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Subject: Re: [PATCH v8 3/6] cpuset: Add cpuset.sched.load_balance flag to v2
To:     Juri Lelli <juri.lelli@redhat.com>
Cc:     Tejun Heo <tj@kernel.org>, Li Zefan <lizefan@huawei.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        kernel-team@fb.com, pjt@google.com, luto@amacapital.net,
        Mike Galbraith <efault@gmx.de>, torvalds@linux-foundation.org,
        Roman Gushchin <guro@fb.com>
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From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <5f409ed7-3850-f1ea-58cf-4326605d1570@redhat.com>
Date:   Thu, 24 May 2018 11:22:13 -0400
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On 05/24/2018 11:16 AM, Juri Lelli wrote:
> On 24/05/18 11:09, Waiman Long wrote:
>> On 05/24/2018 10:36 AM, Juri Lelli wrote:
>>> On 17/05/18 16:55, Waiman Long wrote:
>>>
>>> [...]
>>>
>>>> +	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.
>>> [...]
>>>
>>>> +	/*
>>>> +	 * On default hierachy, a load balance flag change is only allowed
>>>> +	 * in a scheduling domain with no child cpuset.
>>>> +	 */
>>>> +	if (cgroup_subsys_on_dfl(cpuset_cgrp_subsys) && balance_flag_changed &&
>>>> +	   (!is_sched_domain(cs) || css_has_online_children(&cs->css))) {
>>>> +		err = -EINVAL;
>>>> +		goto out;
>>>> +	}
>>> The rule is actually
>>>
>>>  - no child cpuset
>>>  - and it must be a scheduling domain
>>>
>>> Right?
>> Yes, because it doesn't make sense to have a cpu in one cpuset that has
>> loading balance off while, at the same time, in another cpuset with load
>> balancing turned on. This restriction is there to make sure that the
>> above condition will not happen. I may be wrong if there is a realistic
>> use case where the above condition is desired.
> Yep, makes sense to me.
>
> Maybe add the second condition to the comment and documentation.

Sure. Will do.

-Longman