[patch 01/22] GTOD: exponential update_wall_time

From: John Stultz <johnstul@xxxxxxxxxx>

Accumulate time in update_wall_time() exponentially. This avoids long running
loops seen with the dynticks patch as well as the problematic hang seen on
systems with broken clocksources.

NOTE: this only has relevance on dyntick kernels, so the quality of NTP
updates on jiffies-tick systems is unaffected. (non-dyntick kernels call
update_wall_time() in every timer tick)

Signed-off-by: John Stultz <johnstul@xxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
kernel/timer.c | 28 ++++++++++++++++++++--------
1 file changed, 20 insertions(+), 8 deletions(-)

Index: linux-2.6.18-mm3/kernel/timer.c
===================================================================
--- linux-2.6.18-mm3.orig/kernel/timer.c 2006-10-04 18:13:53.000000000 +0200
+++ linux-2.6.18-mm3/kernel/timer.c 2006-10-04 18:13:53.000000000 +0200
@@ -907,6 +907,7 @@ static void clocksource_adjust(struct cl
static void update_wall_time(void)
{
cycle_t offset;
+ int shift = 0;

/* Make sure we're fully resumed: */
if (unlikely(timekeeping_suspended))
@@ -919,28 +920,39 @@ static void update_wall_time(void)
#endif
clock->xtime_nsec += (s64)xtime.tv_nsec << clock->shift;

+ while (offset > clock->cycle_interval << (shift + 1))
+ 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) {
+ if (offset < (clock->cycle_interval << shift)) {
+ shift--;
+ continue;
+ }
+
/* accumulate one interval */
- clock->xtime_nsec += clock->xtime_interval;
- clock->cycle_last += clock->cycle_interval;
- offset -= clock->cycle_interval;
+ clock->xtime_nsec += clock->xtime_interval << shift;
+ clock->cycle_last += clock->cycle_interval << shift;
+ offset -= clock->cycle_interval << shift;

- if (clock->xtime_nsec >= (u64)NSEC_PER_SEC << clock->shift) {
+ while (clock->xtime_nsec >= (u64)NSEC_PER_SEC << clock->shift) {
clock->xtime_nsec -= (u64)NSEC_PER_SEC << clock->shift;
xtime.tv_sec++;
second_overflow();
}

/* interpolator bits */
- time_interpolator_update(clock->xtime_interval
- >> clock->shift);
+ time_interpolator_update((clock->xtime_interval
+ >> clock->shift)<<shift);

/* accumulate error between NTP and clock interval */
- clock->error += current_tick_length();
- clock->error -= clock->xtime_interval << (TICK_LENGTH_SHIFT - clock->shift);
+ clock->error += current_tick_length() << shift;
+ clock->error -= (clock->xtime_interval
+ << (TICK_LENGTH_SHIFT - clock->shift))<<shift;
+
+ shift--;
}

/* correct the clock when NTP error is too big */

--

-
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Relevant Pages

  • [patch 01/23] GTOD: exponential update_wall_time
    ... Accumulate time in update_wall_timeexponentially. ... running loops seen with the dynticks patch as well as the problematic ... this only has relevance on dyntick kernels, ... kernels call update_wall_timein every timer tick) ...
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  • [patch 01/21] GTOD: exponential update_wall_time
    ... Accumulate time in update_wall_timeexponentially. ... running loops seen with the dynticks patch as well as the problematic ... this only has relevance on dyntick kernels, ... kernels call update_wall_timein every timer tick) ...
    (Linux-Kernel)