linux-kernel - Re: [PATCH v9 2/7] cgroup/cpuset: Allow no-task partition to have empty cpuset.cpus.effective

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Date:   Tue, 14 Dec 2021 22:24:22 -0500
From:   Waiman Long <longman@...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>,
        Frederic Weisbecker <frederic@...nel.org>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Michal Koutný <mkoutny@...e.com>
Subject: Re: [PATCH v9 2/7] cgroup/cpuset: Allow no-task partition to have
 empty cpuset.cpus.effective

On 12/13/21 15:45, Tejun Heo wrote:
> On Sun, Dec 05, 2021 at 01:32:15PM -0500, Waiman Long wrote:
>>   	adding = deleting = false;
>>   	old_prs = new_prs = cpuset->partition_root_state;
>>   	if (cmd == partcmd_enable) {
>> +		/*
>> +		 * Enabling partition root is not allowed if not all the CPUs
>> +		 * can be granted from parent's effective_cpus.
>> +		 */
>> +		if (!cpumask_subset(cpuset->cpus_allowed, parent->effective_cpus))
>> +			return -EINVAL;
>> +
>> +		/*
>> +		 * A parent can be left with no CPU as long as there is no
>> +		 * task directly associated with the parent partition. For
>> +		 * such a parent, no new task can be moved into it.
>> +		 */
>> +		if (partition_is_populated(parent, cpuset) &&
>> +		    cpumask_equal(cpuset->cpus_allowed, parent->effective_cpus))
>> +			return -EINVAL;
> So, given that this only happens with threaded domains, can we just not
> allow partitions within threaded domains? The combination doesn't make whole
> lot of sense to me anyway.
AFAICS, there are code in cpuset.c that disallows the an non-child node 
to hold tasks, but the check doesn't cover all the possible cases. I 
remembered that I was able to create such a scenario without using 
threaded domains. That is why I put in this conditional check. It has 
nothing to do with the use of threaded domains.
>> +	/*
>> +	 * 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;
> And then we can avoid this extra restriction too, right?

This check is supposed to prevent a task to be moved to a leaf cpuset 
partition with just offlined cpus and hence no effective cpu. A possible 
alternative is to force the partition to become invalid, but I think not 
allowing the move is easier until one or more offlined cpus are onlined.

Cheers,
Longman