linux-kernel - Re: [RFC PATCH 00/86] Make the kernel preemptible

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Date:   Wed, 8 Nov 2023 10:15:52 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Ankur Arora' <ankur.a.arora@...cle.com>,
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Subject: Re: [RFC PATCH 00/86] Make the kernel preemptible

On Wed, 8 Nov 2023 09:43:10 +0000
David Laight <David.Laight@...LAB.COM> wrote:

> > 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.

What IPI is extra?

> 
> 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.

No, voluntary sleep can not be done in a preemptible section.

-- Steve