Bug in hrtimer_get_next_event?

[Gary King]

I am implementing idle state controls (CPU_IDLE) for Tegra SoCs, and one of the idle states is not awakened by the hrtimer interrupt. There is a system-wide high-resolution timer which can be used as a wakeup source, but I need the high-resolution sleep time to configure the alarm.

To fix this, I want to use hrtimer_get_next_event; however, the code that is in the tree only walks the hrtimer bases when hres mode is not active; when hres mode is active, hrtimer_get_next_event always returns KTIME_MAX. Is there any reason for the negative comparison, or is this a bug?

After changing this locally, I encountered one other problem on dynamic-tick systems: get_next_timer_interrupt is called to determine whether or not it is safe to enter nohz mode; however, hrtimer_get_next_event (which is used by get_next_timer_interrupt) will always return <=1 jiffy, since the emulated tick scheduler event will be armed when tick_nohz_stop_sched_tick queries the sleep time. As a result, tick_nohz_stop_sched_tick will never enter nohz mode.  I can think of a couple ways to address this (cancel the tick timer before querying the event and rearm if necessary from either the arch cpu_idle code or nohz_stop_sched_tick; ignore the tick timer in hrtimer_get_next_event); does anyone have a recommendation for a preferred approach?

- Gary
gking@xxxxxxxxxx
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