PATCH v3 0/3] softirq: Use a dedicated thread for timer wakeups with forced-threading.

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

Hi,

the following was in the PREEMPT_RT queue since last softirq rework. The
result is that timer wake ups (hrtimer, timer_list) happens in hardirq
processing them requires to wake ksoftirqd. ksoftirqd runs at SCHED_OTHER so it
will compete for resources with all other tasks in the system, potentially
delayed the processing further.

The idea was to let the timers be processed by a dedicated thread
running at low SCHED_FIFO priority.
While looking at it again, it might make sense to have the
pending_softirq flag per-thread to avoid threads with higher priority
picking up softirqs from low-priority threads. This isn't yet a problem
because adding softirqs for processing happens only from threaded
interrupts. So the low-priority thread will wait until the high-priority
thread is done. And the high-priority thread will PI-boost the
low-priority thread until it is done. It would only make sense to make
the flags per-thread once the BH lock is gone.

The patch is limited to the forced threaded case.

v2…v3 https://lore.kernel.org/all/20241024150413.518862-1-bigeasy@linutronix.de/
 - Redo the comment in interrupt.h. Make it more verbose and add more
   details. Frederick asked for it.

 - After updating the comment I convinced myself that it also makes
   sense to use this in the forced threaded case.

 - Merged raise_hrtimer_softirq() and raise_timer_softirq() into a
   single function since both use __raise_softirq_irqoff() in the
   alternative case.
 
v1…v2 https://lore.kernel.org/all/20241004103842.131014-1-bigeasy@linutronix.de/
 Frederick's comments:
 - Use __raise_softirq_irqoff() to raise the softirq for !PREEMPT_RT. Also a
   lockdep test to ensure that this is always invoked from an IRQ.
 - Make raise_ktimers_thread() only OR the flag and nothing else to
   align with __raise_softirq_irqoff(). The wake happens on return from
   interrupt anyway.
 - A comment in timersd_setup() and interrupt.h
 - local_pending_timers() => local_timers_pending().

Sebastian