linux-kernel - Re: [PATCH 3/5] cgroup/cpuset: Allow non-top parent partition root to distribute out all CPUs

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Date:   Thu, 17 Jun 2021 16:45:35 -0400
From:   Waiman Long <llong@...hat.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Shuah Khan <shuah@...nel.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>, Phil Auld <pauld@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>
Subject: Re: [PATCH 3/5] cgroup/cpuset: Allow non-top parent partition root to
 distribute out all CPUs

On 6/16/21 4:57 PM, Tejun Heo wrote:
> Hello,
>
> On Thu, Jun 03, 2021 at 05:24:14PM -0400, Waiman Long wrote:
>> @@ -2181,6 +2192,13 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
>>   	    (cpumask_empty(cs->cpus_allowed) || nodes_empty(cs->mems_allowed)))
>>   		goto out_unlock;
>>   
>> +	/*
>> +	 * On default hierarchy, task cannot be moved to a cpuset with empty
>> +	 * effective cpus.
>> +	 */
>> +	if (is_in_v2_mode() && cpumask_empty(cs->effective_cpus))
>> +		goto out_unlock;
>> +
> This is inconsistent with how other events which leave a root partition
> empty is handled. Woudln't it be more consistent to switch the parent to
> PRS_ERROR and behave accordingly but allow it to have valid child roots?

 From my point of view, PRS_ERROR is used when cpus are gone because of 
cpu hotplug (offline). It can be a temporary condition that will be 
corrected later on. I don't want to use PRS_ERROR for the particular 
case that the users have explicitly distributed out all the cpus to 
child partitions. I will clarify it in the next version and double check 
to make sure that this rule is consistently apply.

Thanks,
Longman