[PATCH 0/3] clockevents: Introduce CLOCK_EVT_STATE_ONESHOT_STOPPED

[Viresh Kumar]

Hi Thomas/Ingo/Peter,

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 clockevent device should be stopped
or its interrupts should be masked.

A simple (yet hacky) solution to get this fixed could be: update
hrtimer_force_reprogram() to always reprogram clockevent device and update
clockevent drivers to STOP generating events (or delay it to max time) when
'expires' is set to KTIME_MAX. But the drawback here is that every clockevent
driver has to be hacked for this particular case and its very easy for new ones
to miss this.

However, 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. Third patch stops clockevent devices when no longer required and Second
patch starts them again as and when required.

The review order can be 1,3,2 for better understanding. But patch 2 was required
before 3 to keep 'git bisect' happy, otherwise clockevent device might never get
restarted after stopping it :)

It is also pushed here:

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

Viresh Kumar (3):
  clockevents: Introduce CLOCK_EVT_STATE_ONESHOT_STOPPED state
  clockevents: Restart clockevent device before using it again
  clockevents: Switch state to ONESHOT_STOPPED for unused clockevent
    devices

 include/linux/clockchips.h |  7 ++++++-
 kernel/time/clockevents.c  | 18 +++++++++++++++-
 kernel/time/hrtimer.c      | 51 ++++++++++++++++++++++++++++++++++++++++++----
 kernel/time/tick-sched.c   | 17 +++++++++++++++-
 kernel/time/timer_list.c   |  6 ++++++
 5 files changed, 92 insertions(+), 7 deletions(-)

-- 
2.3.0.rc0.44.ga94655d

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