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Linux/net/ipv4/tcp_cdg.c

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  1 /*
  2  * CAIA Delay-Gradient (CDG) congestion control
  3  *
  4  * This implementation is based on the paper:
  5  *   D.A. Hayes and G. Armitage. "Revisiting TCP congestion control using
  6  *   delay gradients." In IFIP Networking, pages 328-341. Springer, 2011.
  7  *
  8  * Scavenger traffic (Less-than-Best-Effort) should disable coexistence
  9  * heuristics using parameters use_shadow=0 and use_ineff=0.
 10  *
 11  * Parameters window, backoff_beta, and backoff_factor are crucial for
 12  * throughput and delay. Future work is needed to determine better defaults,
 13  * and to provide guidelines for use in different environments/contexts.
 14  *
 15  * Except for window, knobs are configured via /sys/module/tcp_cdg/parameters/.
 16  * Parameter window is only configurable when loading tcp_cdg as a module.
 17  *
 18  * Notable differences from paper/FreeBSD:
 19  *   o Using Hybrid Slow start and Proportional Rate Reduction.
 20  *   o Add toggle for shadow window mechanism. Suggested by David Hayes.
 21  *   o Add toggle for non-congestion loss tolerance.
 22  *   o Scaling parameter G is changed to a backoff factor;
 23  *     conversion is given by: backoff_factor = 1000/(G * window).
 24  *   o Limit shadow window to 2 * cwnd, or to cwnd when application limited.
 25  *   o More accurate e^-x.
 26  */
 27 #include <linux/kernel.h>
 28 #include <linux/random.h>
 29 #include <linux/module.h>
 30 #include <net/tcp.h>
 31 
 32 #define HYSTART_ACK_TRAIN       1
 33 #define HYSTART_DELAY           2
 34 
 35 static int window __read_mostly = 8;
 36 static unsigned int backoff_beta __read_mostly = 0.7071 * 1024; /* sqrt 0.5 */
 37 static unsigned int backoff_factor __read_mostly = 42;
 38 static unsigned int hystart_detect __read_mostly = 3;
 39 static unsigned int use_ineff __read_mostly = 5;
 40 static bool use_shadow __read_mostly = true;
 41 static bool use_tolerance __read_mostly;
 42 
 43 module_param(window, int, 0444);
 44 MODULE_PARM_DESC(window, "gradient window size (power of two <= 256)");
 45 module_param(backoff_beta, uint, 0644);
 46 MODULE_PARM_DESC(backoff_beta, "backoff beta (0-1024)");
 47 module_param(backoff_factor, uint, 0644);
 48 MODULE_PARM_DESC(backoff_factor, "backoff probability scale factor");
 49 module_param(hystart_detect, uint, 0644);
 50 MODULE_PARM_DESC(hystart_detect, "use Hybrid Slow start "
 51                  "(0: disabled, 1: ACK train, 2: delay threshold, 3: both)");
 52 module_param(use_ineff, uint, 0644);
 53 MODULE_PARM_DESC(use_ineff, "use ineffectual backoff detection (threshold)");
 54 module_param(use_shadow, bool, 0644);
 55 MODULE_PARM_DESC(use_shadow, "use shadow window heuristic");
 56 module_param(use_tolerance, bool, 0644);
 57 MODULE_PARM_DESC(use_tolerance, "use loss tolerance heuristic");
 58 
 59 struct minmax {
 60         union {
 61                 struct {
 62                         s32 min;
 63                         s32 max;
 64                 };
 65                 u64 v64;
 66         };
 67 };
 68 
 69 enum cdg_state {
 70         CDG_UNKNOWN = 0,
 71         CDG_NONFULL = 1,
 72         CDG_FULL    = 2,
 73         CDG_BACKOFF = 3,
 74 };
 75 
 76 struct cdg {
 77         struct minmax rtt;
 78         struct minmax rtt_prev;
 79         struct minmax *gradients;
 80         struct minmax gsum;
 81         bool gfilled;
 82         u8  tail;
 83         u8  state;
 84         u8  delack;
 85         u32 rtt_seq;
 86         u32 undo_cwnd;
 87         u32 shadow_wnd;
 88         u16 backoff_cnt;
 89         u16 sample_cnt;
 90         s32 delay_min;
 91         u32 last_ack;
 92         u32 round_start;
 93 };
 94 
 95 /**
 96  * nexp_u32 - negative base-e exponential
 97  * @ux: x in units of micro
 98  *
 99  * Returns exp(ux * -1e-6) * U32_MAX.
