Sender: Tejun Heo <htejun@gmail.com>
Date:   Mon, 13 Dec 2021 10:45:33 -1000
From:   Tejun Heo <tj@kernel.org>
To:     Waiman Long <longman@redhat.com>
Cc:     Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Shuah Khan <shuah@kernel.org>, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kselftest@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Roman Gushchin <guro@fb.com>, Phil Auld <pauld@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>
Subject: Re: [PATCH v9 2/7] cgroup/cpuset: Allow no-task partition to have
 empty cpuset.cpus.effective
Message-ID: <Ybew7d2oE2gLcLNO@slm.duckdns.org>
References: <20211205183220.818872-1-longman@redhat.com>
 <20211205183220.818872-3-longman@redhat.com>
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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