Re: CONFIG_PREEMPT_DYNAMIC=y?

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From	Emanuele Rocca <ema@debian.org>
Newsgroups	linux.debian.kernel
Subject	Re: CONFIG_PREEMPT_DYNAMIC=y?
Date	Tue, 10 Oct 2023 12:10:01 +0200
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Hi,

On 2023-10-08 07:17, Diederik de Haas wrote:
> $ grep PREEMPT_DYNAMIC /boot/config-6.5.0-1-amd64 
> CONFIG_PREEMPT_DYNAMIC=y
> CONFIG_HAVE_PREEMPT_DYNAMIC=y
> CONFIG_HAVE_PREEMPT_DYNAMIC_CALL=y

[...]

> $ grep PREEMPT_DYNAMIC /boot/config-6.1.0-13-arm64 
> # CONFIG_PREEMPT_DYNAMIC is not set
> CONFIG_HAVE_PREEMPT_DYNAMIC=y
> CONFIG_HAVE_PREEMPT_DYNAMIC_KEY=y

CONFIG_PREEMPT_DYNAMIC is set to 'y' by default on amd64 due to
HAVE_PREEMPT_DYNAMIC_CALL being 'y', see:
https://sources.debian.org/src/linux/6.5.6-1/kernel/Kconfig.preempt/?hl=101#L101

arm64 does not have PREEMPT_DYNAMIC_CALL, this is why PREEMPT_DYNAMIC is
not set by default there. However, there's another feature which is
functionally equivalent, PREEMPT_DYNAMIC_KEY. While _CALL and _KEY are
equivalent, in theory _KEY may have more overhead. See:
https://lore.kernel.org/lkml/20220214165216.2231574-1-mark.rutland@arm.com/#t

SUSE and Fedora both seem to have CONFIG_HAVE_PREEMPT_DYNAMIC=y on
arm64, see:
https://github.com/SUSE/kernel-source/blob/SLE15-SP6/config/arm64/default#L123
https://src.fedoraproject.org/rpms/kernel/blob/f34/f/kernel.spec#_4458

Setting CONFIG_HAVE_PREEMPT_DYNAMIC=y on arm64 is likely safe for us
too, but we may want to run some benchmark first to see if there are any
noticeable slowdowns.

Thread

CONFIG_PREEMPT_DYNAMIC=y? Johannes Schauer Marin Rodrigues <josch@debian.org> - 2023-10-08 18:30 +0200
  Re: CONFIG_PREEMPT_DYNAMIC=y? Diederik de Haas <didi.debian@cknow.org> - 2023-10-08 19:30 +0200
    Re: CONFIG_PREEMPT_DYNAMIC=y? Emanuele Rocca <ema@debian.org> - 2023-10-10 12:10 +0200
      Re: CONFIG_PREEMPT_DYNAMIC=y? Johannes Schauer Marin Rodrigues <josch@debian.org> - 2023-10-10 12:20 +0200
        Re: CONFIG_PREEMPT_DYNAMIC=y? Emanuele Rocca <ema@debian.org> - 2023-10-10 12:40 +0200
          Re: CONFIG_PREEMPT_DYNAMIC=y? Marc Haber <mh+debian-kernel@zugschlus.de> - 2023-10-10 15:50 +0200
      Re: CONFIG_PREEMPT_DYNAMIC=y? Diederik de Haas <didi.debian@cknow.org> - 2023-10-10 14:00 +0200
        Re: CONFIG_PREEMPT_DYNAMIC=y? Vincent Blut <vincent.debian@free.fr> - 2023-10-10 14:20 +0200
          Re: CONFIG_PREEMPT_DYNAMIC=y? Diederik de Haas <didi.debian@cknow.org> - 2023-10-10 15:20 +0200
        Re: CONFIG_PREEMPT_DYNAMIC=y? Emanuele Rocca <ema@debian.org> - 2023-10-10 15:00 +0200
          Re: CONFIG_PREEMPT_DYNAMIC=y? Diederik de Haas <didi.debian@cknow.org> - 2023-10-10 23:30 +0200
            Re: CONFIG_PREEMPT_DYNAMIC=y? Diederik de Haas <didi.debian@cknow.org> - 2023-10-17 20:00 +0200
  Re: CONFIG_PREEMPT_DYNAMIC=y? Emanuele Rocca <ema@debian.org> - 2023-10-20 20:40 +0200

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