On 2017-10-24 00:23, Thomas Gleixner wrote:
On Mon, 23 Oct 2017, Sodagudi Prasad wrote:
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;
}
Right, because this code does not have access to the broadcast device
at
all. It doesn't even know and care about it.
Some of the drivers in cpuidle frame works are using
tick_nohz_get_sleep_length API
to find maximum idle/sleep time from ts->sleep_length.
And also the estimated sleep length until the next timer in
tick_nohz_stop_sched_tick
not getting updated at the end.
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.
If the device is shutdown, then next_event does not matter. But yes,
for
consistency reasons we could set it to KTIME_MAX.
Thanks tglx for your quick reply on this and I will send patch for the
same.
Thanks,
tglx
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