Hi,

Linux 5.12 is going to support changing the PREEMPT setting at runtime (as a boot argument).
See http://lkml.iu.edu/hypermail/linux/kernel/2102.2/01311.html for the details on that.

A small note on PREEMPT.
For desktop purposes, you want to have it enabled. Like arch does in it's kernel config:
CONFIG_PREEMPT=y

But for server setups you want to change that to:
CONFIG_PREEMPT_VOLUNTARY=y
# CONFIG_PREEMPT=y

So just voluntary PREEMPT.

But not only servers want this. If you run a KVM with Windows 10 as a guest and a boat load of memory (say anything upward of 10GB), your boot time will be insanely long with the default PREEMPT.
I tried this with a server and assigning it 50GB of memory. The boot took about 6 minutes and during that time it has 100% cpu usage, with PREEMPT set to voluntary it booted in ~20 seconds. Thus there too you want to change it to "CONFIG_PREEMPT_VOLUNTARY=y"

Changing this, currently, requires to rebuild the kernel.
A step i'd like to prevent.

My request is to, as soon as the 5.12 kernel is out, that Arch will default to:
CONFIG_PREEMPT_DYNAMIC=y

And comment out the current "CONFIG_PREEMPT=y".

If no other change is done the system will behave exactly as it currently does. The default PREEMPT of the dynamic mode is full.
A user can then change that in the boot arguments:
preempt=none/voluntary/full

Best regards,
Mark