clock event deviceâs next_event

[Sodagudi Prasad]

Hi Viresh and Thomas,

In the functions tick_nohz_stop_sched_tick(), when expires = KTIME_MAX 
we are canceling the tick_sched_timer timer but we are not updating the 
clock event deviceâs next_event to KTIME_MAX.
Due to that broadcast deviceâs next_event is not programmed properly and 
resulting unnecessary wakeups for this cpu.

        /*
         * If the expiration time == KTIME_MAX, then we simply stop
         * the tick timer.
         */
        if (unlikely(expires == KTIME_MAX)) {
                if (ts->nohz_mode == NOHZ_MODE_HIGHRES)
                        hrtimer_cancel(&ts->sched_timer);
                goto out;
        }


After digging further, I see that following call flow is updating 
tick_cpu_device state to shutdown state but clock event device  
next_event is not updated to KTIME_MAX.
hrtimer_cancel -> __remove_hrtimer -> hrtimer_force_reprogram -> 
tick_program_event.

int tick_program_event(ktime_t expires, int force)
{
        struct clock_event_device *dev = 
__this_cpu_read(tick_cpu_device.evtdev);

        if (unlikely(expires == KTIME_MAX)) {
                /*
                 * We don't need the clock event device any more, stop 
it.
                 */
                clockevents_switch_state(dev, 
CLOCK_EVT_STATE_ONESHOT_STOPPED);
                return 0;
        }
In the above tick_program_event() function clock event deviceâs 
next_event is not getting updated as clockevents_program_event() 
function not called after state update.

-thanks, Prasad


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