Re: CONFIG_PREEMPT_DYNAMIC=y?

From	Emanuele Rocca <ema@debian.org>
Newsgroups	linux.debian.kernel
Subject	Re: CONFIG_PREEMPT_DYNAMIC=y?
Date	2023-10-20 20:40 +0200
Message-ID	<Hr2zD-heZn-9@gated-at.bofh.it> (permalink)
References	<HmEPf-eqdx-7@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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Hi,

On 2023-10-08 06:20, Johannes Schauer Marin Rodrigues wrote:
> I'm wondering whether it'd make sense for the Debian kernel to be built with
> CONFIG_PREEMPT_DYNAMIC=y.

On amd64 we already have PREEMPT_DYNAMIC=y, so I thought of running some
benchmarks on my Ryzen system to evaluate the performance overhead it
introduces.

To my surprise, I found that in Debian you get a 20% (!) performance
improvement on kernel-heavy workloads simply by setting PREEMPT_DYNAMIC
to 'n'. I brought up the topic on LKML [1], and it turns out that what
is really eating performance for us is DEBUG_PREEMPT=y. There is a
performance penalty in PREEMPT_DYNAMIC=y as well, but it's much smaller,
around 3 or 4%.

The Kconfig help entry for DEBUG_PREEMPT also clearly mentions the
performance overhead:

 This option has potential to introduce high runtime overhead,
 depending on workload as it triggers debugging routines for each
 this_cpu operation. It should only be used for debugging purposes.

Regardless of what we are going to decide about PREEMPT_DYNAMIC, we
surely have to disable DEBUG_PREEMPT. I opened [2] for that.

[1] https://lore.kernel.org/lkml/ZTJFA_Ac6nWawIHb@ariel/
[2] https://salsa.debian.org/kernel-team/linux/-/merge_requests/874

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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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