Dynamic preemption support in Linux 5.12 kernel

[Artem Tim]

Hello.

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.

RT kernel FAQ: https://rt.wiki.kernel.org/index.php/Frequently_Asked_Questions

[1]: http://lkml.iu.edu/hypermail/linux/kernel/2102.2/01311.html
[2]: https://copr.fedorainfracloud.org/coprs/atim/kernel-futex/
[3]: https://wiki.archlinux.org/index.php/Professional_audio#Realtime_kernel

[Reon Beon]

Yes, please.

[Chris Murphy]

On Sun, Apr 25, 2021 at 12:39 PM Artem Tim <ego.cordatus(a)gmail.com&gt; wrote:
>  Hello.
>
>  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.


-- 
Chris Murphy

[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

[Vitaly Zaitsev]

On 25.04.2021 20:39, Artem Tim wrote:
>  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. 
+1 for this. Significantly reduces X11 input lag in full-screen games.

-- 
Sincerely,
   Vitaly Zaitsev (vitaly(a)easycoding.org)

[Justin Forbes]

On Mon, Apr 26, 2021 at 11:25 AM Vitaly Zaitsev via devel
<devel(a)lists.fedoraproject.org&gt; wrote:
>  On 25.04.2021 20:39, Artem Tim wrote:
>  > 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.
>
>  +1 for this. Significantly reduces X11 input lag in full-screen games.
>  
From a kernel standpoint, I do not think turning on CONFIG_PREEMPT is
desired. That sets the *default* to PREEMPT, and changes the current
situation. I plan to keep CONFIG_PREEMPT_VOLUNTARY as the default. The
code introduced is odd, in that you can choose other options (none,
voluntary, full) based on that starting point, but it doesn't allow
you to default to anything else and keep CONFIG_PREEMPT_DYNAMIC on to
choose more preemption than the default.  I will see if I can hack
something together and get it upstream that would allow us to keep the
voluntary default and move to preempt with the command line.  Moving
forward, if something like that is not accepted, this might be a valid
change for F35, but I don't see it reasonable to change preemption in
stable releases (and the F34 ship has sailed).  Even if I do get such
a patch upstream, the default would remain voluntary, and the Desktop
SIG could add a preempt= command line by default for F35+.

Justin

[Artem Tim]

Good to know. I am crossing fingers 🤞 and hope you could improve current defaults and this
will be upstreamed. So at least users who want PREEMPT could enable it in runtime and not
need to rebuild custom kernel every time for this.

[Andrew Lutomirski]

On Mon, Apr 26, 2021 at 1:29 PM Justin Forbes <jmforbes(a)linuxtx.org&gt; wrote:
>  On Mon, Apr 26, 2021 at 11:25 AM Vitaly Zaitsev via devel
>  <devel(a)lists.fedoraproject.org&gt; wrote:
>  >
>  > On 25.04.2021 20:39, Artem Tim wrote:
>  > > 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.
>  >
>  > +1 for this. Significantly reduces X11 input lag in full-screen games.
>  >
>
>  From a kernel standpoint, I do not think turning on CONFIG_PREEMPT is
>  desired. That sets the *default* to PREEMPT, and changes the current
>  situation. I plan to keep CONFIG_PREEMPT_VOLUNTARY as the default. 
I'm curious: why do you prefer voluntary as the default instead of full preempt?

>  The
>  code introduced is odd, in that you can choose other options (none,
>  voluntary, full) based on that starting point, but it doesn't allow
>  you to default to anything else and keep CONFIG_PREEMPT_DYNAMIC on to
>  choose more preemption than the default. 
I've pinged the right people.  I'll try to make this happen.

[Justin Forbes]

On Mon, Apr 26, 2021 at 5:12 PM Andrew Lutomirski <luto(a)mit.edu&gt; wrote:
>  On Mon, Apr 26, 2021 at 1:29 PM Justin Forbes <jmforbes(a)linuxtx.org&gt; wrote:
>  >
>  > On Mon, Apr 26, 2021 at 11:25 AM Vitaly Zaitsev via devel
>  > <devel(a)lists.fedoraproject.org&gt; wrote:
>  > >
>  > > On 25.04.2021 20:39, Artem Tim wrote:
>  > > > 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.
>  > >
>  > > +1 for this. Significantly reduces X11 input lag in full-screen games.
>  > >
>  >
>  > From a kernel standpoint, I do not think turning on CONFIG_PREEMPT is
>  > desired. That sets the *default* to PREEMPT, and changes the current
>  > situation. I plan to keep CONFIG_PREEMPT_VOLUNTARY as the default.
>
>  I'm curious: why do you prefer voluntary as the default instead of full preempt?
It has always been a nice balance for Fedora where we do have a lot of
desktop users, but also a lot of server users. And it has been well
tested as the Fedora default for quite some time, and the default for
RHEL as well.  I don't specifically dislike the full preempt, but
introducing such a change to rebases on release Fedora is a risk I
don't think we should take.  I might be willing to make such a change
for Fedora 35, where we have plenty of time to test on various
workloads, but that doesn't help anyone for a good 6 months.  I would
much rather get the option for users now (I will even put the patch in
5.12 if I know it is headed upstream).

>  > The
>  > code introduced is odd, in that you can choose other options (none,
>  > voluntary, full) based on that starting point, but it doesn't allow
>  > you to default to anything else and keep CONFIG_PREEMPT_DYNAMIC on to
>  > choose more preemption than the default.
>
>  I've pinged the right people.  I'll try to make this happen. 
Thanks.  If not I can take a look.  The code looks good, it is just an
unfortunate lack of options for distros who would like to use dynamic
without disrupting defaults.

Justin

[Chris Murphy]

I created a Fedora Workstation issue to track the progress, and figure
out if any or all of the desktops want to do full preemption by
default.
https://pagure.io/fedora-workstation/issue/228

[Justin Forbes]

On Tue, Apr 27, 2021 at 2:30 PM Chris Murphy <lists(a)colorremedies.com&gt; wrote:
>  I created a Fedora Workstation issue to track the progress, and figure
>  out if any or all of the desktops want to do full preemption by
>  default.
>  https://pagure.io/fedora-workstation/issue/228 
Unless I missed a commit, it is x86_64 only so far.

Justin

[Justin Forbes]

On Mon, Apr 26, 2021 at 5:12 PM Andrew Lutomirski <luto(a)mit.edu&gt; wrote:
>  On Mon, Apr 26, 2021 at 1:29 PM Justin Forbes <jmforbes(a)linuxtx.org&gt; wrote:
>  >
>  > On Mon, Apr 26, 2021 at 11:25 AM Vitaly Zaitsev via devel
>  > <devel(a)lists.fedoraproject.org&gt; wrote:
>  > >
>  > > On 25.04.2021 20:39, Artem Tim wrote:
>  > > > 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.
>  > >
>  > > +1 for this. Significantly reduces X11 input lag in full-screen games.
>  > >
>  >
>  > From a kernel standpoint, I do not think turning on CONFIG_PREEMPT is
>  > desired. That sets the *default* to PREEMPT, and changes the current
>  > situation. I plan to keep CONFIG_PREEMPT_VOLUNTARY as the default.
>
>  I'm curious: why do you prefer voluntary as the default instead of full preempt?
>
>  > The
>  > code introduced is odd, in that you can choose other options (none,
>  > voluntary, full) based on that starting point, but it doesn't allow
>  > you to default to anything else and keep CONFIG_PREEMPT_DYNAMIC on to
>  > choose more preemption than the default.
>
>  I've pinged the right people.  I'll try to make this happen. 
Any update on this?

Thanks,
Justin