100  */
101 static u32 __pure nexp_u32(u32 ux)
102 {
103         static const u16 v[] = {
104                 /* exp(-x)*65536-1 for x = 0, 0.000256, 0.000512, ... */
105                 65535,
106                 65518, 65501, 65468, 65401, 65267, 65001, 64470, 63422,
107                 61378, 57484, 50423, 38795, 22965, 8047,  987,   14,
108         };
109         u32 msb = ux >> 8;
110         u32 res;
111         int i;
112 
113         /* Cut off when ux >= 2^24 (actual result is <= 222/U32_MAX). */
114         if (msb > U16_MAX)
115                 return 0;
116 
117         /* Scale first eight bits linearly: */
118         res = U32_MAX - (ux & 0xff) * (U32_MAX / 1000000);
119 
120         /* Obtain e^(x + y + ...) by computing e^x * e^y * ...: */
121         for (i = 1; msb; i++, msb >>= 1) {
122                 u32 y = v[i & -(msb & 1)] + U32_C(1);
123 
124                 res = ((u64)res * y) >> 16;
125         }
126 
127         return res;
128 }
129 
130 /* Based on the HyStart algorithm (by Ha et al.) that is implemented in
131  * tcp_cubic. Differences/experimental changes:
132  *   o Using Hayes' delayed ACK filter.
133  *   o Using a usec clock for the ACK train.
134  *   o Reset ACK train when application limited.
135  *   o Invoked at any cwnd (i.e. also when cwnd < 16).
136  *   o Invoked only when cwnd < ssthresh (i.e. not when cwnd == ssthresh).
137  */
138 static void tcp_cdg_hystart_update(struct sock *sk)
139 {
140         struct cdg *ca = inet_csk_ca(sk);
141         struct tcp_sock *tp = tcp_sk(sk);
142 
143         ca->delay_min = min_not_zero(ca->delay_min, ca->rtt.min);
144         if (ca->delay_min == 0)
145                 return;
146 
147         if (hystart_detect & HYSTART_ACK_TRAIN) {
148                 u32 now_us = div_u64(local_clock(), NSEC_PER_USEC);
149 
150                 if (ca->last_ack == 0 || !tcp_is_cwnd_limited(sk)) {
151                         ca->last_ack = now_us;
152                         ca->round_start = now_us;
153                 } else if (before(now_us, ca->last_ack + 3000)) {
154                         u32 base_owd = max(ca->delay_min / 2U, 125U);
155 
156                         ca->last_ack = now_us;
157                         if (after(now_us, ca->round_start + base_owd)) {
158                                 NET_INC_STATS_BH(sock_net(sk),
159                                                  LINUX_MIB_TCPHYSTARTTRAINDETECT);
160                                 NET_ADD_STATS_BH(sock_net(sk),
161                                                  LINUX_MIB_TCPHYSTARTTRAINCWND,
162                                                  tp->snd_cwnd);
163                                 tp->snd_ssthresh = tp->snd_cwnd;
164                                 return;
165                         }
166                 }
167         }
168 
169         if (hystart_detect & HYSTART_DELAY) {
170                 if (ca->sample_cnt < 8) {
171                         ca->sample_cnt++;
172                 } else {
173                         s32 thresh = max(ca->delay_min + ca->delay_min / 8U,
174                                          125U);
175 
176                         if (ca->rtt.min > thresh) {
177                                 NET_INC_STATS_BH(sock_net(sk),
178                                                  LINUX_MIB_TCPHYSTARTDELAYDETECT);
179                                 NET_ADD_STATS_BH(sock_net(sk),
180                                                  LINUX_MIB_TCPHYSTARTDELAYCWND,
181                                                  tp->snd_cwnd);
182                                 tp->snd_ssthresh = tp->snd_cwnd;
183                         }
184                 }
185         }
186 }
187 
188 static s32 tcp_cdg_grad(struct cdg *ca)
189 {
190         s32 gmin = ca->rtt.min - ca->rtt_prev.min;
191         s32 gmax = ca->rtt.max - ca->rtt_prev.max;
192         s32 grad;
193 
194         if (ca->gradients) {
195                 ca->gsum.min += gmin - ca->gradients[ca->tail].min;
196                 ca->gsum.max += gmax - ca->gradients[ca->tail].max;
197                 ca->gradients[ca->tail].min = gmin;
198                 ca->gradients[ca->tail].max = gmax;
199                 ca->tail = (ca->tail + 1) & (window - 1);
200                 gmin = ca->gsum.min;
201                 gmax = ca->gsum.max;
202         }
203 
204         /* We keep sums to ignore gradients during cwnd reductions;
205          * the paper's smoothed gradients otherwise simplify to:
206          * (rtt_latest - rtt_oldest) / window.
