1 // SPDX-License-Identifier: GPL-2.0
2 /* Copyright (c) 2019 Facebook
3 *
4 * This program is free software; you can redistribute it and/or
5 * modify it under the terms of version 2 of the GNU General Public
6 * License as published by the Free Software Foundation.
7 *
8 * Sample Host Bandwidth Manager (HBM) BPF program.
9 *
10 * A cgroup skb BPF egress program to limit cgroup output bandwidth.
11 * It uses a modified virtual token bucket queue to limit average
12 * egress bandwidth. The implementation uses credits instead of tokens.
13 * Negative credits imply that queueing would have happened (this is
14 * a virtual queue, so no queueing is done by it. However, queueing may
15 * occur at the actual qdisc (which is not used for rate limiting).
16 *
17 * This implementation uses 3 thresholds, one to start marking packets and
18 * the other two to drop packets:
19 * CREDIT
20 * - <--------------------------|------------------------> +
21 * | | | 0
22 * | Large pkt |
23 * | drop thresh |
24 * Small pkt drop Mark threshold
25 * thresh
26 *
27 * The effect of marking depends on the type of packet:
28 * a) If the packet is ECN enabled and it is a TCP packet, then the packet
29 * is ECN marked.
30 * b) If the packet is a TCP packet, then we probabilistically call tcp_cwr
31 * to reduce the congestion window. The current implementation uses a linear
32 * distribution (0% probability at marking threshold, 100% probability
33 * at drop threshold).
34 * c) If the packet is not a TCP packet, then it is dropped.
35 *
36 * If the credit is below the drop threshold, the packet is dropped. If it
37 * is a TCP packet, then it also calls tcp_cwr since packets dropped by
38 * by a cgroup skb BPF program do not automatically trigger a call to
39 * tcp_cwr in the current kernel code.
40 *
41 * This BPF program actually uses 2 drop thresholds, one threshold
42 * for larger packets (>= 120 bytes) and another for smaller packets. This
43 * protects smaller packets such as SYNs, ACKs, etc.
44 *
45 * The default bandwidth limit is set at 1Gbps but this can be changed by
46 * a user program through a shared BPF map. In addition, by default this BPF
47 * program does not limit connections using loopback. This behavior can be
48 * overwritten by the user program. There is also an option to calculate
49 * some statistics, such as percent of packets marked or dropped, which
50 * the user program can access.
51 *
52 * A latter patch provides such a program (hbm.c)
53 */
54
55 #include "hbm_kern.h"
56
57 SEC("cgroup_skb/egress")
58 int _hbm_out_cg(struct __sk_buff *skb)
59 {
60 struct hbm_pkt_info pkti;
61 int len = skb->len;
62 unsigned int queue_index = 0;
63 unsigned long long curtime;
64 int credit;
65 signed long long delta = 0, new_credit;
66 int max_credit = MAX_CREDIT;
67 bool congestion_flag = false;
68 bool drop_flag = false;
69 bool cwr_flag = false;
70 bool ecn_ce_flag = false;
71 struct hbm_vqueue *qdp;
72 struct hbm_queue_stats *qsp = NULL;
73 int rv = ALLOW_PKT;
74
75 qsp = bpf_map_lookup_elem(&queue_stats, &queue_index);
76 if (qsp != NULL && !qsp->loopback && (skb->ifindex == 1))
77 return ALLOW_PKT;
78
79 hbm_get_pkt_info(skb, &pkti);
80
81 // We may want to account for the length of headers in len
82 // calculation, like ETH header + overhead, specially if it
83 // is a gso packet. But I am not doing it right now.
84
85 qdp = bpf_get_local_storage(&queue_state, 0);
86 if (!qdp)
87 return ALLOW_PKT;
88 else if (qdp->lasttime == 0)
89 hbm_init_vqueue(qdp, 1024);
90
91 curtime = bpf_ktime_get_ns();
92
93 // Begin critical section
94 bpf_spin_lock(&qdp->lock);
95 credit = qdp->credit;
96 delta = curtime - qdp->lasttime;
97 /* delta < 0 implies that another process with a curtime greater
98 * than ours beat us to the critical section and already added
99 * the new credit, so we should not add it ourselves
100 */
101 if (delta > 0) {
102 qdp->lasttime = curtime;
103 new_credit = credit + CREDIT_PER_NS(delta, qdp->rate);
104 if (new_credit > MAX_CREDIT)
105 credit = MAX_CREDIT;
106 else
107 credit = new_credit;
108 }
109 credit -= len;
110 qdp->credit = credit;
111 bpf_spin_unlock(&qdp->lock);
112 // End critical section
113
114 // Check if we should update rate
115 if (qsp != NULL && (qsp->rate * 128) != qdp->rate) {
116 qdp->rate = qsp->rate * 128;
117 bpf_printk("Updating rate: %d (1sec:%llu bits)\n",
118 (int)qdp->rate,
119 CREDIT_PER_NS(1000000000, qdp->rate) * 8);
120 }
121
122 // Set flags (drop, congestion, cwr)
123 // Dropping => we are congested, so ignore congestion flag
124 if (credit < -DROP_THRESH ||
125 (len > LARGE_PKT_THRESH && credit < -LARGE_PKT_DROP_THRESH)) {
126 // Very congested, set drop packet
127 drop_flag = true;
128 if (pkti.ecn)
129 congestion_flag = true;
130 else if (pkti.is_tcp)
131 cwr_flag = true;
132 } else if (credit < 0) {
133 // Congested, set congestion flag
134 if (pkti.ecn || pkti.is_tcp) {
135 if (credit < -MARK_THRESH)
136 congestion_flag = true;
137 else
138 congestion_flag = false;
139 } else {
140 congestion_flag = true;
141 }
142 }
143
144 if (congestion_flag) {
145 if (bpf_skb_ecn_set_ce(skb)) {
146 ecn_ce_flag = true;
147 } else {
148 if (pkti.is_tcp) {
149 unsigned int rand = bpf_get_prandom_u32();
150
151 if (-credit >= MARK_THRESH +
152 (rand % MARK_REGION_SIZE)) {
153 // Do congestion control
154 cwr_flag = true;
155 }
156 } else if (len > LARGE_PKT_THRESH) {
157 // Problem if too many small packets?
158 drop_flag = true;
159 }
160 }
161 }
162
163 if (qsp != NULL)
164 if (qsp->no_cn)
165 cwr_flag = false;
166
167 hbm_update_stats(qsp, len, curtime, congestion_flag, drop_flag,
168 cwr_flag, ecn_ce_flag, &pkti, credit);
169
170 if (drop_flag) {
171 __sync_add_and_fetch(&(qdp->credit), len);
172 rv = DROP_PKT;
173 }
174
175 if (cwr_flag)
176 rv |= 2;
177 return rv;
178 }
179 char _license[] SEC("license") = "GPL";
180
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