genirq/timings: Add infrastructure to track the interrupt timings

The interrupt framework gives a lot of information about each interrupt. It
does not keep track of when those interrupts occur though, which is a
prerequisite for estimating the next interrupt arrival for power management
purposes.

Add a mechanism to record the timestamp for each interrupt occurrences in a
per-CPU circular buffer to help with the prediction of the next occurrence
using a statistical model.

Each CPU can store up to IRQ_TIMINGS_SIZE events <irq, timestamp>, the
current value of IRQ_TIMINGS_SIZE is 32.

Each event is encoded into a single u64, where the high 48 bits are used
for the timestamp and the low 16 bits are for the irq number.

A static key is introduced so when the irq prediction is switched off at
runtime, the overhead is near to zero.

It results in most of the code in internals.h for inline reasons and a very
few in the new file timings.c. The latter will contain more in the next patch
which will provide the statistical model for the next event prediction.

Signed-off-by: Daniel Lezcano <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Acked-by: Nicolas Pitre <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: Hannes Reinecke <[email protected]>
Cc: Vincent Guittot <[email protected]>
Cc: "Rafael J . Wysocki" <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Bjorn Helgaas <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]

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