softirq: Use a dedicated thread for timer wakeups.

A timer/hrtimer softirq is raised in-IRQ context. With threaded
interrupts enabled or on PREEMPT_RT this leads to waking the ksoftirqd
for the processing of the softirq.
Once the ksoftirqd is marked as pending (or is running) it will collect
all raised softirqs. This in turn means that a softirq which would have
been processed at the end of the threaded interrupt, which runs at an
elevated priority, is now moved to ksoftirqd which runs at SCHED_OTHER
priority and competes with every regular task for CPU resources.
This introduces long delays on heavy loaded systems and is not desired
especially if the system is not overloaded by the softirqs.

Split the TIMER_SOFTIRQ and HRTIMER_SOFTIRQ processing into a dedicated
timers thread and let it run at the lowest SCHED_FIFO priority.
RT tasks are are woken up from hardirq context so only timer_list timers
and hrtimers for "regular" tasks are processed here. The higher priority
ensures that wakeups are performed before scheduling SCHED_OTHER tasks.

Using a dedicated variable to store the pending softirq bits values
ensure that the timer are not accidentally picked up by ksoftirqd and
other threaded interrupts.
It shouldn't be picked up by ksoftirqd since it runs at lower priority.
However if the timer bits are ORed while a threaded interrupt is
running, then the timer softirq would be performed at higher priority.
The new timer thread will block on the softirq lock before it starts
softirq work. This "race window" isn't closed because while timer thread
is performing the softirq it can get PI-boosted via the softirq lock by
a random force-threaded thread.
The timer thread can pick up pending softirqs from ksoftirqd but only
if the softirq load is high. It is not be desired that the picked up
softirqs are processed at SCHED_FIFO priority under high softirq load
but this can already happen by a PI-boost by a force-threaded interrupt.

Reported-by: kernel test robot <lkp@intel.com> [ static timer_threads ]
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>