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

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Message-ID: <ZXJKAnrRjBUmKx1V@slm.duckdns.org>
Date:   Thu, 7 Dec 2023 12:41:06 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Gianfranco Dutka <gianfranco.dutka@...sta.com>,
        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>,
        "Pandruvada, Srinivas" <srinivas.pandruvada@...el.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Waiman Long <longman@...hat.com>
Subject: Re: Modifying isolcpus, nohz_full, and rcu_nocb kernel parameters at
 runtime

(cc'ing Waiman)

On Thu, Dec 07, 2023 at 05:32:15PM +0100, Vincent Guittot wrote:
> 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:
...
> > 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.

FWIW, Waiman has been improving both the usability and level of isolation
with cpuset, so it should be better now.

Thanks.

-- 
tejun