linux-kernel - Re: [PATCH v7 2/5] cpuset: Add cpuset.sched_load

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Date:   Wed, 2 May 2018 09:47:00 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 v7 2/5] cpuset: Add cpuset.sched_load_balance to v2

On 05/02/2018 09:42 AM, Peter Zijlstra wrote:
> On Wed, May 02, 2018 at 09:29:54AM -0400, Waiman Long wrote:
>> On 05/02/2018 06:24 AM, Peter Zijlstra wrote:
>>> On Thu, Apr 19, 2018 at 09:47:01AM -0400, Waiman Long wrote:
>>>> +  cpuset.sched_load_balance
>>>> +	A read-write single value file which exists on non-root cgroups.
>>> Uhhm.. it should very much exist in the root group too. Otherwise you
>>> cannot disable it there, which is required to allow smaller groups to
>>> load-balance between themselves.
>>>
>>>> +	The default is "1" (on), and the other possible value is "0"
>>>> +	(off).
>>>> +
>>>> +	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.
>>>> +
>>>> +	This flag is hierarchical and is inherited by child cpusets. It
>>>> +	can be turned off only when the CPUs in this cpuset aren't
>>>> +	listed in the cpuset.cpus of other sibling cgroups, and all
>>>> +	the child cpusets, if present, have this flag turned off.
>>>> +
>>>> +	Once it is off, it cannot be turned back on as long as the
>>>> +	parent cgroup still has this flag in the off state.
>>> That too is wrong and broken. You explicitly want to turn it on for
>>> children.
>>>
>>> So the idea is that you can have:
>>>
>>> 		R
>>> 	      /   \
>>>             A       B
>>>
>>> With:
>>>
>>> 	R cpus=0-3, load_balance=0
>>> 	A cpus=0-1, load_balance=1
>>> 	B cpus=2-3, load_balance=1
>>>
>>> Which will allow all tasks in A,B (and its children) to load-balance
>>> across 0-1 or 2-3 resp.
>>>
>>> If you don't allow the root group to disable load_balance, it will
>>> always be the largest group and load-balancing will always happen system
>>> wide.
>> If you look at the remaining patches in the series, I was proposing a
>> different way to support isolcpus and separate sched domains with
>> turning off load balancing in the root cgroup.
>>
>> For me, it doesn't feel right to have load balancing disabled in the
>> root cgroup as we probably cannot move all the tasks away from the root
>> cgroup anyway. I am going to update the current patchset to incorporate
>> suggestion from Tejun. It will probably be ready sometime next week.
>>
> I've read half of the next patch that adds the isolation thing. And
> while that kludges around the whole root cgorup is magic thing, it
> doesn't help if you move the above scenario on level down:
>
>
> 	R
>      /    \
>    A        B
>           /   \
>         C       D
>
>
> R: cpus=0-7, load_balance=0
> A: cpus=0-1, load_balance=1
> B: cpus=2-7, load_balance=0
> C: cpus=2-3, load_balance=1
> D: cpus=4-7, load_balance=1
>
>
> Also, I feel we should strive to have a minimal amount of tasks that
> cannot be moved out of the root group; the current set is far too large.

What exactly is the use case you have in mind with loading balancing
disabled in B, but enabled in C and D? We would like to support some
sensible use cases, but not every possible combinations.

Cheers,
Longman