Optimize spa_get_random().

In all places except two spa_get_random() is used for small values,
and the consumers do not require well seeded high quality values.
Switch those two exceptions directly to random_get_pseudo_bytes()
and optimize spa_get_random().

On FreeBSD directly map spa_get_random() to new prng32_bounded() KPI
added in FreeBSD 13.  On Linux and in user-space just reduce the type
used to uint32_t to avoid more expensive 64bit division.

Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored-By: iXsystems, Inc.

include/os/freebsd/spl/sys/random.h

@@ -30,6 +30,9 @@
 #define	_OPENSOLARIS_SYS_RANDOM_H_
 
 #include_next <sys/random.h>
+#if  __FreeBSD_version >= 1300108
+#include <sys/prng.h>
+#endif
 
 static inline int
 random_get_bytes(uint8_t *p, size_t s)
@@ -45,4 +48,23 @@ random_get_pseudo_bytes(uint8_t *p, size_t s)
 	return (0);
 }
 
+static inline uint32_t
+spa_get_random(uint32_t range)
+{
+#if  __FreeBSD_version >= 1300108
+	return (prng32_bounded(range));
+#else
+	uint32_t r;
+
+	ASSERT(range != 0);
+
+	if (range == 1)
+		return (0);
+
+	(void) random_get_pseudo_bytes((void *)&r, sizeof (r));
+
+	return (r % range);
+#endif
+}
+
 #endif	/* !_OPENSOLARIS_SYS_RANDOM_H_ */

include/os/linux/spl/sys/random.h

@@ -36,4 +36,19 @@ random_get_bytes(uint8_t *ptr, size_t len)
 
 extern int random_get_pseudo_bytes(uint8_t *ptr, size_t len);
 
+static __inline__ uint32_t
+spa_get_random(uint32_t range)
+{
+	uint32_t r;
+
+	ASSERT(range != 0);
+
+	if (range == 1)
+		return (0);
+
+	(void) random_get_pseudo_bytes((void *)&r, sizeof (r));
+
+	return (r % range);
+}
+
 #endif	/* _SPL_RANDOM_H */

include/sys/spa.h

@@ -1066,7 +1066,6 @@ extern spa_t *spa_by_guid(uint64_t pool_guid, uint64_t device_guid);
 extern boolean_t spa_guid_exists(uint64_t pool_guid, uint64_t device_guid);
 extern char *spa_strdup(const char *);
 extern void spa_strfree(char *);
-extern uint64_t spa_get_random(uint64_t range);
 extern uint64_t spa_generate_guid(spa_t *spa);
 extern void snprintf_blkptr(char *buf, size_t buflen, const blkptr_t *bp);
 extern void spa_freeze(spa_t *spa);

include/sys/zfs_context.h

@@ -646,6 +646,21 @@ extern int lowbit64(uint64_t i);
 extern int random_get_bytes(uint8_t *ptr, size_t len);
 extern int random_get_pseudo_bytes(uint8_t *ptr, size_t len);
 
+static __inline__ uint32_t
+spa_get_random(uint32_t range)
+{
+	uint32_t r;
+
+	ASSERT(range != 0);
+
+	if (range == 1)
+		return (0);
+
+	(void) random_get_pseudo_bytes((void *)&r, sizeof (r));
+
+	return (r % range);
+}
+
 extern void kernel_init(int mode);
 extern void kernel_fini(void);
 extern void random_init(void);

module/zfs/spa_misc.c

@@ -1494,32 +1494,21 @@ spa_strfree(char *s)
 	kmem_free(s, strlen(s) + 1);
 }
 
-uint64_t
-spa_get_random(uint64_t range)
-{
-	uint64_t r;
-
-	ASSERT(range != 0);
-
-	if (range == 1)
-		return (0);
-
-	(void) random_get_pseudo_bytes((void *)&r, sizeof (uint64_t));
-
-	return (r % range);
-}
-
 uint64_t
 spa_generate_guid(spa_t *spa)
 {
-	uint64_t guid = spa_get_random(-1ULL);
+	uint64_t guid;
 
 	if (spa != NULL) {
-		while (guid == 0 || spa_guid_exists(spa_guid(spa), guid))
-			guid = spa_get_random(-1ULL);
+		do {
+			(void) random_get_pseudo_bytes((void *)&guid,
+			    sizeof (guid));
+		} while (guid == 0 || spa_guid_exists(spa_guid(spa), guid));
 	} else {
-		while (guid == 0 || spa_guid_exists(guid, 0))
-			guid = spa_get_random(-1ULL);
+		do {
+			(void) random_get_pseudo_bytes((void *)&guid,
+			    sizeof (guid));
+		} while (guid == 0 || spa_guid_exists(guid, 0));
 	}
 
 	return (guid);

module/zfs/zil.c

@@ -205,8 +205,10 @@ zil_init_log_chain(zilog_t *zilog, blkptr_t *bp)
 {
 	zio_cksum_t *zc = &bp->blk_cksum;
 
-	zc->zc_word[ZIL_ZC_GUID_0] = spa_get_random(-1ULL);
-	zc->zc_word[ZIL_ZC_GUID_1] = spa_get_random(-1ULL);
+	(void) random_get_pseudo_bytes((void *)&zc->zc_word[ZIL_ZC_GUID_0],
+	    sizeof (zc->zc_word[ZIL_ZC_GUID_0]));
+	(void) random_get_pseudo_bytes((void *)&zc->zc_word[ZIL_ZC_GUID_1],
+	    sizeof (zc->zc_word[ZIL_ZC_GUID_1]));
 	zc->zc_word[ZIL_ZC_OBJSET] = dmu_objset_id(zilog->zl_os);
 	zc->zc_word[ZIL_ZC_SEQ] = 1ULL;
 }