207          *
208          * We also drop division by window here.
209          */
210         grad = gmin > 0 ? gmin : gmax;
211 
212         /* Extrapolate missing values in gradient window: */
213         if (!ca->gfilled) {
214                 if (!ca->gradients && window > 1)
215                         grad *= window; /* Memory allocation failed. */
216                 else if (ca->tail == 0)
217                         ca->gfilled = true;
218                 else
219                         grad = (grad * window) / (int)ca->tail;
220         }
221 
222         /* Backoff was effectual: */
223         if (gmin <= -32 || gmax <= -32)
224                 ca->backoff_cnt = 0;
225 
226         if (use_tolerance) {
227                 /* Reduce small variations to zero: */
228                 gmin = DIV_ROUND_CLOSEST(gmin, 64);
229                 gmax = DIV_ROUND_CLOSEST(gmax, 64);
230 
231                 if (gmin > 0 && gmax <= 0)
232                         ca->state = CDG_FULL;
233                 else if ((gmin > 0 && gmax > 0) || gmax < 0)
234                         ca->state = CDG_NONFULL;
235         }
236         return grad;
237 }
238 
239 static bool tcp_cdg_backoff(struct sock *sk, u32 grad)
240 {
241         struct cdg *ca = inet_csk_ca(sk);
242         struct tcp_sock *tp = tcp_sk(sk);
243 
244         if (prandom_u32() <= nexp_u32(grad * backoff_factor))
245                 return false;
246 
247         if (use_ineff) {
248                 ca->backoff_cnt++;
249                 if (ca->backoff_cnt > use_ineff)
250                         return false;
251         }
252 
253         ca->shadow_wnd = max(ca->shadow_wnd, tp->snd_cwnd);
254         ca->state = CDG_BACKOFF;
255         tcp_enter_cwr(sk);
256         return true;
257 }
258 
259 /* Not called in CWR or Recovery state. */
260 static void tcp_cdg_cong_avoid(struct sock *sk, u32 ack, u32 acked)
261 {
262         struct cdg *ca = inet_csk_ca(sk);
263         struct tcp_sock *tp = tcp_sk(sk);
264         u32 prior_snd_cwnd;
265         u32 incr;
266 
267         if (tp->snd_cwnd < tp->snd_ssthresh && hystart_detect)
268                 tcp_cdg_hystart_update(sk);
269 
270         if (after(ack, ca->rtt_seq) && ca->rtt.v64) {
271                 s32 grad = 0;
272 
273                 if (ca->rtt_prev.v64)
274                         grad = tcp_cdg_grad(ca);
275                 ca->rtt_seq = tp->snd_nxt;
276                 ca->rtt_prev = ca->rtt;
277                 ca->rtt.v64 = 0;
278                 ca->last_ack = 0;
279                 ca->sample_cnt = 0;
280 
281                 if (grad > 0 && tcp_cdg_backoff(sk, grad))
282                         return;
283         }
284 
285         if (!tcp_is_cwnd_limited(sk)) {
286                 ca->shadow_wnd = min(ca->shadow_wnd, tp->snd_cwnd);
287                 return;
288         }
289 
290         prior_snd_cwnd = tp->snd_cwnd;
291         tcp_reno_cong_avoid(sk, ack, acked);
292 
293         incr = tp->snd_cwnd - prior_snd_cwnd;
294         ca->shadow_wnd = max(ca->shadow_wnd, ca->shadow_wnd + incr);
295 }
296 
297 static void tcp_cdg_acked(struct sock *sk, u32 num_acked, s32 rtt_us)
298 {
299         struct cdg *ca = inet_csk_ca(sk);
300         struct tcp_sock *tp = tcp_sk(sk);
301 
302         if (rtt_us <= 0)
303                 return;
304 
305         /* A heuristic for filtering delayed ACKs, adapted from:
306          * D.A. Hayes. "Timing enhancements to the FreeBSD kernel to support
307          * delay and rate based TCP mechanisms." TR 100219A. CAIA, 2010.
308          */
309         if (tp->sacked_out == 0) {
310                 if (num_acked == 1 && ca->delack) {
311                         /* A delayed ACK is only used for the minimum if it is
312                          * provenly lower than an existing non-zero minimum.
