Re: Dynamic preemption support in Linux 5.12 kernel

[Michal Schmidt]

On Mon, Apr 26, 2021 at 3:31 AM Chris Murphy <lists(a)colorremedies.com&gt;
wrote:

>  On Sun, Apr 25, 2021 at 12:39 PM Artem Tim
> <ego.cordatus(a)gmail.com&gt; wrote:
>  > Upcoming 5.12 allow building with dynamic preemption support which
>  allows changing mode at boot/run-time so finally no need to rebuild or make
>  alternative kernel build anymore[1]. Responsive system is a must have for
>  gaming and good desktop experience overall, not only for professional
>  audio. For example difference in input mouse lag is like day and night with
>  PREEMPT mode. Maybe on cheap mouse this is hard to notice but on 500-1000Hz
>  mouse really easy. You can test this yourself with experimental kernel
>  build on COPR[2]. Some Linux distros[3] already using PREEMPT mode by
>  default for a decent period of time. Fedora kernel build with
>  CONFIG_PREEMPT_VOLUNTARY but this is not enough for desktop use case.
>  >
>  > Please provide full PREEMPT mode by default in 5.12 kernel for desktop
>  variants. With 5.12 it is possible for user to change it without efforts if
>  they need this.
>  >
>
>  I see CONFIG_HAVE_PREEMPT_DYNAMIC=y with kernel
>  5.12.0-0.rc8.191.fc35.x86_64+debug.
>  
CONFIG_HAVE_PREEMPT_DYNAMIC only says the architecture supports the
feature. This is hard-coded by arch/x86/Kconfig.
To actually enable the feature, CONFIG_PREEMPT=y would have to be set. The
combination of CONFIG_PREEMPT and CONFIG_HAVE_PREEMPT_DYNAMIC then selects
CONFIG_PREEMPT_DYNAMIC. See kernel/Kconfig.preempt.

Michal