locking/ww_mutex/test: Use prng instead of rng to avoid hangs at bootup

Booting w/ qemu without kvm, and with 64 cpus, I noticed we'd
sometimes hung task watchdog splats in get_random_u32_below()
when using the test-ww_mutex stress test.

While entropy exhaustion is no longer an issue, the RNG may be
slower early in boot. The test-ww_mutex code will spawn off
128 threads (2x cpus) and each thread will call
get_random_u32_below() a number of times to generate a random
order of the 16 locks.

This intense use takes time and without kvm, qemu can be slow
enough that we trip the hung task watchdogs.

For this test, we don't need true randomness, just mixed up
orders for testing ww_mutex lock acquisitions, so it changes
the logic to use the prng instead, which takes less time
and avoids the watchdgos.

Feedback would be appreciated!
Signed-off-by: John Stultz <jstultz@google.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20230922043616.19282-2-jstultz@google.com

kernel/locking/test-ww_mutex.c

@@ -9,7 +9,7 @@
 #include <linux/delay.h>
 #include <linux/kthread.h>
 #include <linux/module.h>
-#include <linux/random.h>
+#include <linux/prandom.h>
 #include <linux/slab.h>
 #include <linux/ww_mutex.h>
 
@@ -386,6 +386,19 @@ struct stress {
 	int nlocks;
 };
 
+struct rnd_state rng;
+DEFINE_SPINLOCK(rng_lock);
+
+static inline u32 prandom_u32_below(u32 ceil)
+{
+	u32 ret;
+
+	spin_lock(&rng_lock);
+	ret = prandom_u32_state(&rng) % ceil;
+	spin_unlock(&rng_lock);
+	return ret;
+}
+
 static int *get_random_order(int count)
 {
 	int *order;
@@ -399,7 +412,7 @@ static int *get_random_order(int count)
 		order[n] = n;
 
 	for (n = count - 1; n > 1; n--) {
-		r = get_random_u32_below(n + 1);
+		r = prandom_u32_below(n + 1);
 		if (r != n) {
 			tmp = order[n];
 			order[n] = order[r];
@@ -625,6 +638,8 @@ static int __init test_ww_mutex_init(void)
 
 	printk(KERN_INFO "Beginning ww mutex selftests\n");
 
+	prandom_seed_state(&rng, get_random_u64());
+
 	wq = alloc_workqueue("test-ww_mutex", WQ_UNBOUND, 0);
 	if (!wq)
 		return -ENOMEM;