linux-kernel - Re: [PATCH 1/1] cgroup/cpuset: Rebuild sched domains if isolated partition changed

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Date:   Sun, 17 Sep 2023 12:51:43 -0400
From:   Waiman Long <longman@...hat.com>
To:     Pierre Gondois <pierre.gondois@....com>,
        linux-kernel@...r.kernel.org
Cc:     rui.zhang@...el.com, aaron.lu@...el.com,
        Zefan Li <lizefan.x@...edance.com>, Tejun Heo <tj@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>, cgroups@...r.kernel.org
Subject: Re: [PATCH 1/1] cgroup/cpuset: Rebuild sched domains if isolated
 partition changed

On 9/15/23 11:45, Pierre Gondois wrote:
> When an isolated parition is created, the sched domains (sds) are
> rebuilt and the sds of the isolated CPUs are detached. This only
> happens at the creation of the isolated parition. Updating
> the cpuset of the partition doesn't rebuild the sds:
>
> To reproduce:
>    # ls /sys/kernel/debug/sched/domains/cpu0/
>    domain0
>    # ls /sys/kernel/debug/sched/domains/cpu1/
>    domain0
>    #
>    # mkdir cgroup
>    # mount -t cgroup2 none cgroup/
>    # mkdir cgroup/A1/
>    # echo "+cpuset" > cgroup/cgroup.subtree_control
>    # echo 0-3 > cgroup/A1/cpuset.cpus
>    # echo isolated > cgroup/A1/cpuset.cpus.partition
>    #
>    # ls /sys/kernel/debug/sched/domains/cpu0/
>    # ls /sys/kernel/debug/sched/domains/cpu1/
>    #
>    # echo 0 > cgroup/A1/cpuset.cpus
>    # ls /sys/kernel/debug/sched/domains/cpu0/
>    # ls /sys/kernel/debug/sched/domains/cpu1/
>    #
>
> Here CPU1 should have a sched domain re-attached.
>
> Signed-off-by: Pierre Gondois <pierre.gondois@....com>
> ---
>   kernel/cgroup/cpuset.c | 12 ++++++++----
>   1 file changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 58e6f18f01c1..e3eb27ff9b68 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -1680,11 +1680,15 @@ static void update_cpumasks_hier(struct cpuset *cs, struct tmpmasks *tmp,
>   		 * empty cpuset is changed, we need to rebuild sched domains.
>   		 * On default hierarchy, the cpuset needs to be a partition
>   		 * root as well.
> +		 * Also rebuild sched domains if the cpuset of an isolated
> +		 * partition changed.
>   		 */
> -		if (!cpumask_empty(cp->cpus_allowed) &&
> -		    is_sched_load_balance(cp) &&
> -		   (!cgroup_subsys_on_dfl(cpuset_cgrp_subsys) ||
> -		    is_partition_valid(cp)))
> +		if ((!cpumask_empty(cp->cpus_allowed) &&
> +		     is_sched_load_balance(cp) &&
> +		     (!cgroup_subsys_on_dfl(cpuset_cgrp_subsys) ||
> +		      is_partition_valid(cp))) ||
> +		    (cp->partition_root_state == PRS_ISOLATED ||
> +		     cp->partition_root_state == PRS_INVALID_ISOLATED))
>   			need_rebuild_sched_domains = true;
>   
>   		rcu_read_lock();

Thanks for spotting the problem and sending out a patch to fix it. 
However, it should be done in the update_cpumask(). The sched_domain 
rebuild in update_cpumasks_hier() is supposed to be used for impacted 
descendant cpusets lower down in the hierarchy.

Anyway, I believe this problem should have been fixed in commit 
a86ce68078b2 ("cgroup/cpuset: Extract out CS_CPU_EXCLUSIVE & 
CS_SCHED_LOAD_BALANCE handling") recently merged into the v6.6 kernel. 
Would you mind testing the latest upstream kernel to see if this problem 
is gone or not?

Thanks,
Longman