Re: Selecting the scheduler from boot menu

On Wed, 18 Feb 2026 12:05:05 +0100
Milan Obuch <[email protected]> wrote:

[ snip ]

> I had today an opportunity to do some boot menu hacking as I could do
> some reboots for tests, and I found it is quite easy to add a menu
> item into loader menu. See attached patch for result.
> 
> First chunk in this patch just places new entry after kernel select
> item, the second one actually defines new item's look and function.
> 
> It works - partially. When 4BSD is selected, kernel environment is
> changed:
> 
> # kenv | grep kern.sched
> kern.sched.name="4BSD"
> 
> (new variable is created), when this options is not touched, this
> variable does not exist. It is created, however, if I change scheduler
> option to 4BSD and then back to ULE:
> 
> # kenv | grep kern.sched
> kern.sched.name="ULE"
> 
> In all cases, however, actual change is not made, sysctl is not
> changed, every time I get
> 
> # sysctl kern.sched.name
> kern.sched.name: ULE
> 
> So we have simple method to do the loader menu stuff, next step is to
> find where actual change could be done (and how). Also, I did my tests
> on 15.0-STABLE built from sources from February 1st. It did not work
> for me even when entering
> 
> set kern.sched.name="4BSD"
> 
> from loader prompt and then booting my system.
> 
> Maybe test with 16-CURRENT is necessary for this to work... I have no
> box for this test right now.

I did a small test with 16-CURRENT installer image - I wrote an image
to USB flash disk, applied my patch to /boot/lua/menu.lua file and
verify it works as expected, namely when 4BSD scheduler is selected, I
see

# sysctl kern.sched.name
kern.sched.name: 4BSD

and that's it. This setting is temporary, if desired to change
scheduler to 4BSD, you still need to write it into /boot/loader.conf
file, as before:

kern.sched.name="4BSD"

Caveat: with this setting, boot menu still displays ULE as scheduler to
be used. If you really would like to select ULE with this setting
active in /boot/loader.conf file, you need to press 7 *twice*.

I am going to try to find a fix for this annoyance, some more testing
is necessary. But I think it is already usefull for someone even with
this flaw, I hope :)

Regards,
Milan