gettimeofday resolution seriously degraded in test9

[Joe Korty <joe.korty@ccur.com>]

[ 2nd posting, the first seems to have been lost ]

Linus,
 This bit of -test9 code reduces the resolution of gettimeofday(2) from
1 microsecond to 1 millisecond whenever a negative time adjustment is
in progress.  This seriously damages efforts to measure time intervals
accurately with gettimeofday.  Please consider backing it out.

Joe


diff -Nura linux-2.6.0-test8/arch/i386/kernel/time.c linux-2.6.0-test9/arch/i386/kernel/time.c
--- linux-2.6.0-test8/arch/i386/kernel/time.c	2003-10-17 17:43:11.000000000 -0400
+++ linux-2.6.0-test9/arch/i386/kernel/time.c	2003-10-25 14:43:37.000000000 -0400
@@ -104,6 +104,15 @@
 		lost = jiffies - wall_jiffies;
 		if (lost)
 			usec += lost * (1000000 / HZ);
+
+		/*
+		 * If time_adjust is negative then NTP is slowing the clock
+		 * so make sure not to go into next possible interval.
+		 * Better to lose some accuracy than have time go backwards..
+		 */
+		if (unlikely(time_adjust < 0) && usec > tickadj)
+			usec = tickadj;
+
 		sec = xtime.tv_sec;
 		usec += (xtime.tv_nsec / 1000);
 	} while (read_seqretry(&xtime_lock, seq));