linux-kernel - Re: [RFC PATCH 4/4] cpuset: Support RCU-NOCB toggle on v2 root partitions

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Date:   Thu, 26 May 2022 08:21:13 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Phil Auld <pauld@...hat.com>,
        Zefan Li <lizefan.x@...edance.com>,
        Waiman Long <longman@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...nel.org>,
        Nicolas Saenz Julienne <nsaenz@...nel.org>,
        rcu@...r.kernel.org
Subject: Re: [RFC PATCH 4/4] cpuset: Support RCU-NOCB toggle on v2 root
 partitions

On Thu, May 26, 2022 at 12:10:55AM +0200, Frederic Weisbecker wrote:
> Introduce a new "isolation.rcu_nocb" file within a cgroup2/cpuset
> directory which provides support for a set of CPUs to either enable ("1")
> or disable ("0") RCU callbacks offloading (aka. RCU NOCB). This can
> overwrite previous boot settings towards "rcu_nocbs=" kernel parameter.
> 
> The file is only writeable on "root" type partitions to exclude any
> overlap. The deepest root type partition has the highest priority.
> This means that given the following setting:
> 
>                     Top cpuset (CPUs: 0-7)
>                     cpuset.isolation.rcu_nocb = 0
>                               |
>                               |
>                     Subdirectory A (CPUs: 5-7)
>                     cpuset.cpus.partition = root
>                     cpuset.isolation.rcu_nocb = 0
>                               |
>                               |
>                     Subdirectory B (CPUs: 7)
>                     cpuset.cpus.partition = root
>                     cpuset.isolation.rcu_nocb = 1
> 
> the result is that only CPU 7 is in rcu_nocb mode.
> 
> Note that "rcu_nocbs" kernel parameter must be passed on boot, even
> without a cpulist, so that nocb support is enabled.

Does it even make sense to make this hierarchical? What's wrong with a
cpumask under sys/ or proc/?

Thanks.

-- 
tejun