The long road to lazy preemption in the Linux CPU scheduler

> A higher level of preemption enables the system to respond more quickly to events; whether an event is the movement of a mouse or an "imminent meltdown" signal from a nuclear reactor, faster response tends to be more gratifying. But a higher level of preemption can hurt the overall throughput of the system; workloads with a lot of long-running, CPU-intensive tasks tend to benefit from being disturbed as little as possible. More frequent preemption can also lead to higher lock contention. That is why the different modes exist; the optimal preemption mode will vary for different workloads.

Why isn't the level of preemption a property of the specific event, rather than of some global mode? Some events need to be handled with less latency than others.

You need CPU time to evaluate the priority of the event. This can't happen until after you've interrupted whatever process is currently on the CPU. And so the highest possible priority an event can happen is limited by how short a time slice a program gets before it has to go through a context switch.

To stand ready to reliably respond to any one kind of event with low latency, every CPU intensive program must suffer a performance penalty all the time. And this is true no matter how rare those events may be.

> You need CPU time to evaluate the priority of the event.

Not necessarily. The CPU can do it in hardware. As a simple example, the 6502 had separate “interrupt request” (IRQ) and “non-maskable interrupts (NMI) pins, supporting two interrupt levels. The former could be disabled; the latter could not.

A programmable interrupt controller (https://en.wikipedia.org/wiki/Programmable_interrupt_control...) also could ‘know’ that it need not immediately handle some interrupts.

The user you replied to likely means something different: The priority of the event often depends on the exact contents on the event and not the hardware event source. For example, say you receive a "read request completed" interrupt from a storage device. The kernel now needs to pass on the data to the process which originally requested it. In order to know how urgent the original request and thus the handling of the interrupt is, the kernel needs to check which sector was read and associate it with a process. Merely knowing that it came from a specific storage device is not sufficient.

By the way, NMI still exist on x86 to this day, but AFAIK they're only used for serious machine-level issues and watchdog timeouts.

This, too, can be done in hardware (if nothing else, with a small coprocessor).

This doesn't shed light

Generally, any given software can be done in hardware.

Specifically, we could attach small custom coprocessors to everything for the Linux kernel, and Linux could require them to do any sort of multitasking.

In practice, software allows us to customize these things and upgrade them and change them without tightly coupling us to a specific kernel and hardware design.