hrtimer: Provide softirq context hrtimers

There are quite some places in the kernel which use a combination of hrtimers and tasklets to make use of the precise expiry of hrtimers, which schedule a tasklet to bring the actual function into softirq context.

This was introduced when the previous hrtimer softirq code was removed. That code was implemented by expiring the timer in hard irq context and then deferring the execution of the callback into softirq context. That caused a lot of pointless shuffling between the rbtree and a linked list.

In recent discussions it turned out that more potential users of hrtimers in softirq context might come up. Aside of that the RT patches need this functionality as well to defer hrtimers into softirq context if their callbacks are not interrupt safe on RT.

This series implements a new approach by adding SOFT hrtimer mode and instead of doing the list shuffle, timers started with this mode are put into separate soft expiry hrtimer queues. These queues are evaluated only when the hardirq context detects that the first expiring timer in the softirq queues has expired. That makes the overhead in the hardirq context minimal.

The series reworks the code to reuse as much as possible from the existing facilities for the new softirq hrtimers and integrates them with all flavours of hrtimers (HIGH_RES=y/n - NOHZ=y/n).

To achieve this quite some of the conditionals in the existing code are removed for the price of adding some pointless data and state tracking to the HIGH_RES=n case. That's minimal, but well worth it as it increases the readability and maintainability of the code.

The first part of the series implements the new functionality and the second part converts the hrtimer/tasklet users to make use of it and removes struct hrtimer_tasklet and the surrounding helper functions.

This series is available from git as well:

git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git WIP.timers

Thanks,

Anna-Maria

---
v1..v2:

- integration of Peter Zijlstras patch:

    https://lkml.kernel.org/r/[email protected]
  - using hrtimer_mode instead of additional hrtimer clock bases
  - folding the fix for updating the base offsets:
    https://lkml.kernel.org/r/[email protected]

- rework of 08/25 - 10/25 (all of those patches are facing reduction of conditional code) and make hrtimer_force_reprogram() unconditional as well
- integration of new versions of "ALSA/dummy: Replace tasklet with softirq hrtimer" and "net/cdc_ncm: Replace tasklet with softirq hrtimer"
- additional hrtimer/tasklet user conversion: "net/mvpp2: Replace tasklet with softirq hrtimer"
- additional fixes of several wrong comments
- update hrtimer tracing (mode and clock bases)

 drivers/net/ethernet/marvell/mvpp2.c  |   62 +--
 drivers/net/usb/cdc_ncm.c             |   38 --
 drivers/net/wireless/mac80211_hwsim.c |   44 +-
 drivers/usb/gadget/function/f_ncm.c   |   30 -
 include/linux/hrtimer.h               |  110 +++---
 include/linux/interrupt.h             |   25 -
 include/linux/usb/cdc_ncm.h           |    1 
 include/net/xfrm.h                    |    2 
 include/trace/events/timer.h          |   37 +-
 kernel/softirq.c                      |   51 --
 kernel/time/hrtimer.c                 |  623 +++++++++++++++++++++-------------
 kernel/time/timer.c                   |    6 
 net/can/bcm.c                         |  156 ++------
 net/xfrm/xfrm_state.c                 |   30 -
 sound/drivers/dummy.c                 |   27 -
 15 files changed, 652 insertions(+), 590 deletions(-)