metrics: Add running average of CC cwnd when exiting slow start

Part of #40708

Signed-off-by: David Goulet <dgoulet@torproject.org>

src/core/or/congestion_control_vegas.c

@@ -23,6 +23,7 @@
 #include "core/or/channel.h"
 #include "feature/nodelist/networkstatus.h"
 #include "feature/control/control_events.h"
+#include "lib/math/stats.h"
 
 #define OUTBUF_CELLS (2*TLS_RECORD_MAX_CELLS)
 
@@ -49,6 +50,11 @@
 #define VEGAS_DELTA_ONION_DFLT (9*OUTBUF_CELLS)
 #define VEGAS_SSCAP_ONION_DFLT (600)
 
+/** Moving average of the cc->cwnd from each circuit exiting slowstart. */
+double cc_stats_vegas_exit_ss_cwnd_ma = 0;
+/* Running count of this moving average. Needed so we can update it. */
+static double stats_cwnd_exit_ss_ma_count = 0;
+
 /**
  * The original TCP Vegas congestion window BDP estimator.
  */
@@ -243,6 +249,11 @@ congestion_control_vegas_exit_slow_start(const circuit_t *circ,
   cc->next_cc_event = CWND_UPDATE_RATE(cc);
   congestion_control_vegas_log(circ, cc);
 
+  /* Update running cc->cwnd average for metrics. */
+  stats_cwnd_exit_ss_ma_count++;
+  STATS_UPDATE_AVG(cc_stats_vegas_exit_ss_cwnd_ma,
+                   cc->cwnd, stats_cwnd_exit_ss_ma_count);
+
   /* We need to report that slow start has exited ASAP,
    * for sbws bandwidth measurement. */
   if (CIRCUIT_IS_ORIGIN(circ)) {

src/core/or/congestion_control_vegas.h

@@ -12,6 +12,8 @@
 #include "core/or/crypt_path_st.h"
 #include "core/or/circuit_st.h"
 
+extern double cc_stats_vegas_exit_ss_cwnd_ma;
+
 /* Processing SENDME cell. */
 int congestion_control_vegas_process_sendme(struct congestion_control_t *cc,
                                             const circuit_t *circ,

src/feature/relay/relay_metrics.c

@@ -14,16 +14,18 @@
 #include "core/mainloop/connection.h"
 #include "core/mainloop/mainloop.h"
 #include "core/or/congestion_control_common.h"
+#include "core/or/congestion_control_vegas.h"
 #include "core/or/circuitlist.h"
 #include "core/or/dos.h"
 #include "core/or/relay.h"
 
 #include "app/config/config.h"
 
-#include "lib/malloc/malloc.h"
 #include "lib/container/smartlist.h"
-#include "lib/metrics/metrics_store.h"
 #include "lib/log/util_bug.h"
+#include "lib/malloc/malloc.h"
+#include "lib/math/fp.h"
+#include "lib/metrics/metrics_store.h"
 
 #include "feature/hs/hs_dos.h"
 #include "feature/nodelist/nodelist.h"
@@ -370,6 +372,16 @@ fill_cc_values(void)
   metrics_store_entry_add_label(sentry,
           metrics_format_label("action", "rtt_reset"));
   metrics_store_entry_update(sentry, congestion_control_get_num_rtt_reset());
+
+  sentry = metrics_store_add(the_store, rentry->type, rentry->name,
+                             rentry->help);
+
+  metrics_store_entry_add_label(sentry,
+          metrics_format_label("state", "slow_start_exit"));
+  metrics_store_entry_add_label(sentry,
+          metrics_format_label("action", "cwnd"));
+  metrics_store_entry_update(sentry,
+                             tor_llround(cc_stats_vegas_exit_ss_cwnd_ma));
 }
 
 /** Helper: Fill in single stream metrics output. */

src/lib/math/include.am

@@ -20,4 +20,5 @@ src_lib_libtor_math_testing_a_CFLAGS = $(AM_CFLAGS) $(TEST_CFLAGS)
 noinst_HEADERS +=				\
 		src/lib/math/fp.h		\
 		src/lib/math/laplace.h  \
-		src/lib/math/prob_distr.h
+		src/lib/math/prob_distr.h \
+		src/lib/math/stats.h

src/lib/math/stats.h

@@ -0,0 +1,19 @@
+/* Copyright (c) 2022, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+/**
+ * \file stats.h
+ *
+ * \brief Header for stats.c
+ **/
+
+#ifndef TOR_STATS_H
+#define TOR_STATS_H
+
+/** Update an average making it a "running average". The "avg" is the current
+ * value that will be updated to the new one. The "value" is the new value to
+ * add to the average and "n" is the new count as in including the "value". */
+#define STATS_UPDATE_AVG(avg, value, n) \
+  avg = ((avg) * ((n) - 1) + (value)) / (n)
+
+#endif /* !defined(TOR_STATS_H) */