[RFC PATCH 15/30 v3] Fixup merge between xtime_cache and timkkeeping starvation fix

Steven Rostedt
2008-01-15T20:54:31Z

[-- Attachment #1: fixup-merge-between-xtime-cache-and-starvation.patch --]
[-- Type: text/plain, Size: 3141 bytes --]

> void update_wall_time(void)
> {
> - cycle_t offset;
> + cycle_t cycle_now, offset;
>
> /* Make sure we're fully resumed: */
> if (unlikely(timekeeping_suspended))
> return;
>
> #ifdef CONFIG_GENERIC_TIME
> - offset = (clocksource_read(clock) - clock->cycle_last) & clock->mask;
> + cycle_now = clocksource_read(clock);
> #else
> - offset = clock->cycle_interval;
> + cycle_now = clock->cycle_last + clock->cycle_interval;
> #endif
> + offset = (cycle_now - clock->cycle_last) & clock->mask;

It seems this offset addition was to merge against the colliding
xtime_cache changes in mainline. However, I don't think its quite right,
and might be causing incorrect time() or vtime() results if NO_HZ is
enabled.

> + clocksource_accumulate(clock, cycle_now);
> +
> clock->xtime_nsec += (s64)xtime.tv_nsec << clock->shift;
>
> /* normally this loop will run just once, however in the
> * case of lost or late ticks, it will accumulate correctly.
> */
> - while (offset >= clock->cycle_interval) {
> + while (clock->cycle_accumulated >= clock->cycle_interval) {
> /* accumulate one interval */
> clock->xtime_nsec += clock->xtime_interval;
> - clock->cycle_last += clock->cycle_interval;
> - offset -= clock->cycle_interval;
> + clock->cycle_accumulated -= clock->cycle_interval;
>
> if (clock->xtime_nsec >= (u64)NSEC_PER_SEC << clock->shift) {
> clock->xtime_nsec -= (u64)NSEC_PER_SEC << clock->shift;
> @@ -482,7 +485,7 @@ void update_wall_time(void)
> }
>
> /* correct the clock when NTP error is too big */
> - clocksource_adjust(offset);
> + clocksource_adjust(clock->cycle_accumulated);

I suspect the following is needed, but haven't been able to test it yet.

Fixup merge between xtime_cache and timekeeping starvation fix.

Signed-off-by: John Stultz <johnstul@us.ibm.com>
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
---
kernel/time/timekeeping.c | 5 ++---
1 file changed, 2 insertions(+), 3 deletions(-)

Index: linux-compile.git/kernel/time/timekeeping.c
===================================================================
--- linux-compile.git.orig/kernel/time/timekeeping.c 2008-01-15 10:20:15.000000000 -0500
+++ linux-compile.git/kernel/time/timekeeping.c 2008-01-15 10:20:50.000000000 -0500
@@ -449,7 +449,7 @@ static void clocksource_adjust(s64 offse
*/
void update_wall_time(void)
{
- cycle_t cycle_now, offset;
+ cycle_t cycle_now;

/* Make sure we're fully resumed: */
if (unlikely(timekeeping_suspended))
@@ -460,7 +460,6 @@ void update_wall_time(void)
#else
cycle_now = clock->cycle_last + clock->cycle_interval;
#endif
- offset = (cycle_now - clock->cycle_last) & clock->mask;
clocksource_accumulate(clock, cycle_now);

clock->xtime_nsec += (s64)xtime.tv_nsec << clock->shift;
@@ -491,7 +490,7 @@ void update_wall_time(void)
xtime.tv_nsec = (s64)clock->xtime_nsec >> clock->shift;
clock->xtime_nsec -= (s64)xtime.tv_nsec << clock->shift;

- update_xtime_cache(cyc2ns(clock, offset));
+ update_xtime_cache(cyc2ns(clock, clock->cycle_accumulated));

/* check to see if there is a new clocksource to use */
change_clocksource();

--