clockevents: Stop unused clockevent devices

Hi,

A clockevent device is used to service timers/hrtimers requests and the next event (when it should fire) is decided by the timer/hrtimer expiring next. When no timers/hrtimers are pending to be serviced, the expiry time is set to a special value: KTIME_MAX.

This would normally happen with NO_HZ_{IDLE|FULL} in both LOWRES/HIGHRES modes.

When 'expiry == KTIME_MAX', we either cancel the 'tick-sched' hrtimer (NOHZ_MODE_HIGHRES) or skip reprogramming clockevent device (NOHZ_MODE_LOWRES). But, the clockevent device is already reprogrammed from the tick-handler for next tick.

As the clock event device is programmed in ONESHOT mode it will at least fire one more time (unnecessarily). Timers on few implementations (like arm_arch_timer, etc.) only support PERIODIC mode and their drivers emulate ONESHOT over that. Which means that on these platforms we will get spurious interrupts periodically (at last programmed interval rate, normally tick rate).

In order to avoid spurious interrupts, the unused clockevent device should be stopped or its interrupts should be masked.

Thomas suggested to add an optional state ONESHOT_STOPPED to solve this problem:
lkml.org/lkml/2014/5/9/508.

First patch implements the required infrastructure to start/stop clockevent device. The second patch stops/starts clockevent devices as and when required.

It is also pushed here:

git://git.kernel.org/pub/scm/linux/kernel/git/vireshk/linux.git tick/oneshot-stopped

@Preeti: I have applied your Reviewed-by only for the first commit as others have changed significantly. Feel free to give it again :)

V1->V2:
- Peter & Ingo suggested to to start/stop clockevent device from tick_program_event() instead of the call sites that called tick_program_event().

--
viresh

Viresh Kumar (2):
clockevents: Introduce CLOCK_EVT_STATE_ONESHOT_STOPPED state
Clockevents: Stop unused clockevent devices

 include/linux/clockchips.h |  7 ++++++-
 kernel/time/clockevents.c  | 18 +++++++++++++++++-
 kernel/time/hrtimer.c      |  6 ++----
 kernel/time/tick-oneshot.c | 16 ++++++++++++++++
 kernel/time/timer_list.c   |  6 ++++++
 5 files changed, 47 insertions(+), 6 deletions(-)

2.3.0.rc0.44.ga94655d