[$] Per-vector software-interrupt masking

Software interrupts (or 'softirqs') are one of the oldest
deferred-execution mechanisms in the kernel, and that age shows at times.
Some developers have occasionally been heard to mutter about removing them, but
softirqs are too deeply embedded into how the kernel works to be easily ripped
out; most developers just leave them alone. So the recent per-vector
softirq masking patch set from Frederic Weisbecker is noteworthy as an
exception to that rule. Weisbecker is not getting rid of softirqs, but he
is trying to reduce their impact and improve their latency.