a1b..1aca744c220f 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -955,6 +955,13 @@ enum {
BPF_SOCK_OPS_NEEDS_ECN, /* If connection's congestion control
* needs ECN
+ BPF_SOCK_OPS_BASE_RTT, /* Get base RTT. The correct value is
+ * based on the path and may be
+ * dependent on the congestion control
+ * algorithm. In general it indicates
+ * a congestion threshold. RTTs above
+ * this indicate congestion
+ */
#define TCP_BPF_IW 1001 /* Set TCP initial congestion window */