linux-kernel - Re: [PATCH v12 9/9] cpuset: Support forced turning off of partition flag

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Message-ID: <be374350-df64-5685-2802-629d43b0b284@redhat.com>
Date:   Mon, 24 Sep 2018 11:47:11 -0400
From:   Waiman Long <longman@...hat.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Peter Zijlstra <peterz@...radead.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>,
        Patrick Bellasi <patrick.bellasi@....com>
Subject: Re: [PATCH v12 9/9] cpuset: Support forced turning off of partition
 flag

On 09/06/2018 05:20 PM, Waiman Long wrote:
> On 08/27/2018 01:50 PM, Waiman Long wrote:
>> On 08/27/2018 12:40 PM, Tejun Heo wrote:
>>> Hello, Waiman.
>>>
>>> On Mon, Aug 27, 2018 at 10:41:24AM -0400, Waiman Long wrote:
>>>> Cpuset allows arbitrary modification of cpu list in "cpuset.cpus"
>>>> even if the requested CPUs won't be granted in "cpuset.cpus.effective"
>>>> as restricted by its parent. However, the validate_change() function
>>>> will inhibit removal of CPUs that have been used in child cpusets.
>>>>
>>>> Being a partition root, however, limits the kind of cpu list
>>>> modification that is allowed. Adding CPUs is not allowed if the new
>>>> CPUs are not in the parent's effective cpu list that can be put into
>>>> "cpuset.cpus.reserved". In addition, a child partition cannot exhaust
>>>> all the parent's effective CPUs.
>>>>
>>>> Because of the implicit cpu exclusive nature of the partition root,
>>>> cpu changes that break that cpu exclusivity will not be allowed. Other
>>>> changes that break the conditions of being a partition root is generally
>>>> allowed. However, the partition flag of the cpuset as well those of
>>>> the descendant partitions will be forcefully turned off.
>>> First of all, thanks a lot for your persistence.  I'm not necessarily
>>> against the flag being forced off but wonder whether the same
>>> config/effective approach can be used as in .cpus and .mems.  Can you
>>> elaborate a bit on the choice here?
>>>
>>> Thank you.
>> My current code has explicitly assumed the following relationship for
>> partition root.
>>
>>     cpus_allowed = effective_cpus + reserved_cpus
>>
>> Also effective_cpus cannot be empty. Specifically, cpus_allowed has to
>> be equal to effective_cpus before a cpuset can be made a partition root.
>>
>> Any changes that break the above conditions will turn off the partition
>> flag forcefully. The only exception is cpu offlining where cpus_allowed
>>> effective_cpus + reserved_cpus can happen.
>> One reason for doing so is because reserved_cpus is hidden. So the main
>> way to infer that is to do cpus_allowed - effective_cpus.
>>
>> It is probably doable to make cpus_allowed >= effective_cpus +
>> reserved_cpus in general, but we may need to expose reserved_cpus as a
>> read-only file, for instance. There may also be other complications that
>> we will need to take care of if this is supported. My current preference
>> is to not doing that unless there is compelling reason to do so.
>>
>> Cheers,
>> Longman
> For the current patch, one of conditions to allow users to turn on the
> partition flag is cpus_allowed = effective_cpus. Once the partition flag
> is on, the twos have to be the same too except with cpu offlining. This
> patch is added to allow arbitrary modification to the cpuset.cpus file
> except those that have already been forbidden in the existing code. When
> the modification violates the rules of being a partition root, the flag
> will be turned off or we will have to deny the modification request.
>
> I am admitting that it is an easy way out in term of programming effort
> as I am not sure how much effort is needed to enable arbitrary
> modification to cpuset.cpus without force disabling and whether such an
> effort is even worthwhile to do.
>
> We can certainly relax the current restrictions in the future.  Any
> suggestion to improve the patchset is welcome. I would really like to
> see this merged in 4.20 (or maybe 5.0) kernel.
>
> Cheers,
> Longman
>
Tejun,

Sorry for bothering you again.

I would like to know your assessment of this patchset as to whether it
is ready or not.

Thanks,
Longman