linux-kernel - Re: [RFC PATCH 6/6] cpuset: Add cpuset.isolation

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Date:   Wed, 14 Jul 2021 18:52:43 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Alex Belits <abelits@...vell.com>,
        Nitesh Lal <nilal@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Nicolas Saenz <nsaenzju@...hat.com>,
        Christoph Lameter <cl@...two.de>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Zefan Li <lizefan.x@...edance.com>, cgroups@...r.kernel.org
Subject: Re: [RFC PATCH 6/6] cpuset: Add cpuset.isolation_mask file

On Wed, Jul 14, 2021 at 03:54:20PM +0200, Frederic Weisbecker wrote:
> Add a new cpuset.isolation_mask file in order to be able to modify the
> housekeeping cpumask for each individual isolation feature on runtime.
> In the future this will include nohz_full, unbound timers,
> unbound workqueues, unbound kthreads, managed irqs, etc...
> 
> Start with supporting domain exclusion and CPUs passed through
> "isolcpus=".
> 
> The cpuset.isolation_mask defaults to 0. Setting it to 1 will exclude
> the given cpuset from the domains (they will be attached to NULL domain).
> As long as a CPU is part of any cpuset with cpuset.isolation_mask set to
> 1, it will remain isolated even if it overlaps with another cpuset that
> has cpuset.isolation_mask  set to 0. The same applies to parent and
> subdirectories.
> 
> If a cpuset is a subset of "isolcpus=", it automatically maps it and
> cpuset.isolation_mask will be set to 1. This subset is then cleared from
> the initial "isolcpus=" mask. The user is then free to override
> cpuset.isolation_mask to 0 in order to revert the effect of "isolcpus=".
> 
> Here is an example of use where the CPU 7 has been isolated on boot and
> get re-attached to domains later from cpuset:
> 
> 	$ cat /proc/cmdline
> 		isolcpus=7
> 	$ cd /sys/fs/cgroup/cpuset
> 	$ mkdir cpu7
> 	$ cd cpu7
> 	$ cat cpuset.cpus
> 		0-7
> 	$ cat cpuset.isolation_mask
> 		0
> 	$ ls /sys/kernel/debug/domains/cpu7	# empty because isolcpus=7
> 	$ echo 7 > cpuset.cpus
> 	$ cat cpuset.isolation_mask	# isolcpus subset automatically mapped
> 		1
> 	$ echo 0 > cpuset.isolation_mask
> 	$ ls /sys/kernel/debug/domains/cpu7/
> 		domain0  domain1
> 

cpusets already has means to create paritions; why are you creating
something else?