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The SEND_RESCHED is really only interesting if the IPI was generated
by LITMUS^RT. Therefore, we don't need to trace in Linux's
architecture-specific code. Instead, we hook into the preemption state
machine, which is informed about incoming IPIs anyway.
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Preemption state tracing is only useful when debugging preemption-
and IPI-related races. Since it creates a lot of clutter in the logs,
this patch turns it off unless explicitly requested.
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To date, Litmus has just hooked into the smp_send_reschedule() IPI
handler and marked tasks as having to reschedule to implement remote
preemptions. This was never particularly clean, but so far we got away
with it. However, changes in the underlying Linux, and peculartities
of the ARM code (interrupts enabled before context switch) break this
naive approach. This patch introduces new state-machine based remote
preemption support. By examining the local state before calling
set_tsk_need_resched(), we avoid confusing the underlying Linux
scheduler. Further, this patch avoids sending unncessary IPIs.
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