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

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Message-ID: <Ybew7d2oE2gLcLNO@slm.duckdns.org>
Date:   Mon, 13 Dec 2021 10:45:33 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Waiman Long <longman@...hat.com>
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 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.

> +	/*
> +	 * 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?

Thanks.

-- 
tejun