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From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Thu, 7 Dec 2023 17:32:15 +0100
Message-ID: <CAKfTPtAkhfAhFouCGTy7m4swCeeEsu1VdWEX_ahOVDq1U594Dg@mail.gmail.com>
Subject: Re: Modifying isolcpus, nohz_full, and rcu_nocb kernel parameters at runtime
To:     Gianfranco Dutka <gianfranco.dutka@arista.com>
Cc:     linux-kernel@vger.kernel.org, vincent.guittot@linaro.com,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>,
        Tejun Heo <tj@kernel.org>,
        "Pandruvada, Srinivas" <srinivas.pandruvada@intel.com>,
        Frederic Weisbecker <frederic@kernel.org>
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Hi Gianfranco,

+ Adding all scheduler maintainers and few other people that are
working on similar things

On Thu, 7 Dec 2023 at 16:07, Gianfranco Dutka
<gianfranco.dutka@arista.com> wrote:
>
> Good afternoon maintainers and subscribers to the lkml,
>
> I'm a bit new to kernel development but I had a question with respect to =
the kernel parameters: isolcpus, nohz_full, and rcu_nocbs.
>
> Basically the question is this, am I able to modify the three parameters =
I mentioned above at runtime after the kernel has already started/booted? D=
oing some reading online it seems that it=E2=80=99s not possible but I want=
ed to double check with the maintainers if there wasn=E2=80=99t some sort o=
f change in the works that might make it possible. If not, what would be re=
quired to make the change after boot-time through some kind of patch or som=
ething like that? Would that be something that might be valuable upstream?

It's not possible but you can achieve something close with cgroup
although you will still have some housekeeping activities happening in
your partition.

This thread tries to do something similar:
https://lore.kernel.org/lkml/20230804090858.7605-1-rui.zhang@intel.com/


>
> At the moment we are running an application that might see some benefit f=
rom being able to isolate cpus on the fly without having to reboot everythi=
ng every time we want to modify the parameters mentioned above.
>
> Thanks,
> Gianfranco