posix-cpu-timers: Provide mechanisms to defer timer handling to task_work
Running posix CPU timers in hard interrupt context has a few downsides: - For PREEMPT_RT it cannot work as the expiry code needs to take sighand lock, which is a 'sleeping spinlock' in RT. The original RT approach of offloading the posix CPU timer handling into a high priority thread was clumsy and provided no real benefit in general. - For fine grained accounting it's just wrong to run this in context of the timer interrupt because that way a process specific CPU time is accounted to the timer interrupt. - Long running timer interrupts caused by a large amount of expiring timers which can be created and armed by unpriviledged user space. There is no hard requirement to expire them in interrupt context. If the signal is targeted at the task itself then it won't be delivered before the task returns to user space anyway. If the signal is targeted at a supervisor process then it might be slightly delayed, but posix CPU timers are inaccurate anyway due to the fact that they are tied to the tick. Provide infrastructure to schedule task work which allows splitting the posix CPU timer code into a quick check in interrupt context and a thread context expiry and signal delivery function. This has to be enabled by architectures as it requires that the architecture specific KVM implementation handles pending task work before exiting to guest mode. Signed-off-by:Thomas Gleixner <tglx@linutronix.de> Signed-off-by: