linux-kernel - Re: [PATCH v8 3/6] cpuset: Add cpuset.sched.load

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Message-ID: <20180524154341.GJ12198@hirez.programming.kicks-ass.net>
Date:   Thu, 24 May 2018 17:43:41 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        kernel-team@...com, pjt@...gle.com, luto@...capital.net,
        Mike Galbraith <efault@....de>, torvalds@...ux-foundation.org,
        Roman Gushchin <guro@...com>,
        Juri Lelli <juri.lelli@...hat.com>
Subject: Re: [PATCH v8 3/6] cpuset: Add cpuset.sched.load_balance flag to v2

On Thu, May 17, 2018 at 04:55:42PM -0400, Waiman Long wrote:
> The sched.load_balance flag is needed to enable CPU isolation similar to
> what can be done with the "isolcpus" kernel boot parameter. Its value
> can only be changed in a scheduling domain with no child cpusets. On
> a non-scheduling domain cpuset, the value of sched.load_balance is
> inherited from its parent.
> 
> This flag is set by the parent and is not delegatable.
> 
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
>  Documentation/cgroup-v2.txt | 24 ++++++++++++++++++++
>  kernel/cgroup/cpuset.c      | 53 +++++++++++++++++++++++++++++++++++++++++----
>  2 files changed, 73 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt
> index 54d9e22..071b634d 100644
> --- a/Documentation/cgroup-v2.txt
> +++ b/Documentation/cgroup-v2.txt
> @@ -1536,6 +1536,30 @@ Cpuset Interface Files
>  	CPUs of the parent cgroup. Once it is set, this flag cannot be
>  	cleared if there are any child cgroups with cpuset enabled.
>  
> +	A parent cgroup cannot distribute all its CPUs to child
> +	scheduling domain cgroups unless its load balancing flag is
> +	turned off.
> +
> +  cpuset.sched.load_balance
> +	A read-write single value file which exists on non-root
> +	cpuset-enabled cgroups.  It is a binary value flag that accepts
> +	either "0" (off) or a non-zero value (on).  This flag is set
> +	by the parent and is not delegatable.
> +
> +	When it is on, tasks within this cpuset will be load-balanced
> +	by the kernel scheduler.  Tasks will be moved from CPUs with
> +	high load to other CPUs within the same cpuset with less load
> +	periodically.
> +
> +	When it is off, there will be no load balancing among CPUs on
> +	this cgroup.  Tasks will stay in the CPUs they are running on
> +	and will not be moved to other CPUs.
> +
> +	The initial value of this flag is "1".	This flag is then
> +	inherited by child cgroups with cpuset enabled.  Its state
> +	can only be changed on a scheduling domain cgroup with no
> +	cpuset-enabled children.

I'm confused... why exactly do we have both domain and load_balance ?