timers: Optimize collect_expired_timers() for NOHZ

After a NOHZ idle sleep the timer wheel must be forwarded to current jiffies.
There might be expired timers so the current code loops and checks the expired
buckets for timers. This can take quite some time for long NOHZ idle periods.

The pending bitmask in the timer base allows us to do a quick search for the
next expiring timer and therefore a fast forward of the base time which
prevents pointless long lasting loops.

For a 3 seconds idle sleep this reduces the catchup time from ~1ms to 5us.

Signed-off-by: Anna-Maria Gleixner <anna-maria@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Arjan van de Ven <arjan@infradead.org>
Cc: Chris Mason <clm@fb.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: George Spelvin <linux@sciencehorizons.net>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Len Brown <lenb@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: rt@linutronix.de
Link: http://lkml.kernel.org/r/20160704094342.351296290@linutronix.de


Signed-off-by: Ingo Molnar <mingo@kernel.org>