linux-kernel - Re: Modifying isolcpus, nohz_full, and rcu

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

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@...sta.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? Doing some reading online it seems that it’s not possible but I wanted to double check with the maintainers if there wasn’t some sort of change in the works that might make it possible. If not, what would be required to make the change after boot-time through some kind of patch or something 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 from being able to isolate cpus on the fly without having to reboot everything every time we want to modify the parameters mentioned above.
>
> Thanks,
> Gianfranco