313                          */
314                         ca->rtt.min = min(ca->rtt.min, rtt_us);
315                         ca->delack--;
316                         return;
317                 } else if (num_acked > 1 && ca->delack < 5) {
318                         ca->delack++;
319                 }
320         }
321 
322         ca->rtt.min = min_not_zero(ca->rtt.min, rtt_us);
323         ca->rtt.max = max(ca->rtt.max, rtt_us);
324 }
325 
326 static u32 tcp_cdg_ssthresh(struct sock *sk)
327 {
328         struct cdg *ca = inet_csk_ca(sk);
329         struct tcp_sock *tp = tcp_sk(sk);
330 
331         ca->undo_cwnd = tp->snd_cwnd;
332 
333         if (ca->state == CDG_BACKOFF)
334                 return max(2U, (tp->snd_cwnd * min(1024U, backoff_beta)) >> 10);
335 
336         if (ca->state == CDG_NONFULL && use_tolerance)
337                 return tp->snd_cwnd;
338 
339         ca->shadow_wnd = min(ca->shadow_wnd >> 1, tp->snd_cwnd);
340         if (use_shadow)
341                 return max3(2U, ca->shadow_wnd, tp->snd_cwnd >> 1);
342         return max(2U, tp->snd_cwnd >> 1);
343 }
344 
345 static u32 tcp_cdg_undo_cwnd(struct sock *sk)
346 {
347         struct cdg *ca = inet_csk_ca(sk);
348 
349         return max(tcp_sk(sk)->snd_cwnd, ca->undo_cwnd);
350 }
351 
352 static void tcp_cdg_cwnd_event(struct sock *sk, const enum tcp_ca_event ev)
353 {
354         struct cdg *ca = inet_csk_ca(sk);
355         struct tcp_sock *tp = tcp_sk(sk);
356         struct minmax *gradients;
357 
358         switch (ev) {
359         case CA_EVENT_CWND_RESTART:
360                 gradients = ca->gradients;
361                 if (gradients)
362                         memset(gradients, 0, window * sizeof(gradients[0]));
363                 memset(ca, 0, sizeof(*ca));
364 
365                 ca->gradients = gradients;
366                 ca->rtt_seq = tp->snd_nxt;
367                 ca->shadow_wnd = tp->snd_cwnd;
368                 break;
369         case CA_EVENT_COMPLETE_CWR:
370                 ca->state = CDG_UNKNOWN;
371                 ca->rtt_seq = tp->snd_nxt;
372                 ca->rtt_prev = ca->rtt;
373                 ca->rtt.v64 = 0;
374                 break;
375         default:
376                 break;
377         }
378 }
379 
380 static void tcp_cdg_init(struct sock *sk)
381 {
382         struct cdg *ca = inet_csk_ca(sk);
383         struct tcp_sock *tp = tcp_sk(sk);
384 
385         /* We silently fall back to window = 1 if allocation fails. */
386         if (window > 1)
387                 ca->gradients = kcalloc(window, sizeof(ca->gradients[0]),
388                                         GFP_NOWAIT | __GFP_NOWARN);
389         ca->rtt_seq = tp->snd_nxt;
390         ca->shadow_wnd = tp->snd_cwnd;
391 }
392 
393 static void tcp_cdg_release(struct sock *sk)
394 {
395         struct cdg *ca = inet_csk_ca(sk);
396 
397         kfree(ca->gradients);
398 }
399 
400 struct tcp_congestion_ops tcp_cdg __read_mostly = {
401         .cong_avoid = tcp_cdg_cong_avoid,
402         .cwnd_event = tcp_cdg_cwnd_event,
403         .pkts_acked = tcp_cdg_acked,
404         .undo_cwnd = tcp_cdg_undo_cwnd,
405         .ssthresh = tcp_cdg_ssthresh,
406         .release = tcp_cdg_release,
407         .init = tcp_cdg_init,
408         .owner = THIS_MODULE,
409         .name = "cdg",
410 };
411 
412 static int __init tcp_cdg_register(void)
413 {
414         if (backoff_beta > 1024 || window < 1 || window > 256)
415                 return -ERANGE;
416         if (!is_power_of_2(window))
417                 return -EINVAL;
418 
419         BUILD_BUG_ON(sizeof(struct cdg) > ICSK_CA_PRIV_SIZE);
420         tcp_register_congestion_control(&tcp_cdg);
421         return 0;
422 }
423 
424 static void __exit tcp_cdg_unregister(void)
425 {
426         tcp_unregister_congestion_control(&tcp_cdg);
427 }
428 
429 module_init(tcp_cdg_register);
430 module_exit(tcp_cdg_unregister);
431 MODULE_AUTHOR("Kenneth Klette Jonassen");
432 MODULE_LICENSE("GPL");
433 MODULE_DESCRIPTION("TCP CDG");
434 

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