clocksource: Handle negative skews in "skew is too large" messages

The nanosecond-to-millisecond skew computation uses unsigned arithmetic,
which produces user-unfriendly large positive numbers for negative skews.
Therefore, use signed arithmetic for this computation in order to preserve
the negativity.

Reported-by: Chris Bainbridge <[email protected]>
Reported-by: Feng Tang <[email protected]>
Fixes: dd02926 ("clocksource: Improve "skew is too large" messages")
Reviewed-by: Feng Tang <[email protected]>
Tested-by: Chris Bainbridge <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>

Author: paulmckrcu

kernel/time/clocksource.c

@@ -473,8 +473,8 @@ static void clocksource_watchdog(struct timer_list *unused)
 		/* Check the deviation from the watchdog clocksource. */
 		md = cs->uncertainty_margin + watchdog->uncertainty_margin;
 		if (abs(cs_nsec - wd_nsec) > md) {
-			u64 cs_wd_msec;
-			u64 wd_msec;
+			s64 cs_wd_msec;
+			s64 wd_msec;
 			u32 wd_rem;
 
 			pr_warn("timekeeping watchdog on CPU%d: Marking clocksource '%s' as unstable because the skew is too large:\n",
@@ -483,8 +483,8 @@ static void clocksource_watchdog(struct timer_list *unused)
 				watchdog->name, wd_nsec, wdnow, wdlast, watchdog->mask);
 			pr_warn("                      '%s' cs_nsec: %lld cs_now: %llx cs_last: %llx mask: %llx\n",
 				cs->name, cs_nsec, csnow, cslast, cs->mask);
-			cs_wd_msec = div_u64_rem(cs_nsec - wd_nsec, 1000U * 1000U, &wd_rem);
-			wd_msec = div_u64_rem(wd_nsec, 1000U * 1000U, &wd_rem);
+			cs_wd_msec = div_s64_rem(cs_nsec - wd_nsec, 1000 * 1000, &wd_rem);
+			wd_msec = div_s64_rem(wd_nsec, 1000 * 1000, &wd_rem);
 			pr_warn("                      Clocksource '%s' skewed %lld ns (%lld ms) over watchdog '%s' interval of %lld ns (%lld ms)\n",
 				cs->name, cs_nsec - wd_nsec, cs_wd_msec, watchdog->name, wd_nsec, wd_msec);
 			if (curr_clocksource == cs)