Normally pending work is work that cannot be done from NMI context, such
as wakeups and disabling the counter. The pending work is a single
linked list using atomic ops so that it functions from NMI context.

Normally this is called from IRQ context through use of an self-IPI
(x86) or upon enabling hard interrupts (powerpc). Architectures that do
not implement perf_counter_set_pending() nor call
perf_counter_do_pending() upon leaving NMI context will get a polling
fallback from the timer softirq.

However, in -rt we cannot do the wakeup from IRQ context because its a
wait_queue wakup, which can be O(n), so defer all wakeups to the softirq
fallback by creating a second pending list that's only processed from
there.
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@ghostprotocols.net>
LKML-Reference: <1251208265.7538.1157.camel@twins>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>