From 8a34869984c470fb243fc9587c469b316add2f7e Mon Sep 17 00:00:00 2001
From: Azat Khuzhin <azat@libevent.org>
Date: Sun, 17 Nov 2019 18:13:51 +0300
Subject: [PATCH] test-ratelim: calculate timers bias (for slow CPUs) to avoid
 false-positive

This can be/should be done for regression tests too.

Refs: https://ci.appveyor.com/project/libevent/libevent/builds/28916689/job/kg621aa194a0qbym
Refs: https://github.com/libevent/libevent/pull/917#issuecomment-553811834
v2: EVENT_BASE_FLAG_PRECISE_TIMER
---
 test/test-ratelim.c | 83 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 83 insertions(+)

diff --git a/test/test-ratelim.c b/test/test-ratelim.c
index 9ee989bd..e47fd731 100644
--- a/test/test-ratelim.c
+++ b/test/test-ratelim.c
@@ -50,6 +50,10 @@
 #include "event2/listener.h"
 #include "event2/thread.h"
 
+#ifndef MIN
+#define MIN(a,b) (((a)<(b))?(a):(b))
+#endif
+
 static struct evutil_weakrand_state weakrand_state;
 
 static int cfg_verbose = 0;
@@ -85,6 +89,18 @@ struct client_state {
 };
 static const struct timeval *ms100_common=NULL;
 
+/* Timers bias for slow CPUs, affects:
+ * - cfg_connlimit_tolerance  (--check-connlimit)
+ * - cfg_grouplimit_tolerance (--check-grouplimit)
+ * - cfg_stddev_tolerance     (--check-stddev)
+ */
+static int timer_bias_events;
+static struct timeval timer_bias_start;
+double timer_bias_spend;
+/* Real cost is less (approximately ~5 usec),
+ * this macros adjusted to make the bias less */
+#define TIMER_MAX_COST_USEC 10
+
 /* info from check_bucket_levels_cb */
 static int total_n_bev_checks = 0;
 static ev_int64_t total_rbucket_level=0;
@@ -244,6 +260,62 @@ group_drain_cb(evutil_socket_t fd, short events, void *arg)
 	bufferevent_rate_limit_group_decrement_write(ratelim_group, cfg_group_drain);
 }
 
+static void
+timer_bias_cb(evutil_socket_t fd, short events, void *arg)
+{
+	struct event *event = arg;
+	struct timeval end;
+	struct timeval diff;
+
+	/** XXX: use rdtsc? (portability issues?) */
+	evutil_gettimeofday(&end, NULL);
+	evutil_timersub(&end, &timer_bias_start, &diff);
+	timer_bias_spend += diff.tv_sec + diff.tv_usec * 1e6;
+	timer_bias_start = end;
+
+	if (++timer_bias_events == 100)
+		event_del(event);
+}
+static double
+timer_bias_calculate(void)
+{
+	struct event_config *cfg = NULL;
+	struct event_base *base = NULL;
+	struct event *timer = NULL;
+	struct timeval tv = { 0, 1 };
+
+	cfg = event_config_new();
+	if (!cfg)
+		goto err;
+	if (event_config_set_flag(cfg, EVENT_BASE_FLAG_PRECISE_TIMER))
+		goto err;
+	base = event_base_new_with_config(cfg);
+	if (!base)
+		goto err;
+
+	timer = event_new(base, -1, EV_PERSIST, timer_bias_cb, event_self_cbarg());
+	if (!timer || event_add(timer, &tv)) {
+		goto err;
+	}
+
+	evutil_gettimeofday(&timer_bias_start, NULL);
+	event_base_dispatch(base);
+	event_free(timer);
+
+	return MIN(timer_bias_spend / 1e6 / timer_bias_events / TIMER_MAX_COST_USEC, 5);
+
+err:
+	if (cfg)
+		event_config_free(cfg);
+	if (timer)
+		event_free(timer);
+	if (base)
+		event_base_free(base);
+
+	fprintf(stderr, "Couldn't create event for CPU cycle counter bias\n");
+	return -1;
+}
+
 static int
 test_ratelimiting(void)
 {
@@ -266,6 +338,7 @@ test_ratelimiting(void)
 	struct event_config *base_cfg;
 	struct event *periodic_level_check;
 	struct event *group_drain_event=NULL;
+	double timer_bias;
 
 	memset(&sin, 0, sizeof(sin));
 	sin.sin_family = AF_INET;
@@ -275,6 +348,16 @@ test_ratelimiting(void)
 	if (0)
 		event_enable_debug_mode();
 
+	timer_bias = timer_bias_calculate();
+	if (timer_bias > 1) {
+		fprintf(stderr, "CPU is slow, timers bias is %f\n", timer_bias);
+		cfg_connlimit_tolerance  *= timer_bias;
+		cfg_grouplimit_tolerance *= timer_bias;
+		cfg_stddev_tolerance     *= timer_bias;
+	} else {
+		printf("CPU is fast enough, timers bias is %f\n", timer_bias);
+	}
+
 	base_cfg = event_config_new();
 
 #ifdef _WIN32
-- 
2.40.0