[$] Heuristics for software-interrupt processing

The kernel's software-interrupt ("softirq") mechanism was added prior tothe 1.0 kernel release, but it implements a design seen in systems that werealready old when Linux was born. For much of that time, softirqs have beenan impediment to the kernel community's scalability and response-timegoals, but they have proved resistant to removal. A recent discussion on aproposed new heuristic to mitigate a softirq-related performance problemmay have reinvigorated interest in doing something about this subsystemas a whole rather than just tweaking the parameters of how it operates.