Re: gettimeofday nanoseconds patch (makes it possible for theposix-timer functions to return higher accuracy)

[Christoph Lameter]

On Wed, 14 Jul 2004, john stultz wrote:

> On Wed, 2004-07-14 at 13:28, Christoph Lameter wrote:
> > > None the less, I do understand the desire for the change (and am working
> > > to address it in 2.7), so could you at least use a better name then
> > > gettimeofday()? Maybe get_ns_time() or something? Its just too similar
> > > to do_gettimeofday and the syscall gettimeofday().
> >
> > Right. I had it named getnstimeofday before but the feeling was that the
> > patch should not introduce a new name. Any approach that would allow
> > progress on the issue would be fine with me.
>
> Fair enough. getnstimeofday() sounds good enough for me.

Ok. A modified patch is following.

>
> > > Really, I feel the cleaner method is to fix do_gettimeofday() so it
> > > returns a timespec and then convert it to a timeval in
> > > sys_gettimeofday(). However this would add overhead to the syscall, so I
> > > doubt folks would go for it.
> >
> > do_gettimeofday is used all over the linux kernel for a variety of
> > purposes and lots of code depends on the presence of a timeval struct.
>
> Indeed, it would be a decent amount of work to clean that up as well.

The cleanup can be done gradually after this patch is in. I volunteer
to work on this (hoping that my employer may support that  ;-) ).

Signed-off-by: Christoph Lameter <clameter@xxxxxxx>

Index: linux-2.6.7/kernel/timer.c
===================================================================
--- linux-2.6.7.orig/kernel/timer.c
+++ linux-2.6.7/kernel/timer.c
@@ -1241,8 +1241,7 @@
 		 * too.
 		 */

-		do_gettimeofday((struct timeval *)&tp);
-		tp.tv_nsec *= NSEC_PER_USEC;
+		getnstimeofday(&tp);
 		tp.tv_sec += wall_to_monotonic.tv_sec;
 		tp.tv_nsec += wall_to_monotonic.tv_nsec;
 		if (tp.tv_nsec - NSEC_PER_SEC >= 0) {
Index: linux-2.6.7/kernel/posix-timers.c
===================================================================
--- linux-2.6.7.orig/kernel/posix-timers.c
+++ linux-2.6.7/kernel/posix-timers.c
@@ -1168,15 +1168,10 @@
  */
 static int do_posix_gettime(struct k_clock *clock, struct timespec *tp)
 {
-	struct timeval tv;
-
 	if (clock->clock_get)
 		return clock->clock_get(tp);

-	do_gettimeofday(&tv);
-	tp->tv_sec = tv.tv_sec;
-	tp->tv_nsec = tv.tv_usec * NSEC_PER_USEC;
-
+	getnstimeofday(tp);
 	return 0;
 }

@@ -1192,24 +1187,16 @@
 	struct timespec *tp, struct timespec *mo)
 {
 	u64 jiff;
-	struct timeval tpv;
 	unsigned int seq;

 	do {
 		seq = read_seqbegin(&xtime_lock);
-		do_gettimeofday(&tpv);
+		getnstimeofday(tp);
 		*mo = wall_to_monotonic;
 		jiff = jiffies_64;

 	} while(read_seqretry(&xtime_lock, seq));

-	/*
-	 * Love to get this before it is converted to usec.
-	 * It would save a div AND a mpy.
-	 */
-	tp->tv_sec = tpv.tv_sec;
-	tp->tv_nsec = tpv.tv_usec * NSEC_PER_USEC;
-
 	return jiff;
 }

Index: linux-2.6.7/include/linux/time.h
===================================================================
--- linux-2.6.7.orig/include/linux/time.h
+++ linux-2.6.7/include/linux/time.h
@@ -348,6 +348,7 @@
 struct itimerval;
 extern int do_setitimer(int which, struct itimerval *value, struct itimerval *ovalue);
 extern int do_getitimer(int which, struct itimerval *value);
+extern void getnstimeofday (struct timespec *tv);

 static inline void
 set_normalized_timespec (struct timespec *ts, time_t sec, long nsec)
Index: linux-2.6.7/kernel/time.c
===================================================================
--- linux-2.6.7.orig/kernel/time.c
+++ linux-2.6.7/kernel/time.c
@@ -22,6 +22,9 @@
  *	"A Kernel Model for Precision Timekeeping" by Dave Mills
  *	Allow time_constant larger than MAXTC(6) for NTP v4 (MAXTC == 10)
  *	(Even though the technical memorandum forbids it)
+ * 2004-07-14	 Christoph Lameter
+ *	Added getnstimeofday to allow the posix timer functions to return
+ *	with nanosecond accuracy
  */

 #include <linux/module.h>
@@ -421,6 +424,41 @@

 EXPORT_SYMBOL(current_kernel_time);

+#ifdef CONFIG_TIME_INTERPOLATION
+void getnstimeofday (struct timespec *tv)
+{
+	unsigned long seq,sec,nsec;
+
+	do {
+		seq = read_seqbegin(&xtime_lock);
+		sec = xtime.tv_sec;
+		nsec = xtime.tv_nsec+time_interpolator_get_offset();
+	} while (unlikely(read_seqretry(&xtime_lock, seq)));
+
+	while (unlikely(nsec >= NSEC_PER_SEC)) {
+		nsec -= NSEC_PER_SEC;
+		++sec;
+	}
+	tv->tv_sec = sec;
+	tv->tv_nsec = nsec;
+}
+#else
+/*
+ * Simulate gettimeofday using do_gettimeofday which only allows a timeval
+ * and therefore only yields usec accuracy
+ */
+void getnstimeofday(struct timespec *tv)
+{
+	struct timeval x;
+
+	do_gettimeofday(&x);
+	tv->tv_sec = x.tv_sec;
+	tv->tv_nsec = x.tv_usec * NSEC_PER_USEC;
+}
+#endif
+
+EXPORT_SYMBOL(getnstimeofday);
+
 #if (BITS_PER_LONG < 64)
 u64 get_jiffies_64(void)
 {
-
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