scsi: scsi_debug: Randomize command completion time

Add a new command line option (e.g. random=1) and sysfs attribute that
causes subsequent command completion times to be between the current
command delay setting and 0. A uniformly distributed 32 bit, kernel
provided integer is used for this purpose.

Since the existing 'delay' whose units are jiffies (typically milliseconds)
and 'ndelay' (units: nanoseconds) options (and sysfs attributes) span a
range greater than 32 bits, some scaling is required.

The purpose of this patch is to widen the range of testing cases that are
visited in long running tests. Put simply: rarely struct race conditions
are more likely to be found when this facility is used.

The default is the previous case in which all command completions were
roughly equal to (if not, slightly longer) than the value given by the
'delay' or 'ndelay' settings (or their defaults).  This option's default is
equivalent to setting 'random=0' .

[mkp: use kstrtobool()]

Link: https://lore.kernel.org/r/20200421151424.32668-2-dgilbert@interlog.com
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/scsi_debug.c

@@ -39,6 +39,7 @@
 #include <linux/uuid.h>
 #include <linux/t10-pi.h>
 #include <linux/msdos_partition.h>
+#include <linux/random.h>
 
 #include <net/checksum.h>
 
@@ -126,6 +127,7 @@ static const char *sdebug_version_date = "20190125";
 #define DEF_PHYSBLK_EXP 0
 #define DEF_OPT_XFERLEN_EXP 0
 #define DEF_PTYPE   TYPE_DISK
+#define DEF_RANDOM false
 #define DEF_REMOVABLE false
 #define DEF_SCSI_LEVEL   7    /* INQUIRY, byte2 [6->SPC-4; 7->SPC-5] */
 #define DEF_SECTOR_SIZE 512
@@ -656,6 +658,7 @@ static unsigned int sdebug_unmap_max_blocks = DEF_UNMAP_MAX_BLOCKS;
 static unsigned int sdebug_unmap_max_desc = DEF_UNMAP_MAX_DESC;
 static unsigned int sdebug_write_same_length = DEF_WRITESAME_LENGTH;
 static int sdebug_uuid_ctl = DEF_UUID_CTL;
+static bool sdebug_random = DEF_RANDOM;
 static bool sdebug_removable = DEF_REMOVABLE;
 static bool sdebug_clustering;
 static bool sdebug_host_lock = DEF_HOST_LOCK;
@@ -4355,9 +4358,21 @@ static int schedule_resp(struct scsi_cmnd *cmnd, struct sdebug_dev_info *devip,
 		ktime_t kt;
 
 		if (delta_jiff > 0) {
-			kt = ns_to_ktime((u64)delta_jiff * (NSEC_PER_SEC / HZ));
-		} else
-			kt = ndelay;
+			u64 ns = jiffies_to_nsecs(delta_jiff);
+
+			if (sdebug_random && ns < U32_MAX) {
+				ns = prandom_u32_max((u32)ns);
+			} else if (sdebug_random) {
+				ns >>= 12;	/* scale to 4 usec precision */
+				if (ns < U32_MAX)	/* over 4 hours max */
+					ns = prandom_u32_max((u32)ns);
+				ns <<= 12;
+			}
+			kt = ns_to_ktime(ns);
+		} else {	/* ndelay has a 4.2 second max */
+			kt = sdebug_random ? prandom_u32_max((u32)ndelay) :
+					     (u32)ndelay;
+		}
 		if (!sd_dp->init_hrt) {
 			sd_dp->init_hrt = true;
 			sqcp->sd_dp = sd_dp;
@@ -4452,6 +4467,7 @@ module_param_named(opts, sdebug_opts, int, S_IRUGO | S_IWUSR);
 module_param_named(physblk_exp, sdebug_physblk_exp, int, S_IRUGO);
 module_param_named(opt_xferlen_exp, sdebug_opt_xferlen_exp, int, S_IRUGO);
 module_param_named(ptype, sdebug_ptype, int, S_IRUGO | S_IWUSR);
+module_param_named(random, sdebug_random, bool, S_IRUGO | S_IWUSR);
 module_param_named(removable, sdebug_removable, bool, S_IRUGO | S_IWUSR);
 module_param_named(scsi_level, sdebug_scsi_level, int, S_IRUGO);
 module_param_named(sector_size, sdebug_sector_size, int, S_IRUGO);
@@ -4512,6 +4528,7 @@ MODULE_PARM_DESC(opts, "1->noise, 2->medium_err, 4->timeout, 8->recovered_err...
 MODULE_PARM_DESC(physblk_exp, "physical block exponent (def=0)");
 MODULE_PARM_DESC(opt_xferlen_exp, "optimal transfer length granularity exponent (def=physblk_exp)");
 MODULE_PARM_DESC(ptype, "SCSI peripheral type(def=0[disk])");
+MODULE_PARM_DESC(random, "If set, uniformly randomize command duration between 0 and delay_in_ns");
 MODULE_PARM_DESC(removable, "claim to have removable media (def=0)");
 MODULE_PARM_DESC(scsi_level, "SCSI level to simulate(def=7[SPC-5])");
 MODULE_PARM_DESC(sector_size, "logical block size in bytes (def=512)");
@@ -5102,6 +5119,24 @@ static ssize_t map_show(struct device_driver *ddp, char *buf)
 }
 static DRIVER_ATTR_RO(map);
 
+static ssize_t random_show(struct device_driver *ddp, char *buf)
+{
+	return scnprintf(buf, PAGE_SIZE, "%d\n", sdebug_random);
+}
+
+static ssize_t random_store(struct device_driver *ddp, const char *buf,
+			    size_t count)
+{
+	bool v;
+
+	if (kstrtobool(buf, &v))
+		return -EINVAL;
+
+	sdebug_random = v;
+	return count;
+}
+static DRIVER_ATTR_RW(random);
+
 static ssize_t removable_show(struct device_driver *ddp, char *buf)
 {
 	return scnprintf(buf, PAGE_SIZE, "%d\n", sdebug_removable ? 1 : 0);
@@ -5212,6 +5247,7 @@ static struct attribute *sdebug_drv_attrs[] = {
 	&driver_attr_guard.attr,
 	&driver_attr_ato.attr,
 	&driver_attr_map.attr,
+	&driver_attr_random.attr,
 	&driver_attr_removable.attr,
 	&driver_attr_host_lock.attr,
 	&driver_attr_ndelay.attr,