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 * 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 * a user program, such as hbm, can access.
 51 */
 52
 53#include "hbm_kern.h"
 54
 55SEC("cgroup_skb/egress")
 56int _hbm_out_cg(struct __sk_buff *skb)
 57{
 58	long long delta = 0, delta_send;
 59	unsigned long long curtime, sendtime;
 60	struct hbm_queue_stats *qsp = NULL;
 61	unsigned int queue_index = 0;
 62	bool congestion_flag = false;
 63	bool ecn_ce_flag = false;
 64	struct hbm_pkt_info pkti = {};
 65	struct hbm_vqueue *qdp;
 66	bool drop_flag = false;
 67	bool cwr_flag = false;
 68	int len = skb->len;
 69	int rv = ALLOW_PKT;
 70
 71	qsp = bpf_map_lookup_elem(&queue_stats, &queue_index);
 72
 73	// Check if we should ignore loopback traffic
 74	if (qsp != NULL && !qsp->loopback && (skb->ifindex == 1))
 75		return ALLOW_PKT;
 76
 77	hbm_get_pkt_info(skb, &pkti);
 78
 79	// We may want to account for the length of headers in len
 80	// calculation, like ETH header + overhead, specially if it
 81	// is a gso packet. But I am not doing it right now.
 82
 83	qdp = bpf_get_local_storage(&queue_state, 0);
 84	if (!qdp)
 85		return ALLOW_PKT;
 86	if (qdp->lasttime == 0)
 87		hbm_init_edt_vqueue(qdp, 1024);
 88
 89	curtime = bpf_ktime_get_ns();
 90
 91	// Begin critical section
 92	bpf_spin_lock(&qdp->lock);
 93	delta = qdp->lasttime - curtime;
 94	// bound bursts to 100us
 95	if (delta < -BURST_SIZE_NS) {
 96		// negative delta is a credit that allows bursts
 97		qdp->lasttime = curtime - BURST_SIZE_NS;
 98		delta = -BURST_SIZE_NS;
 99	}
100	sendtime = qdp->lasttime;
101	delta_send = BYTES_TO_NS(len, qdp->rate);
102	__sync_add_and_fetch(&(qdp->lasttime), delta_send);
103	bpf_spin_unlock(&qdp->lock);
104	// End critical section
105
106	// Set EDT of packet
107	skb->tstamp = sendtime;
108
109	// Check if we should update rate
110	if (qsp != NULL && (qsp->rate * 128) != qdp->rate)
111		qdp->rate = qsp->rate * 128;
112
113	// Set flags (drop, congestion, cwr)
114	// last packet will be sent in the future, bound latency
115	if (delta > DROP_THRESH_NS || (delta > LARGE_PKT_DROP_THRESH_NS &&
116				       len > LARGE_PKT_THRESH)) {
117		drop_flag = true;
118		if (pkti.is_tcp && pkti.ecn == 0)
119			cwr_flag = true;
120	} else if (delta > MARK_THRESH_NS) {
121		if (pkti.is_tcp)
122			congestion_flag = true;
123		else
124			drop_flag = true;
125	}
126
127	if (congestion_flag) {
128		if (bpf_skb_ecn_set_ce(skb)) {
129			ecn_ce_flag = true;
130		} else {
131			if (pkti.is_tcp) {
132				unsigned int rand = bpf_get_prandom_u32();
133
134				if (delta >= MARK_THRESH_NS +
135				    (rand % MARK_REGION_SIZE_NS)) {
136					// Do congestion control
137					cwr_flag = true;
138				}
139			} else if (len > LARGE_PKT_THRESH) {
140				// Problem if too many small packets?
141				drop_flag = true;
142				congestion_flag = false;
143			}
144		}
145	}
146
147	if (pkti.is_tcp && drop_flag && pkti.packets_out <= 1) {
148		drop_flag = false;
149		cwr_flag = true;
150		congestion_flag = false;
151	}
152
153	if (qsp != NULL && qsp->no_cn)
154			cwr_flag = false;
155
156	hbm_update_stats(qsp, len, curtime, congestion_flag, drop_flag,
157			 cwr_flag, ecn_ce_flag, &pkti, (int) delta);
158
159	if (drop_flag) {
160		__sync_add_and_fetch(&(qdp->lasttime), -delta_send);
161		rv = DROP_PKT;
162	}
163
164	if (cwr_flag)
165		rv |= CWR;
166	return rv;
167}
168char _license[] SEC("license") = "GPL";