dm mpath: only use ktime_get_ns() in historical selector

Mixing sched_clock() and ktime_get_ns() usage will give bad results.

Switch hst_select_path() from using sched_clock() to ktime_get_ns().
Also rename path_service_time()'s 'sched_now' variable to 'now'.

Fixes: 2613eab ("dm mpath: add Historical Service Time Path Selector")
Signed-off-by: Khazhismel Kumykov <khazhy@google.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

Author: khazhyk

drivers/md/dm-ps-historical-service-time.c

@@ -27,7 +27,6 @@
 #include <linux/blkdev.h>
 #include <linux/slab.h>
 #include <linux/module.h>
-#include <linux/sched/clock.h>
 
 
 #define DM_MSG_PREFIX	"multipath historical-service-time"
@@ -433,7 +432,7 @@ static struct dm_path *hst_select_path(struct path_selector *ps,
 {
 	struct selector *s = ps->context;
 	struct path_info *pi = NULL, *best = NULL;
-	u64 time_now = sched_clock();
+	u64 time_now = ktime_get_ns();
 	struct dm_path *ret = NULL;
 	unsigned long flags;
 
@@ -474,7 +473,7 @@ static int hst_start_io(struct path_selector *ps, struct dm_path *path,
 
 static u64 path_service_time(struct path_info *pi, u64 start_time)
 {
-	u64 sched_now = ktime_get_ns();
+	u64 now = ktime_get_ns();
 
 	/* if a previous disk request has finished after this IO was
 	 * sent to the hardware, pretend the submission happened
@@ -483,11 +482,11 @@ static u64 path_service_time(struct path_info *pi, u64 start_time)
 	if (time_after64(pi->last_finish, start_time))
 		start_time = pi->last_finish;
 
-	pi->last_finish = sched_now;
-	if (time_before64(sched_now, start_time))
+	pi->last_finish = now;
+	if (time_before64(now, start_time))
 		return 0;
 
-	return sched_now - start_time;
+	return now - start_time;
 }
 
 static int hst_end_io(struct path_selector *ps, struct dm_path *path,