RE: [RFC PATCH 00/86] Make the kernel preemptible

[David Laight]

From: Ankur Arora
> Sent: 07 November 2023 21:57
...
> There are four main sets of preemption points in the kernel:
> 
>  1. return to user
>  2. explicit preemption points (cond_resched() and its ilk)
>  3. return to kernel (tick/IPI/irq at irqexit)
>  4. end of non-preemptible sections at (preempt_count() == preempt_offset)
> 
...
> Policies:
> 
> A - preemption=none: run to completion
> B - preemption=voluntary: run to completion, unless a task of higher
>     sched-class awaits
> C - preemption=full: optimized for low-latency. Preempt whenever a higher
>     priority task awaits.

If you remove cond_resched() then won't both B and C require an extra IPI.
That is probably OK for RT tasks but could get expensive for
normal tasks that aren't bound to a specific cpu.

I suspect C could also lead to tasks being pre-empted just before
they sleep (eg after waking another task).
There might already be mitigation for that, I'm not sure if
a voluntary sleep can be done in a non-pre-emptible section.

Certainly it should all help the scheduling of RT tasks - which
can currently get delayed by a non-RT task in a slow kernel path.
Although the worst one is the softint code...

	David

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