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

[Tejun Heo <tj@kernel.org>]

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