linux-kernel - Re: [PATCH v8 2/6] cpuset: Add new v2 cpuset.sched.domain flag

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Date:   Thu, 24 May 2018 14:53:31 -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 v8 2/6] cpuset: Add new v2 cpuset.sched.domain flag

On 05/24/2018 11:41 AM, Peter Zijlstra wrote:
> On Thu, May 17, 2018 at 04:55:41PM -0400, Waiman Long wrote:
>> A new cpuset.sched.domain boolean flag is added to cpuset v2. This new
>> flag indicates that the CPUs in the current cpuset should be treated
>> as a separate scheduling domain.
> The traditional name for this is a partition.

Do you want to call it cpuset.sched.partition? That name sounds strange
to me.

>>                                  This new flag is owned by the parent
>> and will cause the CPUs in the cpuset to be removed from the effective
>> CPUs of its parent.
> This is a significant departure from existing behaviour, but one I can
> appreciate. I don't immediately see something terribly wrong with it.
>
>> This is implemented internally by adding a new isolated_cpus mask that
>> holds the CPUs belonging to child scheduling domain cpusets so that:
>>
>> 	isolated_cpus | effective_cpus = cpus_allowed
>> 	isolated_cpus & effective_cpus = 0
>>
>> This new flag can only be turned on in a cpuset if its parent is either
>> root or a scheduling domain itself with non-empty cpu list. The state
>> of this flag cannot be changed if the cpuset has children.
>>
>> Signed-off-by: Waiman Long <longman@...hat.com>
>> ---
>>  Documentation/cgroup-v2.txt |  22 ++++
>>  kernel/cgroup/cpuset.c      | 237 +++++++++++++++++++++++++++++++++++++++++++-
>>  2 files changed, 256 insertions(+), 3 deletions(-)
>>
>> diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt
>> index cf7bac6..54d9e22 100644
>> --- a/Documentation/cgroup-v2.txt
>> +++ b/Documentation/cgroup-v2.txt
>> @@ -1514,6 +1514,28 @@ Cpuset Interface Files
>>  	it is a subset of "cpuset.mems".  Its value will be affected
>>  	by memory nodes hotplug events.
>>  
>> +  cpuset.sched.domain
>> +	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).
> I would be conservative and only allow 0/1.

I stated that because echoing other integer value like 2 into the flag
file won't return any error. I will modify it to say just 0 and 1.

>>                                                  This flag is set
>> +	by the parent and is not delegatable.
>> +
>> +	If set, it indicates that the CPUs in the current cgroup will
>> +	be the root of a scheduling domain.  The root cgroup is always
>> +	a scheduling domain.  There are constraints on where this flag
>> +	can be set.  It can only be set in a cgroup if all the following
>> +	conditions are true.
>> +
>> +	1) The parent cgroup is also a scheduling domain with a non-empty
>> +	   cpu list.
> Ah, so initially I was confused by the requirement for root to have it
> always set, but you'll allow child domains to steal _all_ CPUs, such
> that root ends up with an empty effective set?
>
> What about the (kernel) threads that cannot be moved out of the root
> group?

Actually, the current code won't allow you to take all the CPUs from a
scheduling domain cpuset with load balancing on. So there must be at
least 1 cpu left. You can take all away if load balancing is off.

>> +	2) The list of CPUs are exclusive, i.e. they are not shared by
>> +	   any of its siblings.
> Right.
>
>> +	3) There is no child cgroups with cpuset enabled.
>> +
>> +	Setting this flag will take the CPUs away from the effective
>> +	CPUs of the parent cgroup. Once it is set, this flag cannot be
>> +	cleared if there are any child cgroups with cpuset enabled.
> This I'm not clear on. Why?
>
That is for pragmatic reason as it is easier to code this way. We could
remove this restriction but that will make the code more complex.

Cheers,
Longman