linux-kernel - clock event device’s next

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Date:   Mon, 23 Oct 2017 21:33:09 -0700
From:   Sodagudi Prasad <psodagud@...eaurora.org>
To:     Thomas Gleixner <tglx@...utronix.de>, viresh.kumar@...aro.org,
        fweisbec@...il.com, mingo@...nel.org
Cc:     linux-kernel@...r.kernel.org, psodagud@...eaurora.org
Subject: clock event device’s next_event

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 device’s next_event to KTIME_MAX.
Due to that broadcast device’s 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 device’s 
next_event is not getting updated as clockevents_program_event() 
function not called after state update.

-thanks, Prasad

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