Clockevents and dyntick

In the kernel, why keep watching the clock when you can hit the snooze button?

By Jonathan Corbet, LinuxWorld.com, 02/28/07

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One of the last patch sets to be merged before the 2.6.21 window closed was the clockevents and dyntick work from the real-time tree. These patches have been in the works for some time, and were originally targeted for merging in 2.6.19. In the process, the developers (primarily Ingo Molnar and Thomas Gleixner) discovered one of the fundamental laws of kernel development: if your patches break Andrew Morton's laptop, they are unlikely to make it into the mainline. That little difficulty has now been overcome, with the result that 2.6.21 will include some interesting core changes.

Dealing with clock devices has traditionally been handled in the kernel's architecture-specific code. The result has been a lot of duplicated code between architectures (there are more architectures than common timer devices) and no uniform interface for the core kernel to make use of these devices. John Stultz's generic time of day infrastructure resolved a number of those problems, at least for the timekeeping task, but anybody who wanted to program timer devices in a more general way still ended up dealing with architecture-specific code.

The "clockevents" patch set finishes this job. At its core, clockevents creates a driver API for devices which can deliver interrupts at a specific time in the future. The API tracks the capabilities of each timer (resolution and whether it can do one-shot or periodic interrupts, for example) and provides a simple interface for arming the timer. This API is defined in the core kernel, with only a low-level driver remaining in the architecture-specific code. The end result is that the kernel now has the means to query and use timer capabilities in an architecture-independent manner.

With the clockevents mechanism in place, it becomes possible to support truly high-resolution timers. When such a timer is requested, all that is required is to pick a suitable clockevent device and arm it for the desired time. These devices can deliver interrupts with a high degree of precision, with the result that kernel timers, too, can offer high precision - a feature which is of clear utility to real-time users (among others).

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