Linux 6.12 new preemption models

Linux 6.12 new preemption models

[Leo Famulari]

FYI, Linux / linux-libre 6.12 introduces a new set of preemption
options:

------
Preemption Model
  1. No Forced Preemption (Server) (PREEMPT_NONE)
> 2. Voluntary Kernel Preemption (Desktop) (PREEMPT_VOLUNTARY)
  3. Preemptible Kernel (Low-Latency Desktop) (PREEMPT)
  4. Fully Preemptible Kernel (Real-Time) (PREEMPT_RT) (NEW)
------

These options trade between kernel throughput and latency.

PREEMPT_NONE is how the kernels worked before 6.12. 

The trade-off made in PREEMPT_VOLUNTARY is suggested for desktops, so
I'm going to select it for our 6.12 kernel packaging and it will
presumably remain so for future versions as well.

But if you are building Guix kernels for a busy server or HPC, you
should select PREEMPT_NONE. And you might have use cases for the other
options as well.

Let me know if you disagree.

References:
https://cateee.net/lkddb/web-lkddb/PREEMPT_NONE.html
https://cateee.net/lkddb/web-lkddb/PREEMPT_VOLUNTARY.html
https://cateee.net/lkddb/web-lkddb/PREEMPT.html
https://cateee.net/lkddb/web-lkddb/PREEMPT_RT.html

Re: Linux 6.12 new preemption models

[kiasoc5]

On 1/1/25 17:51, Leo Famulari wrote:
> FYI, Linux / linux-libre 6.12 introduces a new set of preemption
> options:
> 
> ------
> Preemption Model
>    1. No Forced Preemption (Server) (PREEMPT_NONE)
>> 2. Voluntary Kernel Preemption (Desktop) (PREEMPT_VOLUNTARY)
>    3. Preemptible Kernel (Low-Latency Desktop) (PREEMPT)
>    4. Fully Preemptible Kernel (Real-Time) (PREEMPT_RT) (NEW)
> ------
> 
> These options trade between kernel throughput and latency.
> 
> PREEMPT_NONE is how the kernels worked before 6.12.
> 
> The trade-off made in PREEMPT_VOLUNTARY is suggested for desktops, so
> I'm going to select it for our 6.12 kernel packaging and it will
> presumably remain so for future versions as well.
> 
> But if you are building Guix kernels for a busy server or HPC, you
> should select PREEMPT_NONE. And you might have use cases for the other
> options as well.
> 
> Let me know if you disagree.
> 
> References:
> https://cateee.net/lkddb/web-lkddb/PREEMPT_NONE.html
> https://cateee.net/lkddb/web-lkddb/PREEMPT_VOLUNTARY.html
> https://cateee.net/lkddb/web-lkddb/PREEMPT.html
> https://cateee.net/lkddb/web-lkddb/PREEMPT_RT.html
> 


How about PREEMPT_DYNAMIC: 
https://cateee.net/lkddb/web-lkddb/PREEMPT_DYNAMIC.html

This enables changing between PREEMPT_NONE, PREEMPT_VOLUNTARY, and 
PREEMPT at boot time. Then the only kernel that needs to be compiled 
separate is PREEMPT_RT.

Re: Linux 6.12 new preemption models

[Leo Famulari]

On Fri, Jan 03, 2025 at 01:15:42PM -0500, kiasoc5 wrote:
> How about PREEMPT_DYNAMIC:
> https://cateee.net/lkddb/web-lkddb/PREEMPT_DYNAMIC.html
> 
> This enables changing between PREEMPT_NONE, PREEMPT_VOLUNTARY, and PREEMPT
> at boot time. Then the only kernel that needs to be compiled separate is
> PREEMPT_RT.

Good suggestion! Turns out it's already enabled :)