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    Subject[RFC] exponential update_wall_time
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    Hey Roman,
    Just wanted to run this by you for comment. Ingo was finding issues w/
    update_wall_clock looping for too long when using dynticks. I also think
    this will avoid the bad clocksource caused hangs that have been
    occasionally reported (instead of looping forever, time will just
    accumulate oddly), which will help point to the issue.

    Anyway, just wanted to get your thoughts before I send it to Andrew for
    more testing.

    thanks
    -john

    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.

    This applies on top of 2.6.18-mm1

    Signed-off-by: John Stultz <johnstul@us.ibm.com>

    kernel/timer.c | 28 ++++++++++++++++++++--------
    1 file changed, 20 insertions(+), 8 deletions(-)

    Index: linux-2.6.18/kernel/timer.c
    ===================================================================
    --- linux-2.6.18.orig/kernel/timer.c 2006-09-20 16:17:54.000000000 +0200
    +++ linux-2.6.18/kernel/timer.c 2006-09-20 16:17:59.000000000 +0200
    @@ -902,6 +902,7 @@ 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))
    @@ -914,28 +915,39 @@ 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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