Re: CONFIG_PREEMPT_DYNAMIC=y?

From	Emanuele Rocca <ema@debian.org>
Newsgroups	linux.debian.kernel
Subject	Re: CONFIG_PREEMPT_DYNAMIC=y?
Date	2023-10-10 15:00 +0200
Message-ID	<Hnkv7-eS5C-3@gated-at.bofh.it> (permalink)
References	<HmEPf-eqdx-7@gated-at.bofh.it> <HmFLj-eqM9-7@gated-at.bofh.it> <HnhQB-eQol-7@gated-at.bofh.it> <Hnjz3-eRsk-1@gated-at.bofh.it>
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Hi Diederik,

On 2023-10-10 01:54, Diederik de Haas wrote:
> On Tuesday, 10 October 2023 12:10:07 CEST Emanuele Rocca wrote:
> > 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.
> 
> Neither does amd64. So it appears something else is causing 
> CONFIG_PREEMPT_DYNAMIC to be enabled on amd64.

Here is where HAVE_PREEMPT_DYNAMIC_CALL is set on x86:
https://sources.debian.org/src/linux/6.5.6-1/arch/x86/Kconfig/?hl=271#L271

> I think it should be explicitly enabled (or disabled) and not be it dependent
> on some other, possibly unrelated, Kconfig option being enabled

Right, but HAVE_PREEMPT_DYNAMIC_CALL is not a config option. It's a
setting that defines whether the architecture can use inline static
calls to handle the preemption model passed to the kernel command line
with preempt=

To my understanding, only x86 can do that. From the PREEMPT_DYNAMIC
Kconfig help:

 The runtime overhead is negligible with HAVE_STATIC_CALL_INLINE enabled
 but if runtime patching is not available for the specific architecture
 then the potential overhead should be considered.

> ad 1) I _think_ running benchmarks is not that useful as its result will/should
> vary based on the default preemption model and whether that is overridden
> on the kernel command line

Sorry, I wasn't clear in my previous message about what we should be
benchmarking. The current Debian kernel on arm64 has PREEMPT_VOLUNTARY=y
and PREEMPT_DYNAMIC not set. We want to run benchmarks to see if a
kernel with PREEMPT_DYNAMIC=y booted with preempt=voluntary has any
significant slowdowns compared to the current kernels. We don't want to
benchmark the various preemption models.

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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