timers: iprovements squashed

timers: Create detach_if_pending() and use it

Most callers of detach_timer() have the same pattern around
them. Check whether the timer is pending and eventually updating
base->next_timer.

Create detach_if_pending() and replace the duplicated code.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Gilad Ben-Yossef <gilad@benyossef.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Link: http://lkml.kernel.org/r/20120525214819.131246037@linutronix.de

timers: Consolidate base->next_timer update

Another bunch of mindlessly copied code. All callers of
internal_add_timer() except the recascading code updates
base->next_timer.

Move this into internal_add_timer() and let the cascading code call
__internal_add_timer().

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Gilad Ben-Yossef <gilad@benyossef.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Link: http://lkml.kernel.org/r/20120525214819.189946224@linutronix.de

timers: Add accounting of non deferrable timers

The code in get_next_timer_interrupt() is suboptimal as it has to run
through the cascade to find the next expiring timer. On a completely
idle core we should only do that when there is an active timer
enqueued and base->next_timer does not give us a fast answer.

Add accounting of the active timers to the now consolidated
attach/detach code. I deliberately avoided sanity checks because the
code is fully symetric and any fiddling with timers w/o using the API
functions will lead to cute explosions anyway. ulong is big enough
even on 32bit and if we really run into the situation to have more
than 1<<32 timers enqueued there, then we are definitely not in a
state to go idle and run through that code.

This allows us to fix another shortcoming of get_next_timer_interrupt().

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Gilad Ben-Yossef <gilad@benyossef.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Link: http://lkml.kernel.org/r/20120525214819.236377028@linutronix.de

timers: Improve get_next_timer_interrupt()

Gilad reported at

 http://lkml.kernel.org/r/1336056962-10465-2-git-send-email-gilad@benyossef.com

"Current timer code fails to correctly return a value meaning that
 there is no future timer event, with the result that the timer keeps
 getting re-armed in HZ one shot mode even when we could turn it off,
 generating unneeded interrupts.

 What is happening is that when __next_timer_interrupt() wishes
 to return a value that signifies "there is no future timer
 event", it returns (base->timer_jiffies + NEXT_TIMER_MAX_DELTA).

 However, the code in tick_nohz_stop_sched_tick(), which called
 __next_timer_interrupt() via get_next_timer_interrupt(),
 compares the return value to (last_jiffies + NEXT_TIMER_MAX_DELTA)
 to see if the timer needs to be re-armed.

 base->timer_jiffies != last_jiffies and so tick_nohz_stop_sched_tick()
 interperts the return value as indication that there is a distant
 future event 12 days from now and programs the timer to fire next
 after KTIME_MAX nsecs instead of avoiding to arm it. This ends up
 causing a needless interrupt once every KTIME_MAX nsecs."

Fix this by using the new active timer accounting. This avoids scans
when no active timer is enqueued completely, so we don't have to rely
on base->timer_next and base->timer_jiffies anymore.

Reported-by: Gilad Ben-Yossef <gilad@benyossef.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Link: http://lkml.kernel.org/r/20120525214819.317535385@linutronix.de