Date:   Tue, 3 May 2022 14:39:23 -0400
From:   Waiman Long <longman@...hat.com>
To:     Phil Auld <pauld@...hat.com>
Cc:     Tejun Heo <tj@...nel.org>, 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>,
        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 v10 3/8] cgroup/cpuset: Allow no-task partition to have
 empty cpuset.cpus.effective

On 5/3/22 13:54, Phil Auld wrote:
> Hi Waiman
>
> On Tue, May 03, 2022 at 12:21:44PM -0400 Waiman Long wrote:
>> Currently, a partition root cannot have empty "cpuset.cpus.effective".
>> As a result, a parent partition root cannot distribute out all its CPUs
>>   to child partitions with no CPUs left. However in most cases, there
>> shouldn't be any tasks associated with intermediate nodes of the default
>>   hierarchy. So the current rule is too restrictive and can waste valuable
>>   CPU resource.
>>
>> To address this issue, we are now allowing a partition to have empty
>> "cpuset.cpus.effective" as long as it has no task. Therefore, a parent
>> partition with no task can now have all its CPUs distributed out to its
>> child partitions. The top cpuset always have some house-keeping tasks
>> running and so its list of effective cpu can't never be empty.
> s/never/ever/
It is a double negative. I think I will just remove "never".
>> Once a partition with empty "cpuset.cpus.effective" is formed, no
>> new task can be moved into it until "cpuset.cpus.effective" becomes
>> non-empty.
>>
>> Signed-off-by: Waiman Long <longman@...hat.com>
>> ---
>>   kernel/cgroup/cpuset.c | 111 +++++++++++++++++++++++++++++++----------
>>   1 file changed, 84 insertions(+), 27 deletions(-)
>>
>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>> index d156a39d7a08..7d9abd50a1b9 100644
>> --- a/kernel/cgroup/cpuset.c
>> +++ b/kernel/cgroup/cpuset.c
>> @@ -412,6 +412,41 @@ static inline bool is_in_v2_mode(void)
>>   	      (cpuset_cgrp_subsys.root->flags & CGRP_ROOT_CPUSET_V2_MODE);
>>   }
>>   
>> +/**
>> + * partition_is_populated - check if partition has tasks
>> + * @cs: partition root to be checked
>> + * @excluded_child: a child cpuset to be excluded in task checking
>> + * Return: true if there are tasks, false otherwise
>> + *
>> + * It is assumed that @cs is a valid partition root. @excluded_child should
>> + * be non-NULL when this cpuset is going to become a partition itself.
>> + */
>> +static inline bool partition_is_populated(struct cpuset *cs,
>> +					  struct cpuset *excluded_child)
>> +{
>> +	struct cgroup_subsys_state *css;
>> +	struct cpuset *child;
>> +
>> +	if (cs->css.cgroup->nr_populated_csets)
>> +		return true;
>> +	if (!excluded_child && !cs->nr_subparts_cpus)
>> +		return cgroup_is_populated(cs->css.cgroup);
>> +
>> +	rcu_read_lock();
>> +	cpuset_for_each_child(child, css, cs) {
>> +		if (child == excluded_child)
>> +			continue;
>> +		if (is_partition_valid(child))
>> +			continue;
>> +		if (cgroup_is_populated(child->css.cgroup)) {
>> +			rcu_read_unlock();
>> +			return true;
>> +		}
>> +	}
>> +	rcu_read_unlock();
>> +	return false;
>> +}
>> +
>>   /*
>>    * Return in pmask the portion of a task's cpusets's cpus_allowed that
>>    * are online and are capable of running the task.  If none are found,
>> @@ -1252,22 +1287,25 @@ static int update_parent_subparts_cpumask(struct cpuset *cs, int cmd,
>>   	if ((cmd != partcmd_update) && css_has_online_children(&cs->css))
>>   		return -EBUSY;
>>   
>> -	/*
>> -	 * Enabling partition root is not allowed if not all the CPUs
>> -	 * can be granted from parent's effective_cpus or at least one
>> -	 * CPU will be left after that.
>> -	 */
>> -	if ((cmd == partcmd_enable) &&
>> -	   (!cpumask_subset(cs->cpus_allowed, parent->effective_cpus) ||
>> -	     cpumask_equal(cs->cpus_allowed, parent->effective_cpus)))
>> -		return -EINVAL;
>> -
>> -	/*
>> -	 * A cpumask update cannot make parent's effective_cpus become empty.
>> -	 */
>>   	adding = deleting = false;
>>   	old_prs = new_prs = cs->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(cs->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, cs) &&
>> +		    cpumask_equal(cs->cpus_allowed, parent->effective_cpus))
>> +			return -EINVAL;
>> +
> You might consider switching these around to check the cpumasks first.
Good point, partition_is_populated() is more expensive.
>
>
>>   		cpumask_copy(tmp->addmask, cs->cpus_allowed);
>>   		adding = true;
>>   	} else if (cmd == partcmd_disable) {
>> @@ -1289,9 +1327,10 @@ static int update_parent_subparts_cpumask(struct cpuset *cs, int cmd,
>>   		adding = cpumask_andnot(tmp->addmask, tmp->addmask,
>>   					parent->subparts_cpus);
>>   		/*
>> -		 * Return error if the new effective_cpus could become empty.
>> +		 * Return error if the new effective_cpus could become empty
>> +		 * and there are tasks in the parent.
>>   		 */
>> -		if (adding &&
>> +		if (adding && partition_is_populated(parent, cs) &&
>>   		    cpumask_equal(parent->effective_cpus, tmp->addmask)) {
> Same.
Thanks,
Longman

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