linux-kernel - Re: [PATCH v9 3/7] cgroup/cpuset: Refining features and constraints of a partition

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Date:   Wed, 15 Dec 2021 15:49:44 +0100
From:   Michal Koutný <mkoutny@...e.com>
To:     Waiman Long <longman@...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>, 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>
Subject: Re: [PATCH v9 3/7] cgroup/cpuset: Refining features and constraints
 of a partition

On Sun, Dec 05, 2021 at 01:32:16PM -0500, Waiman Long <longman@...hat.com> wrote:
> @@ -1455,34 +1450,16 @@ static void update_cpumasks_hier(struct cpuset *cs, struct tmpmasks *tmp)
>  			switch (parent->partition_root_state) {
> [...]
> -
>  			case PRS_ENABLED:
> -				if (update_parent_subparts_cpumask(cp, partcmd_update, NULL, tmp))
> -					update_tasks_cpumask(parent);
> +				update_parent = true;
> [...]
> +		if (update_parent) {
> +			if (update_parent_subparts_cpumask(cp, partcmd_update, NULL, tmp))
> +				update_tasks_cpumask(parent);
> +			/*
> +			 * The cpuset partition_root_state may be changed
> +			 * to PRS_ERROR. Capture it.
> +			 */
> +			new_prs = cp->partition_root_state;
> +		}

IIUC, this ensures that when a parent becomes partition root again, this
would propagate downwards to invalidated children. 

However, the documentation says:

> +       Changing a partition root (valid or invalid) to "member" is
> +       always allowed.  If there are child partition roots underneath
> +       it, they will become invalid and unrecoverable.  So care must
> +       be taken to double check for this condition before disabling
> +       a partition root.

I.e. it suggests a child can be trapped in the unrecoverable state (i.e.
not fixable by writing into cpuset.cpus.partition).
But this does not happen, right?

Michal