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

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Message-ID: <4cddddc3-699b-44d6-97a4-9964ca2e60b2@redhat.com>
Date:   Fri, 8 Dec 2023 10:39:50 -0500
From:   Waiman Long <longman@...hat.com>
To:     Gianfranco Dutka <gianfranco.dutka@...sta.com>
Cc:     Tejun Heo <tj@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        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>
Subject: Re: Modifying isolcpus, nohz_full, and rcu_nocb kernel parameters at
 runtime


On 12/8/23 09:18, Gianfranco Dutka wrote:
>
>> The isolcpus, nohz_full and rcu_nocbs are boot-time kernel 
>> parameters. I am in the process of improving dynamic CPU isolation at 
>> runtime. Right now, we are able to do isolcpus=domain with the 
>> isolated cpuset partition functionality. Other aspects of CPU 
>> isolation are being looked at with the goal of reducing the gap of 
>> what one can do at boot time versus what can be done at run time. It 
>> will certain take time to reach that goal.
>>
>> Cheers, Longman
>>
>
> Thank you Waiman for the response. It would seem that getting similar 
> functionality through cgroups/cpusets is the only option at the 
> moment. Is it completely out of the question to possibly patch the 
> kernel to modify these parameters at runtime? Or would that entail a 
> significant change that might not be so trivial to accomplish? For 
> instance, the solution wouldn’t be as simple as patching the kernel to 
> make these writeable and then calling the same functions which run at 
> boot-time when these parameters are originally written?

I would say that using cgroup/cpusets is probably the most you can do 
with dynamic CPU isolation at the moment. OTOH, it may not be a good 
idea to have more than one way of doing the same thing as it will lead 
to code duplication and inconsistency. I don't think it is that easy to 
make CPU isolation fully dynamic and we must be careful not to introduce 
any regression.

Cheers,
Longman