From 06e69b556ce5839f74d82698350301d50e3b46ca Mon Sep 17 00:00:00 2001
From: Willy Tarreau <w@1wt.eu>
Date: Tue, 2 Mar 2021 14:01:35 +0100
Subject: [PATCH] REORG: tools: promote the debug PRNG to more general use as a
 statistical one

We frequently need to access a simple and fast PRNG for statistical
purposes. The debug_prng() function did exactly this using a xorshift
generator but its use was limited to debug only. Let's move this to
tools.h and tools.c to make it accessible everywhere. Since it needs to
be fast, its state is thread-local. An initialization function starts a
different initial value for each thread for better distribution.
---
 include/haproxy/tools.h | 16 ++++++++++++++++
 src/debug.c             | 14 ++------------
 src/tools.c             | 16 ++++++++++++++++
 3 files changed, 34 insertions(+), 12 deletions(-)

diff --git a/include/haproxy/tools.h b/include/haproxy/tools.h
index 46fef67c5..ae900adfb 100644
--- a/include/haproxy/tools.h
+++ b/include/haproxy/tools.h
@@ -1055,4 +1055,20 @@ static inline int32_t ha_random()
 	return ha_random32() >> 1;
 }
 
+extern THREAD_LOCAL unsigned int statistical_prng_state;
+
+/* Xorshift RNGs from http://www.jstatsoft.org/v08/i14/paper.
+ * This has a (2^32)-1 period, only zero is never returned.
+ */
+static inline unsigned int statistical_prng()
+{
+	unsigned int x = statistical_prng_state;
+
+	x ^= x << 13;
+	x ^= x >> 17;
+	x ^= x << 5;
+	return statistical_prng_state = x;
+}
+
+
 #endif /* _HAPROXY_TOOLS_H */
diff --git a/src/debug.c b/src/debug.c
index 209199ab0..914c80de8 100644
--- a/src/debug.c
+++ b/src/debug.c
@@ -42,16 +42,6 @@
 volatile unsigned long threads_to_dump = 0;
 unsigned int debug_commands_issued = 0;
 
-/* Xorshift RNGs from http://www.jstatsoft.org/v08/i14/paper */
-static THREAD_LOCAL unsigned int y = 2463534242U;
-static unsigned int debug_prng()
-{
-        y ^= y << 13;
-        y ^= y >> 17;
-        y ^= y << 5;
-        return y;
-}
-
 /* dumps a backtrace of the current thread that is appended to buffer <buf>.
  * Lines are prefixed with the string <prefix> which may be empty (used for
  * indenting). It is recommended to use this at a function's tail so that
@@ -748,7 +738,7 @@ static struct task *debug_task_handler(struct task *t, void *ctx, unsigned short
 	t->expire = tick_add(now_ms, inter);
 
 	/* half of the calls will wake up another entry */
-	rnd = debug_prng();
+	rnd = statistical_prng();
 	if (rnd & 1) {
 		rnd >>= 1;
 		rnd %= tctx[0];
@@ -770,7 +760,7 @@ static struct task *debug_tasklet_handler(struct task *t, void *ctx, unsigned sh
 
 	/* wake up two random entries */
 	for (i = 0; i < 2; i++) {
-		rnd = debug_prng() % tctx[0];
+		rnd = statistical_prng() % tctx[0];
 		rnd = tctx[rnd + 2];
 
 		if (rnd & 1)
diff --git a/src/tools.c b/src/tools.c
index 1c43354bd..d3ee4265b 100644
--- a/src/tools.c
+++ b/src/tools.c
@@ -78,6 +78,12 @@ THREAD_LOCAL int itoa_idx = 0; /* index of next itoa_str to use */
 THREAD_LOCAL char quoted_str[NB_QSTR][QSTR_SIZE + 1];
 THREAD_LOCAL int quoted_idx = 0;
 
+/* thread-local PRNG state. It's modified to start from a different sequence
+ * on all threads upon startup. It must not be used or anything beyond getting
+ * statistical values as it's 100% predictable.
+ */
+THREAD_LOCAL unsigned int statistical_prng_state = 2463534242U;
+
 /*
  * unsigned long long ASCII representation
  *
@@ -5363,6 +5369,16 @@ size_t sanitize_for_printing(char *line, size_t pos, size_t width)
 	return pos - shift;
 }
 
+static int init_tools_per_thread()
+{
+	/* Let's make each thread start from a different position */
+	statistical_prng_state += tid * MAX_THREADS;
+	if (!statistical_prng_state)
+		statistical_prng_state++;
+	return 1;
+}
+REGISTER_PER_THREAD_INIT(init_tools_per_thread);
+
 /*
  * Local variables:
  *  c-indent-level: 8