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corbet

Jonathan Corbet

On the radar: reconsidering the kernel's preemption models.

It all started in a discussion on optimizing string operations on x86, but that led to finding ways to allow preemption for long-running operations even in non-preempable kernels.

You see, the kernel offers a number of different models for when kernel code itself can be preempted to run something with a higher priority. All the way from PREEMPT_NONE (no preemption at all) through PREEMPT_VOLUNTARY (preemption at explicitly marked points) and plain PREEMPT (anytime not in a critical section) through to PREEMPT_RT for realtime. Linus was getting grumpy about the scattering of voluntary preemption points, and eventually came around to the idea of maybe dropping PREEMPT_NONE and PREEMPT_VOLUNTARY altogether:

https://lwn.net/ml/linux-kernel/CAHk-=whpYjm_AizQij6XEfTd7xvGjrVCx5gzHcHm=2Xijt+Kyg@mail.gmail.com/

I doubt that's going to happen, but we may see a reduction of options in favor of PREEMPT_DYNAMIC, which allows choosing between voluntary and full preemption at boot time.
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